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Sed.Home | Alph | Intro | BasicClass | QFLClass | Keys | Depo.Envir | Evol | Evol.Model | Sed.Tect | Self Tests

A Basic Classification Derived From theSimple Ideal Model

How You Classify It, Is How You Think About It

A Basic Sedimentary Rock ClassificationA good classification is based on some theory that explains

how the rocks form, and are related to each other. We want togroup together rocks that form by similar processes. The mostgeneral theoretical model we have for sedimentary rocks is thesimple ideal model . The basic classification is based on thatmodel.

The complication with sedimentary rocks is they form fromsuch a diversity of processes that straight forwardclassifications are difficult. They are unlike igneous rockswhere a relatively straight forward Texture and Composition classification leading directly to interpretation is possible.

For the basic sedimentary rock classification you can use

the table at the top (go to enlarged clickable version ), or usethe key (also linked in the box above left), or go to the alphabetical listing to directly studyindividual sedimentary rocks. An explanation of the theory behind the basic classification is

below.More sophisticated classifications are described in other pages ( Clastic {QFL key, or QFL

ternary}, and Carbonate ).

Reasoning Behind the Basic Classification

CLASSIFICATIONSYSTEMS:

Theory of ClassificationBasic (this page)

Reading Ternary Diagrams: Clastic:

>> QFL Key>> QFL Ternary

Carbonate IDENTIFICATION K EYS:

Basic Clastic Rock Chemical/Biochemical Carbonate

Alphabetical List of Rocks

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The simple ideal model for the evolution of sedimentary rocks says there are three end products, three attractors, that all sedimentary processes are working to reach - quartzsandstone, shale, and limestone. {html version of all illustrations on this page combined; pdf version for

printing}

The three attractors in the simple ideal model are not isolated, however; each one standsfor a class of weathering products. For example:Quartz sandstone = all visible grains , including such ones as incompletely weatheredfeldspar from the granodiorite in the simple ideal model.

Shale = all clay sized grains (clay is a generic name; there are many kinds of clay mineralsas well as other minerals that are clay sized)

Limestone = all dissolved minerals , including not only calcite CaCO 3, but also halite (tablesalt; NaCl), and gypsum (CaSO 4 . H 2O) among others.

On the chart below we can see these additional weathering products.

OBSERVE: A source of confusion is the switch from mineral names to rock names. Clay is a mineral that forms from the weathering of feldspars such as orthoclase . Clay is extremely fine grained; it is the mud on your shoes,or the mud (suspended clay) in a river or pond. When clay is deposited it becomes the rock shale Calcite (CaCO 3 ) is a mineral with rhombohedral cleavage that reacts with dilute hydrochloric acid. As a

sedimentary mineral it exists either as micrite (" lime mud", that is as fine grained as clay) or large pieces suchas animal skeletons. These together form the rock limestone . Quartz sand is, or course, just quartz sand. It is released as grains from the parent rock by weathering, and after that is little changed. See the grains in this alkali granite . All but the quartz will weathering into

something else.In addition, some mineral names also refer to the rock, so gypsum the mineral is also gypsum the rock.

Sometimes we also make a distinction, such as dolomite the mineral and dolostone the rock, or halite themineral and rock salt the rock.

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So far, so good. But now we have to do some mixing and splitting. Sedimentary rocks aregenerally divided into three great categories, siliciclastic (or simply, clastic ) rocks ,chemical rocks and biochemical rocks . Their relationships to the three divisions fromthe simple ideal model are shown in the figure below. Observe how visible grains and clay

sized grains mix together to form clastic rocks, while minerals in solution split to formchemical and biochemical rocks.

(Silici)Clastic rocks are composed of weathering products that do not dissolved into water,have silica (SiO 2) as one of their major components, and are transported either by rollingalong the bottom, or in suspension. Because of this the VISIBLE GRAIN and CLAY SIZEDGRAIN weathering products in the chart above tend to be mixed, and deposited together. Sowe group them together as SILICICLASTIC rocks. ( Further explanation below ).

Minerals IN SOLUTION on the other hand tend to be deposited together, and are notgenerally deposited in the presence of siliciclastic rocks. Indeed, these rocks are so differentfrom clastic rocks that geologists often specialize in studying either one group or the other.Thus we split these out from clastics.

To deposit minerals that are in solution, they must some how come out of solution and thishappens two ways. Either they precipitate directly from sea water (usually because it isevaporating and concentrating the salts) - CHEMICAL ROCKS .

Or " plants" and " animals" extract the dissolved minerals from the sea water to makeskeletons, their skeletons eventually becoming part of the sediment as BIOCHEMICALROCKS . So, minerals in solution split into two categories.

Note that we still end up with a classification with three main categories, it is just aslightly different three categories than the simple ideal model.

CHEMICAL/BIOCHEMICAL ROCKS

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Chemical/biochemical rocks fall intoseveral groups, depending on how they form.

CARBONATES : These are composedof the mineral calcite (CaCO 3 - calcium

carbonate), and are thus all known ascarbonates . Note on the chart that there aremany of these, that they form by bothchemical and biochemical processes, andthat they tend to be mixed together in various

combinations in the rocks. They are extremely abundant and important.

O THER C HEMICAL R OCKS: These rocks fall into two categories. Chert is asiliceous rock (composed of SiO 2) that forms from the recrystalized skeletons of " animals "(single celled radiolarians, and glass sponges) or single celled " plants " (diatoms,silicoflagellates). And although the silica comes from skeletons to become chert it must be

chemically recrystallized, thus putting it in the chemical category (but it is confusing). Rock salt (halite; NaCl) and gypsum (CaSO 4 . H 2O) originally are dissolved in the seawater, thus making the sea salty. When sea water evaporates in a closed area, such as alagoon, the salt concentration becomes very high, supersaturated, and precipitates out. The

process is common in desert areas, with examples today in the Red Sea and Dead Sea in theMiddle East, both highly saline.

O THER BIOCHEMICAL ROCKS: Peat and coal because they come from plantremains are biochemical rocks, but unlike all the other chemical/biochemical rocks peat andcoal always form in the presence of clastic rocks - sandstones and shales.

This discussionillustrates part of the difficulty of developing a completely consistentclassification. Sedimentary rocks form in so many different ways, from so many different

processes that coming up with one scheme that is inclusive and yet straight forward is noteasy. There always seem to be exceptions to the rule that have to have to be explainedindividually.

(SILICI ) CLASTIC ROCKSClastic rocks (sandstones, shales, etc.) are classified on two criteria - texture (grain size),

and composition (that is, QFL ). Shale , of course, is both fine grained, and composed of clay, so the name "shale"incorporates both properties of texture and composition.

For the coarser sizes, where the mineral grains can be seen by eye or with a handlens, a

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complete name consists of two parts. For example, an " arkose sandstone " is a rock composed of sand sized particles, and a large percentage of those particles are composed of feldspar.

For this basic classification we are only concerned with particle size; composition will besaved for the QFL (quartz, feldspar, lithics) classification .

Clastic particles are divided into size categories based on the WENTWORTH SCALE .This scale has been in use for over a hundred years and is universally recognized. The middlesize is 2 mm, the boundary between sand and gravel. Note that the size categories getgeometrically larger, and smaller, from 2 mm. Sand, for example, ranges from 1/16 mm to 2mm, while granules range from 2 mm to 4 mm, and pebbles from 4 mm to 64 mm. Thereason for this is we want to group together particle sizes that can moved more or lesstogether by running water.

The Wentworth scale is straight forward, and with a ruler for scale it is relatively easy toclassify the rock. One simplification, and one complication, though. The simplification is, wegenerally group all the greater than sand sized particles into one category - gravel , unless

there is a specific need to distinguish these grain sizes. The complication is, gravel is dividedinto whether the grains are angular (breccia), or rounded (conglomerate).

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Sed.Home | Alph | Intro | BasicClass | QFLClass | Keys | Depo.Envir | Evol | Evol.Model | Sed.Tect | Self Tests

I NTRODUCTION TO QFLQFL stands for Quartz, Feldspar, Lithics

Sedimentary rocks are classified on the basis of the texture (grain size) of the rock, andcomposition .

The basic classification only concerned texture, using the Wentworth size scale . But any full

rock name must specify both texture and composition. Thus, an arkose sandstone is a rock of sand sized particles, with a high percentage of those particles being feldspar.

It might seem that an unlimited variety of particles could end up in a sedimentary rock. After all, there are over 6000 known minerals. In addition, any incompletely weathered piece of igneous, sedimentary, or metamorphic rock can also be found in a sedimentary rock.

A composition classification could become very complicated if all of these different particleswere considered. But in most cases rock composition can be defined by four compositionalcomponents:>>> Quartz>>> Feldspar>>> Lithic fragments (including rock fragments and mineral grains other than quartz)

>>> Matrix (a catchall for the silt and clay grains that cannot be easily seen by eye).Descriptions of each category are below. You will learn to recognize and identify them as

you study the rocks.

COMPONENTS OF SILICICLASTIC SEDIMENTARY ROCK COMPOSITION

Quartz Since quartz , for all practical purposes, does not weather into anything else, and will remainafter everything else is weathered or sorted out, it is one of the most important of the four components of sedimentary rock composition.

Pure quartz sandstones are rare. Usually quartz is mixed with one or more of the remainingthree components. Pure quartz sandstones form only under great tectonic stability when the land isnot high enough for rocks to be exposed to weathering, such as during Wilson Cycle Stage A andStage I .

Feldspar Feldspars are some of the most abundant minerals in the earth's crust. With only a fewexceptions all igneous rocks have large amounts of feldspar; e.g. calcium plagioclase in gabbro ,and sodium plagioclase and orthoclase in granite . Medium to high grade metamorphic rocks alsohave large amounts of feldspar.

Sediments near high mountains frequently have large percentages of feldspar as batholiths andregional metamorphic rocks are uplifted and eroded (Wilson Cycle Stage F and Stage H ). Riftsystems ( Stage B and Stage C ) frequently also have large amounts of feldspar.

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Lithics Very simply, if a siliciclastic particle is not quartz or feldspar it is classified a lithic fragment.Lithic means "rock" and all mechanically weathered pieces of another rock, or non-feldspar minerals weathered from a rock, are included here. Frequently they are small, dark in color, anddifficult or impossible to specifically identify in hand specimen. The exception to this isconglomerates and breccias. Lithic fragments are especially abundant in volcanic arc systems(Stage E ), but are common in most collision mountain buildings ( Stage F and Stage H ).

Matrix Matrix is the finer material in which larger particles are embedded. So, in a sandstone the matrixis silt and clay. In a gravel the matrix may be a sand. However, since all minerals other than quartzwill eventually weather into silt or clay sized particles, silt or clay is very common in sedimentaryrocks.

QFL CLASSIFICATIONAND IDENTIFICATION SYSTEMS

A Key For Clastic Sedimentary Rock IdentificatonThe simplest way to identify clastic rocks from composition and texture is with a key. To use

the key you might need several things:>>> A handlens to identify feldspar and lithics, especially with sand sized grains.>>> A metric ruler or grain size chart (link).>>> Some experience in identifying minerals and rock fragments in a rock.

Identifying QFL in a rock usually takes practice. One way to proceed is to have hand samplesof quartz and the feldspars to compare with. A second option is to go to (or print out in color)Bowen's Reaction Series where the minerals are listed, with large scale pictures available for comparison.

Recognizing quartz often gives new people trouble; they often want to identify it as feldspar.The problem is quartz can be clear and glassy , or rounded and frosted. It also varies in color from gray, to white, to pink, etc. But, with rare exceptions, most of the minerals in a rock will bequartz since it is the most abundant sand sized particle, and as weathering takes place it just keepincreasing in abundance.

Feldspars, on the other hand, have cleavage faces ( quartz has no cleavage) that can sometimes be seen with careful looking, such as in Ca plagioclase , Na plagioclase , and orthoclase .

The key is going to drop the rocks into relatively clear-cut categories of arkosic, lithic, andquartz rich, and further subdivide them on grain size.

A Ternary SystemFor Clastic Sedimentary Rock Identificaton

The classification key used above seems very straight forward - until one begins to look atactual rocks. Then we begin to see that not all rocks are pure sandstone, or shale, but are oftenmixtures of grain sizes. Likewise, rocks are often not pure arkoses (feldspar rich) but may alsohave mixtures of lithics too.

Thus, we need a classification system that not only can handle the pure rocks, but also all the

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combinations in between. And it should be easy to use.The universal system used by geologists is based on the

ternary diagram. Ternary diagrams are not hard to read, but if you are not familiar with them read the short primer to ternary diagrams .

This classification requires two ternary diagrams, onefor determining texture, a second for determining QFL.These diagrams are discussed separately below, but arealso combined into one diagram . And sometimes we needa chart to help estimate percent abundance

QFL Diagram:The QFL diagram is to the right. Observe the following:>>>> Quartz is at the top, feldspar on the lower left, andlithics on the lower right. It is always done this way.

>>>> The ternary diagram is divided into 5 fields, herecolor coded. The boundaries among the fields, left andright, are at the 50% boundary, and up and down at the75% and 90% boundaries.

>>>> As you travel toward any apex the quantity of Q,F, or L increases accordingly, with 100% being, of course,

right at the apex.

>>>> Notice that as we travel vertically the amount of quartz in the rock increases, and at the90% boundary and above the rock has so much quartz the rock becomes a "quartz something",such as a quartz sandstone or quartz conglomerate.

>>>> The lower two fields contain rocks that are felspar (red) or lithic (blue) rich. That is, theserocks have more than 25% feldspar or lithics, that is, 25-100% feldspar or lithics.

Rocks with this composition have such names as feldspathic (arkosic) sandstone (both termsare used interchangably) and lithic sandstone .

>>>> Remember that all feldspar and lithic fragments are going to weather and disappear (toshale or dissolved minerals), leaving only quartz. On the QFL diagram, however, we can only

plot the abundance of sand (or larger) particles of various compositions. So, on this diagram, asfeldspar and lithics weather the composition of the remaining sandstone migrates toward thequartz apex. No matter where you start on the diagram the sediment is going to evolve in almosta straight line right to the top.

One of the things we are very interested in is how close the sediment has gotten along its pathof evolution. This is the concept of sediment maturity . Thus, above the arkosic and lithic fields,

but below the quartz field are two more fields, subarkosic and sublithic . Rocks in these fieldshave between 10-25 % feldspar or lithics and are thus farther along in their evolution toward

pure quartz than feldspathic or lithic rocks.

>>>> In other classsification systems, the boundaries among the fields sometimes differ from

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Answers to HowGran ite Would Evolveon a Ternary Diagram

Look at apexes of the ternarydiagram to the right. Note quartz isat the to p, but feldspar and lithicsare com bined in the lower left; thisis becau se both of these weather into som ething else - matrix. So,matrix is in the lower right.

Questio n: >>>>> Now, assume agranite h as 25% quartz and the restfeldspar and lithics (mafic

minerals). Where would you p lot it on the diagram?

Question 2: >>>> > Now ass ume the granite is going to w eather, but not betransported or sort ed. Ho w wo uld the composition change on the diagram?

Question 3: >>>>> Now tran sport and sort the sediment. What paths would thecomponents take on the diagram?

Analysis: A granite is an igneous rock, and therefore has no matrix. If it has 25%quartz then we plot granite on the left side at the 25% level.

As the granite weathers, feldspar/lithics turn into matrix, so the compositionmoves toward the right side, the matrix side. Since there is no sorting, the

feldspar/lithics and their weathering product, matrix, remain mixed with the quartz. And since quartz does not weather, its abundance must remain at 25%. Thus theevolution line can only move horizontally, and the farther it moves to the right theless feldspar/lithics there is and the more matrix, until on the right line all

feldspar/lithics would be turned into matrix.

Finally, we transport and sort the sediment, such as at the shore, where quartz remains on the beach while the matrix (shale) dr ifts off shore to the shelf.

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T he point is, everything we have learned about sedimentary rocks over the past

pages and pages of text and diagrams can all be summarized in this one simplediagram - if you understand and know how to read it. Such diagrams are quick, easy,

and slick, and tell a geologist at a glance a world of information - without gettingmired in all the technical details.

In the Simple Ideal Model for the Evolution of Sedimentary Rocks we said that if weathering, transportation, and sorting go to completion all that remains are three endmember rock compositions.

Siliciclastic Rocks Carbonate Rocks

QUARTZ arenite SHALE LIMESTONE/DOLOMITE

Limestones are not single composition rocks but a group of related rocks all composedof CaCO 3 and reacting with dilute HCl acid. Limestone [CaCO 3] is also chemically

related to dolomite [CaMg(CO 3)2]. Because all these rocks have CO 3- -

in common theyare called the carbonates . The composition of most carbonates is derived from acombination of biological and chemical components.

Two carbonate classification systems are in common use today, one by R.L. Folk andthe second by R.J. Dunham . The Dunham system is based on depositional texture (thatis, the amount of matrix surrounding the grains at the time of deposition). It uses suchnames as mudstone, wackestone, packstone, grainstone, and boundstone. Eachclassification has its strengths, but we use the Folk system here.

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Carbonate rock names (limestones and dolomites) consist of a conjunction of twonames, one describing the ALLOCHEMS, the large pieces, the other describing theINTERSTITIAL MATERIAL. Allochems are equivalent to gravel, sand, lithics or feldspars in the siliciclastics. Interstitial material is equivalent to clay or cements in

clastics. There are four kinds of allochems:

ALLOCHEMS

Fossils - may be whole fossils, or broken and abraded fossils;all are called "bio" fragments

Oolites - small, pearl-like spheres

Pellets - fecal pellets produced by invertebrate animals; look superficially like oolites but are dull and not pearl-like

Intraclasts - chunks of eroded limestone deposited as aconglomerate

Micrite and Sparite Interstitial Material Micrite is "lime mud", the dense, dull-looking sediment made of clay sized crystals of CaCO 3. Much micrite today forms from the breakdown of calcareous algae skeletons. Itis not clear if all ancient micrites formed in the same way. Many carbonates arecomposed of nearly 100% micrite. Such rocks are simply called micrites.

With carbonates containing allochems the question is whether micrite is present or absent as an interstitial material, and if present by how much. If micrite is present duringdeposition then it fills the spaces between the allochems and the rock will be given aname which describes the allochems in a micrite matrix. For example, a rock with fossilfragments embedded in micrite is called a " biomicrite ". Biomicrite is analogous to asiliciclastic wacke, sand imbedded in a lot of matrix.

If, on the other hand, the depositional environment has strong currents, onlyallochems may be deposited. If we could see the sediment during deposition all theallochems would be loose, like a pure sand or gravel. This is analogous to a 100%siliciclastic sand on a beach with no silt or clay. (With carbonates, though, such "clean"sands are not confined to beaches.) Micrite in these cases, being clay sized, has beenwashed away. The rock formed is then composed only of allochems, held together byclear to translucent calcite crystals with rhombohedral cleavage (called SPAR or SPARITE) acting as a cement. The spar is precipitated from fresh or marine water

percolating through the sediment after deposition, but before final cementation. Thisoosparite shows well the spar cement.

Classification of CarbonatesThe classification of carbonates using the allochem/interstitial material system (the

Folk System) is very systematic and straight forward. The allochem name is combinedwith the interstitial name (micrite or spar). The table below shows the major categories of carbonate rocks based on their allochems and interstitial material.

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But what happens if there is more than one allochem in the rock, or there is a mixtureof micrite and spar? This classification system has great flexibility and creativity. Youcan easily build your own descriptive rock names. The name is built up by stringingtogether all the allochem names in order from least to most abundant, and then adding theinterstitial material name ("matrix" below for short). For example, a rock like this:

Oolites + Fossils + Spar matrix = Oo bio spariteThe name is written as one word, Oobiosparite .

Another example (again allochems from least to most abundant):

Pellets + Oolites + Fossils + Micrite matrix = pel oo bio micriteThe name is written as one word, Peloobiomicrite .

But what if there is both micrite and spar matrix? The system is the same; just listthem from least to most abundant.

Fossils + Spar matrix + Micrite matrix = bio spar micrite

This system goves through other levels of refinement, such as the table below wherethe abundance of allochems is dealt with. Other modifiers deal with different sizes of allochems. But all this is beyond our discussion here.

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A classification such as this one works well if you want to construct rock names fromobservations. The system, however, does not lend itself well to constructing keys for classification. A key requires the establishment of arbitrary categories of rocks, and asystem like the one above deal with all the myriad combinations that are possible.

Nonetheless, we have constructed a key to identify rocks based on their allochems andinterstitial material. Just be aware that its main weakness is that there are always rocksthat do not fit easily into its simple categories.

Carbonate Identification Key

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pdf version - (What is pdf? )

S implistically the earth's surface is divided into two broad divisions: (1) Sourcelands - areaswhich are sources of sediment, usually meaning mountainous regions, but including any placeerosion and sediment removal takes place, and (2) Depositional Basins - areas which receiveeroded sediment. We are interested in both sourcelands and depositional basins, but depositional

basins are our primary interest here.Depositional basins are subdivided a number of ways. One is by the predominant sediment

type found in the basin; thus, Siliciclastic Dominated Systems and Carbonate Dominated

Systems .Carbonate dominated systems occur in tectonically stable regions where the supply of siliciclastic sediment is minimal or non-existent. Most of the remaining discussion is for siliciclastic dominated systems.

A second way of partitioning depositional basins is into terrestrial, transitional (shoreline andnearshore), and marine (epicontinental sea to deep water ocean basins) divisions.

A third way of dividing depositional basins is into depositional environments. All thesedivisions are interrelated, but depositional environments are the primary discussion here.

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Depositional Environments A depositional environment is a portion of the earth's surface characterized by a unique

combination of physical, chemical, and biological processes . These processes control howsediment is transported and deposited, what chemical modification it undergoes, and what kind of

organisms live in and affect the sediment. The processes result in the characteristic anddistinctive sedimentary deposits typical of each environment by which we recognize and identifythem.

Classification of Environments by Depositional Processes

Terrestrial Transitional Marine

Alluvial FanBraided River Meandering River (including alluvial plain)

Lake

Delta ComplexBeachLagoonTidal Flat

Dunes

Shelf (storm and tidal dominated)Submarine fanBasin

Carbonate

Tidal FlatLagoon

Carbonate Barrier (reef)Shelf Basin

Several dozen depositional environments have been identified. The more common ones arelisted in the table above. Most environments you will find familiar, although you have probablynot thought of them from a process point of view. Of the many depositional environments known,a smaller number are of special importance because they cover large portions of the earth's

surface, or are common in the rock record. It is this smaller number of environments we explorehere.

OBSERVE: In everyday speech we talk of a beach, for example, as a place we can go visit, andour language encourages us to think of environments as places. You will probably have toremind yourself that in geology depositional environments are defined by processes.

Systematic Changes From Sourceland to Depositional BasinDepositional environments are not distributed randomly or haphazardly. Each exists in a

specific location for specific reasons. A general observation we can make is: sediment changesfrom the sourceland to the depositional basin such that depositional environments evolve insystematic and predictable sequences from sourceland to depositional basin. In general,depositional environments and thus rock types are predictable from sourceland to depositional

basin.For example, large siliciclastic particles dominate near the sourceland in alluvial fans and

braided rivers, and fine siliciclastic particles dominate at the end of the sequence in ocean basinsfar away from the sourceland. An ideal downstream sequence of environments is illustrated on

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the Siliciclastic Depositional Systemsmodel at the top of the page.

Long SystemsThe complete sequence of

depositional environments frommountain to ocean basis is what we call a"long system". The sediments at the

beginning of long systems arechemically and mechanically unstable incomposition, and are generally poorly

sorted, immature gravels. Within the ideal long system the distance or time of final depositionfrom sourceland to basin is great. Thus, by the end of the long system the sediments arechemically and mechanically stable in composition, and are well sorted into the end member sizes of sand and clay. Sandstones at the end of the long system are mature quartz arenites.

Although complete long systems are found, many variations on the model occur, usually

because environments are missing from the sequence. There is one inviolate observation,however:

OBSERVATION: Individual environments can be missing from the ideal long sequence, but theoverall sequence of environments is never rearranged so that a downstream environment comes

before an upstream one.

Short Systems" Short systems" are environmental sequences in which only a few environments from the

ideal long sequence are present. Short system environmental sequences are abbreviated because,unlike the long system, the siliciclastic sourceland is proximal to (close to) the basin. Sediments

across the entire system are chemically and texturally immature. They are generally poorly sortedand range in size from gravel to coarse sand.

Short systems are commonly observed in tectonically active regions. Depending oncircumstances, virtually any of the environments from the long system can be missing. For example, in the Basin and Range province of the western United States large alluvial fans form atthe base of fault block mountains and everything else downstream from the mountains is missing;a very short system.

One type of short system is common enough it has a special name - Fan Delta. Fan deltas formwhere high mountains stand abruptly out of the sea, such as in rift valleys (see Wilson CycleStage B ), or next to subduction-created volcanic arcs (see Wilson Cycle Stage E ), or next to thethrust belt terrain (see Wilson Cycle Stage F ). A typical sequence of depositional environments in

a fan delta is alluvial fan and/or braided-river to a gravel beach to a submarine fan to a basin.

Carbonate SystemsCarbonate systems develop where siliciclastic sourcelands are low and/or very distant, the

water is shallow, and climates are tropical to subtropical. In the warm, clear, shallow water calcareous algae flourish and generate micrite, while invertebrate animal skeletons accumulate assedimentary particles (bioclasts). The oolitic, pelletal, and intraclastic allochems are also

produced locally, depending on conditions.

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It is not unusual for carbonates to be generated distally in long and short siliciclastic systems.Under these conditions, however, the volume of siliciclastic sediment usually overwhelms thecarbonates. The major exception is extreme distal shelves of both long and short systems wherethe siliciclastic sediment does not reach.

Carbonate systems are, of course, not going to have terrestrial environments (unless we

include supertidal flats). Transitional and marine environments dominate. Carbonateenvironments are modeled as "belts" running parallel tothe coastline. The full model has 9environmental belts, but here it is simplified to five: tidal flat, lagoon, reef, shelf, and basin (seecarbonate depositional environments ).

The " Introduction to Sedimentary Rocks " presents a simple, ideal modelillustrating the origin of the three most abundant kinds of sediment: quartz sand, shale,and limestone . Yet in the real world sedimentary particles and rocks come in muchgreater variety than just these three, as we can see from just the alphabetical list of sedimentary rocks .

Here we develop further models to explain how some of the diverse sedimentaryrocks are related to each other. Furthermore, these sediments can be related totectonics, depositional environments, sedimentary sequence position, sea level changesand the whole collection of processes by which sediments and environmentssystematically evolve downstream within ideal long and short depositional systems.

A diagram for the evolution of sedimentary rocks in both long and short systems isillustrated at this evolutionary diagram . The discussion begins with the long systemand then examines a short system. The models are ideal, and makes some assumptionsyou should be aware of.First Assumption: Sediments, sedimentary structures and sequences, anddepositional environments all evolve in unison downstream. In the real world thesemay not evolve synchronously.

Second Assumption: The sourcelands are simple; they are uniform in composition,even if of mixed parent rocks, and do not have complex tectonic histories.

Third Assumption: Environments are found in their ideal state without significanttransitions from environment to environment.

You may want to print out " The Evolution of Depositional Environments, Sedimentary Rocks, and Rock Sequences &quot. Refer to it frequently while reading about the model; the model is as muchvisual as verbal. (Note that the model is in landscape mode and you will have to set your printer accordingly.)

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The Long-System Model Upper Path on evlutionary diagram :We begin the long-system model in the upper left with a tectonically active fault-

block mountain built on a continental sourceland. On a continent the mountains arecomposed of felsic igneous rocks (e.g. granites, granodiorites, syenites). They undergo

primarily mechanical weathering to produce coarse, angular weathering products.These angular fragments are deposited rapidly at the base of the mountain in alluvialfan environments during torrential rains, producing deposits of arkose (feldspathic)

breccia. Because the sediments are dumped rapidly they form coarse grained,disorganized, matrix supported deposits called debris flows.

Some of the debris flow deposits get buried in the alluvial fan and become part of the geologic record. Most of the sediment continues to move downstream. Duringtransportation the angular breccia fragments are abraded and rounded and the rock evolves into an arkose conglomerate. Arkose conglomerates are common in alluvialfan and braided river environments.

Transportation in a braided river is very different from a debris flow. In a braidedriver gravel and sand particles roll and bounce along the bottom in such a way that twocharacteristic types of bedding called L-Bars (gravel beds) and T-Bars (large planar cross beds) form. The gravel in the L-Bars is grain supported, meaning the cobbles,

pebbles, boulders are touching and support each other. This is in contrast to the debrisflow which are typically matrix supported with the gravel not touching and supported

by the smaller particles in between.Near the sourceland, in alluvial fan and braided river environments, energy is high

and sediment moves by a combination of water pushing the grains and gravity helpingthe particles slide and role down steep gradients. The farther downstream however, theless the gradient, and the less gravity is able to help move the particles. More and more

the primary energy moving the particles is the force of running water. Although thisforce is great during a flood and easily moves cobbles and boulders, during morenormal flow the gravel is sorted out and left behind upstream. By the time the distal

braided river is reached most of the gravel is gone (meaning L-Bars no longer form)and T-Bars made of coarse arkose sandstone dominate as the environmental energyfalls.

By the distal braided river chemical weathering is replacing mechanical weatheringas the major process. The feldspars so common in the arkose sediments upstream aredecomposing to clays, and minerals in solution. The result is the sediments areevolving from feldspar rich arkose sandstones to more clay rich subarkose wackes.That is, as the feldspars weather they turn into clay, and as the amount of feldspar sand

declines the relative amount of quartz sand increases.Subarkose wackes in particular are common in the meandering river environment because meandering rivers form best from sediments with abundant silt and clay. Look at the point bar sequence typical of meandering rivers. Notice all the silts and clays atthe top of the sequence which are deposited in the flood plain. Chemical weatheringcontinues in the meandering river so that eventually all the felspar is gone and thesediments consists of only quartz and clay, a quartz wacke.

Thus, by the time the meandering

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river reaches the shoreline transitional environments we are dealing with just twoundissolved products, quartz and clay. At the beach, or wave-washed mouth of a delta then the high energy of the waves continuously and efficiently sorts the sedimentleaving the sand behind on or near the shore while the clay drifts offshore. (Note thatsilt is also a common component here, it consisting largely of QFL grains smaller than

sand.) And since by this time weathering is complete the beach sands are quartzarenites , although some may still contain minor percentages of feldspar.In this long system model, once we reach the beach we have also reached an

epicontinental sea on a tectonically stable craton. Under these conditions the simple,ideal model for the evolution of sedimentary rocks is in force and the result is a quartzsandstone >> shale >> limestone sequence deposited on the beach, shelf, and far shelf environments. If the shelf is frequented by storms then the sediments are deposited inhummocky sequences.

The limestone deposited in the far shelf comes from CaCO 3 dissolved in the river waters during the weathering of feldspars. Limestones (carbonates) typically do notform in the presence of clastics, such as sandstone and shale, and so are not deposited

until the far shelf environment, beyond the reaches of shale deposition. Much or mostof the carbonate is generated when organisms extract it from sea water to form their skeletons, although some, mainly micrites, may be biochemically precipitated.

Lower Path:We now switch to the lower left of the evolutionary model and begin the sediment

evolution process with a complex sourceland of igneous, sedimentary andmetamorphic rocks, such as are produced by a mountain building process. Thissourceland would most likely contain feldspars , but the weathering of all thecomplexity of rocks here would produce mostly abundant lithic fragments so the

model begins with a mechanically weathered, coarse grained lithic breccia .But just as with the arkose breccia above, this sediment begins its transportationdown a long system, undergoing all the transformations of sorting and chemicalweathering that happen along the way. So the lithic breccia will evolve by roundinginto a lithic conglomerate , and by sorting into a lithic sandstone , and by chemicalweathering into a sublithic wacke, and finally to a quartz wacke deposited in ameandering river.

But again, as in the arkosic long system above, these final weathering products getdumped on the beach at the edge of an epicontinental sea on a tectonically stablecraton. The final sorting and precipitating processes result in the quartz sandstone >>shale >> limestone sequence of the simple ideal model.

The point here is that it does not matter what kind of sourceland you start with, if all processes of weathering and sorting are allowed to go to completion the end

products are always the same. In this sense the quartz sandstone, shale, and limestoneof the simple ideal model are like attractors, and all sediments regardless of their starting composition are irresistibly " attracted " to these end products because they arethe most stable outcomes of the sedimentary processes.

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The Short-system Model Short-systems form in tectonically active regions where high mountains are built

very near the depositional basin ( evolutionary model - lower left ). Under theseconditions, both the distance and the time to final deposition are short and systematicchanges in texture, sorting, and particle maturity to end member compositions are not

possible.Short systems form under many circumstances, both continental and oceanic. A

continental fault block mountain, like the one in the long-system model above, could be a short system if a sea existed at the base of the mountain. In the CordilleranOrogeny both a short system and a long system are present.

In a short system the number of environments and distance from sourceland to basin is reduced And because these systems rapidly go from high mountains to deep basins the environments that do remain are often shorter than normal. As a result themechanical energy in each environment is dissipated over a shorter distance making itmore difficult for sorting processes to work efficiently. The result is coarser, more

poorly sorted sediments. Also, chemical weathering does not have as much

opportunity to work and so less clay is generated and quartz does not have anopportunity to increase much at the expense of feldspar and lithics. The end result isless mature sediments at each stage in the sequence.

Still, as in the long-system, the first depositional environment is the alluvial fan atthe base of the mountain. The sediment will be coarse-grained, unsorted, unstratified,matrix supported debris flows of lithic (or arkosic) breccias. As the lithic brecciamoves downstream the angular particles become rounded to form lithic (or arkosic)conglomerates. They are deposited mostly in grain supported gravel L-Bars of a

braided river.So far the short-system is similar to the long-system. Below the braided river,

however, the short-system differs dramatically. There is no long, meandering river, or

delta, or shelf environment, and all of the processes of sorting and weathering whichtakes place in these environments do not happen. Instead, the lithic (arkosic)conglomerates and lithic (arkosic) sandstones of the braided river are dumped directlyonto the beach. This is no beach of white quartz sand; it is a gravel and coarse sand

beach of dark lithic (feldspathic) particles.Imagine being able to study samples of the braided river sands/conglomerates and

beach sands/ conglomerates; how could you tell them apart? They may be depositedonly a few dozen yards apart and will look virtually identical.

Offshore, perhaps only a few hundred yards or so from the beach, the floor of thesea drops away suddenly and steeply to a long underwater slope which descends

perhaps thousands of feet to the basin floor below. Because the sourceland is close,large volumes of lithic (arkosic) sediment continue to pour onto the narrow beach,which cannot hold it all. Much of the sediment bypasses the beach to pour down theslope as turbidity currents forming a submarine fan environment .

The sediments in the submarine fan may be lithic conglomerates or sandy lithicconglomerates in the proximal fan, but distally they become (sub)lithic wackes. Finallyon the ocean floor, beyond the outer edges of the fan, silts and shales, black in color from the low oxygen conditions, are deposited in the basin floor environment.

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The most important message of the short-system is that all the processes of sediment evolution are cut short. As a result immature sediments are deposited indepositional environments which may be atypical in their characteristics (larger sizes,

less maturity, etc.). By extension the longer the system the more it will begin toresemble the ideal long-system. Eventually even a sourceland supplying dominantlylithic sediments to a long system will, by the end of the long system, produce theclassic quartz sandstone >> shale >> limestone sequence of the simple ideal model.

There is only two rocks remaining on the evolutionary chart we have not discussed.These are the wacke quartz conglomerates and quartz conglomerate near the top center of the chart. Quartz for all intents and purposes does not weather. Grains of quartzwhich are sand size eventually become sandstones, but what about large chunks of quartz, such as might be formed in a pegmatite (very coarse felsic igneous rock) or vein quartz precipitated by hydrothermal fluids. These will form quartz gravel. This

quartz gravel is not going to go away, and in a complete long-model it is not likely tomake it all the way to the beach. Yet quartz conglomerates are not that uncommon inthe geologic record. What happens is, the quartz conglomerates just sits around,forever, waiting until the mountain is completely eroded to sea level (a peneplain).When all the terrestrial environments have disappeared the quartz gravel remains as alag deposit (because it lags behind until everything else is gone). Initially it might bequartz mixed with clay, but as time goes by the clay is sorted out leaving behind purequartz (sandy) conglomerates.

ConclusionsThe information in this model is summarized with two deductive arguments:

First Deductive Argument FIRST PREMISE: The composition (the size, shape, sorting, and content) of asedimentary rock is largely dependent on the tectonic regimes in which the sedimentforms and the depositional environments in which it was deposited.

SECOND PREMISE: Environments evolve in systematic and predictable ways fromsourceland to basin floor in each tectonic regime.

CONCLUSION: Therefore, the compositional and textural characteristics of asediment change in corresponding systematic and predictable ways from thesourceland.

Sediment texture and composition are rock properties which help determination thegeologic history of a basin. Sediment textures, sedimentary structures, and structuralsequences shed light on the nature of the transporting and sorting agents, and provideclues to the final environment of deposition.

Second Deductive Argument FIRST PREMISE: Sedimentary structures and sequences of sedimentary structuresfound in a sedimentary rock are determined by the processes characteristic of each

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particular depositional environment.

SECOND PREMISE: Depositional environments evolve in systematic and predictableways downstream.

CONCLUSION: Therefore, sedimentary structures and rock sequences of structureschange in corresponding systematic and predictable ways from the sourceland to the basin.

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Geologists have worked long and hard to discover just what kinds of information siliciclastic rock composition can give, and how accurate it is. This subject takes a semester course just to introduce, but twokinds of information can be used for interpretations. The first is the actual mineral and rock fragmentcontent of the rock. Because many minerals form only under specific conditions it is possible to identifysourceland composition with some confidence. Specific rock fragments, of course, tell directly what thesourceland is made of.

The second kind of information is empirical (actually observed) evidence of the QFL composition of sandstones derived from specifically known tectonic regimes. Sandstone samples collected from manyexamples of each of four tectonic regimes have been plotted on a QFL ternary diagram (see illustration

below). The shaded fields on the ternary diagram represent the sediment compositions associated with eachtectonic regime: stable craton, block faulted continent (including continental rifts), volcanic arcs, andrecycled orogens (arc-continent and continent-continent collision mountains). As the ternary diagram

shows these fields tend to be distinct, indicating that sandstone composition can be used, judiciously, tomake tectonic interpretations.

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What are the 4 states of matter?

Any and all substances can exist in one of 4 forms (also called "states" or "phases")

Which state the material is in basically depends upon its temperature. The easiestexample is water (a liquid). Take away heat and it becomes a solid (we call it ice). Addheat and it turns into a gas we call water vapor. Add more heat and it turns into a highenergy form called plasma. In our study of the earth we are most concerned with the firstthree - plasma becomes more important in astronomy and high energy physics. One of the marvels of water is that it can exist in all 3 states on the surface of the earth - even atthe same place and time. Imagine enjoying a glass of ice water on a humid day and tellme you don't believe in magic.

One way to understand how the 3 phases differ is to discuss how they relate to being putinto something. A solid is rigid with a fixed shape which stays the same no matter whatyou put it in. Put a rock on the table, into a box, into a backpack - it's still the same rock.A liquid is fluid, and affected by gravity. Therefore, liquids will conform to the shape of whatever they are put into, on earth filling the container from the bottom up. Try pouringa glass of water into a different shaped container and you'll see what I mean. The higher energy of gas pushes gas molecules away from each other, causing a gas to attempt to fillup whatever space is available. That's why a balloon is full, with equal pressure at alllocations. By the way, wouldn't balloons be weird if all the gas was at the bottom?

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How do composition and texture relate to igneous rocks?

Igneous rocks are crystalline solids which cool from magma: the liquid phase of solid

rock. Magmas occur at depth in the crust, and are said to exist in "magma chambers," arather loose term indicating an area where the temperature is great enough to melt therock, and the pressure is low enough to allow the material to expand and exist in theliquid state. Many different types of igneous rocks can be produced. The key factors touse in determining which rock you have are the rock's texture and composition.

Texture

Texture relates to how large the individual mineral grains are in the final, solid rock. Inmost cases, the resulting grain size depends on how quickly the magma cooled. Ingeneral, the slower the cooling, the larger the crystals in the final rock. Because of this,

we assume that coarse grained igneous rocks are "intrusive," in that they cooled at depthin the crust where they were insulated by layers of rock and sediment. Fine grained rocksare called "extrusive" and are generally produced through volcanic eruptions.

Grain size can vary greatly, from extremely coarse grained rocks with crystals the size of your fist, down to glassy material which cooled so quickly that there are no mineralgrains at all. Coarse grain varieties (with mineral grains large enough to see without amagnifying glass) are called phaneritic. Granite and gabbro are examples of phaneriticigneous rocks. Fine grained rocks, where the individual grains are too small to see, arecalled aphanitic. Basalt is an example. The most common glassy rock is obsidian.Obviously, there are innumerable intermediate stages to confuse the issue.

Composition

The other factor is composition: the elements in the magma directly affect whichminerals are formed when the magma cools. Again, we will describe the extremes, butthere are countless intermediate compositions. (Composition relates to the mafic andfelsic terms discussed in another question. If these terms are confusing, please refer tothat discussion before continuing.)

The composition of igneous magmas is directly related to where the magma is formed.Magmas associated with crustal spreading are generally mafic, and produce basalt if the

magma erupts at the surface, or gabbro if the magma never makes it out of the magmachamber. It is important to remember that basalt and gabbro are two different rocks based purely on textural differences - they are compositionally the same. Intermediate and felsicmagmas are associated with crustal compression and subduction. In these areas, rock andsediment from the surface is subducted back into the crust, where it re-melts. This allowsthe differentiation process to continue, and the resulting magma is enriched in the lighter elements. Intermediate magmas produce diorite (intrusive) and andesite (extrusive).

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Felsic magmas, the final purified result of the differentiation process, lead to theformation of granite (intrusive) or rhyolite (extrusive).

Igneous Rock Classification

Texture vs. composition

Felsic Intermediate Mafic Ultramafic

Aphanitic

fine grainRhyolite Andesite Basalt

Conditions needed to produce ultramaficflows do not exist innature at this time.

Intermediate Dacite Diabase

Phaneritic

coarse grainGranite Diorite Gabbro Peridotite

Glassy Obsidian

Frothy Pumice Scoria

It is important to note that there are many, many intermediate steps between these maindivisions. Geology is a science full of "shades of gray," and the naming of igneous rocks

is certainly no exception.

lements & minerals common to various magmas

Ultramafic magmas

Olivine - Mg2SiO4 to Fe2SiO4

Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6

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Mafic (basaltic) magmas

Olivine - Mg2SiO4 to Fe2SiO4

Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6

Plagioclase - CaAlSi3O8 to NaAlSi3O8

Intermediate magmas

Plagioclase - CaAlSi3O8 to NaAlSi3O8

Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2

Muscovite/Biotite - KAl2(Si3Al)O10(OH)2

Quartz - SiO2

Felsic (granitic) magmas

Potash Feldspar - KAlSi3O8

Quartz - SiO2

Muscovite/Biotite - KAl2(Si3Al)O10(OH)2

Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2

What are the most important types of rock in the crust?

Excluding the rocks between my ears, I'd have to say that basalt and granite have thehonor of being the most important rocks in the crust.

Basalt and granite actually have quite a bit in common. Both are igneous rocks , whichmeans that they cooled from a magma (the earth gets very hot just below the surface, andthere is lots of liquid rock available). Both are made up of minerals from the silicategroup, so both have large amounts of silica and oxygen . Both will hurt if you drop a big

piece on your toe. But there are several important differences, too. These differences helpdefine and explain how the earth works.

Granite is great stuff! Not only is it my personal favorite, it is without a doubt the mostcommon rock type on the continental land masses. Yosemite Valley in the Sierra Nevada

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and Mt. Rushmore are two notable examples of granitic rocks. But granitic "basementrock" can be found just about everywhere east of the Rockies if you're willing to digthrough the dirt and sedimentary rocks at the surface. Granite is intrusive, which meansthat the magma was trapped deep in the crust , and probably took a very long time to cooldown enough to crystallize into solid rock. This allows the minerals which form plenty of

time to grow, and results in a coarse-textured rock in which individual mineral grains areeasily visible.

Granite is the ultimate silicate rock. As discussed elsewhere in greater detail , on averageoxygen and silica account for 75% of the earth's crust. The remaining 25% is split amongseveral other elements, with aluminum and potassium contributing the most to theformation of the continental granitic rocks. Relatively small amounts of iron andmagnesium occur, but since they have generally higher densities it's not surprising thatthere isn't very much in the granite. Due to the process of differentiation , most of theheavier elements are moving towards the core of the earth, allowing the silica and oxygento accumulate on the surface. And accumulate it has. Enough granitic "scum" has

differentiated to the surface to cover 25% to 30% of the earth with the good stuff. We callthis purified material felsic because of the relatively high percentage of silica and oxygen.

Basalt is extrusive. The magma from which it cools breaks through the crust of the earthand erupts on the surface. We call these types of events volcanic eruptions , and there areseveral main types. The volcanoes that make basalt are very common, and tend to formlong and persistent zones of rifting in nearly all of the ocean basins. We now believe thatthese undersea volcanic areas represent huge spreading ridges where the earth's crust isseparating. It's a lot like a cut on your arm, which will bleed until a scab forms. Basalticmagma is like the blood of the earth - it's what comes out when the earth's skin is cut thewhole way through. As an eruption ends, the basalt "scab" heals the wound in the crust,

and the earth adds some new seafloor crust. Because the magma comes out of the earth(and often into water) it cools very quickly, and the minerals have very little opportunityto grow. Basalt is commonly very fine grained , and it is nearly impossible to seeindividual minerals without magnification.

Basalt is considered a mafic silicate rock. Among other characteristics, mafic mineralsand rocks are generally dark in color and high in specific gravity . This is in large part dueto the amount of iron, magnesium, and several other relatively heavy elements which"contaminate" the silica and oxygen. But this heavy stuff really isn't happy near thesurface, and will take any opportunity it can to head for deeper levels . The trick is to heatthe basalt back up again so it can melt and give the iron another shot at the core. It wantsto be there, and heat is the key which unlocks the door.

As it turns out , most of the ocean floor is basalt, and most of the continents are granite.Basaltic crust is dark and thin and heavy, while granite is light and accumulates intocontinent-sized rafts which bob about like corks in this "sea of basalt." When a continentruns into a piece of seafloor , it's much like a Mac truck running into a Volkswagon. Notvery pretty, but at least there's a clear winner. And the seafloor basalt ends up in prettymuch the same position as does the VW - under the truck (or continent, as the case may

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be). This may seem like a drag for the basalt, but remember that it isn't all that happy onthe surface anyway, and this gives it the heat it needs to re-melt and try to complete thedifferentiation process which was so rudely interrupted at the spreading ridge. If successful and allowed to continue, what's left behind is a "purified" magma with most of the iron, magnesium, and other heavy elements removed. When it cools, guess what

forms? And the continental land mass just got a wee bit larger.