Laboratory 3
Weathering of Rocks and
Formation of Sediment

Pamela J. W. Gore

Georgia Perimeter College

Clarkston, GA 30021

Copyright © 1998-2004 Pamela J. W. Gore

See newer version of this page at http://facstaff.gpc.edu/~pgore/geology/historical_lab/Weathering.pdf

This lab introduces the products of rock weathering. Weathering is important because it is the process through which rocks are broken down and sediment is formed. Sediment is loose particulate material which becomes cemented and compacted to form sedimentary rocks.

TYPES OF WEATHERING

There are three major types of weathering:
  1. physical
  2. chemical
  3. biological

Physical weathering breaks rocks down into smaller pieces. Types of physical weathering include frost wedging, exfoliation, and thermal expansion.

Chemical weathering breaks rocks down chemically adding or removing chemical elements, and changes them into other materials. Chemical weathering consists of chemical reactions, most of which involve water. Types of chemical weathering include:

  • dissolution
  • hydrolysis
  • oxidation

Biological weathering is the breakdown of rock caused by the action of living organisms, including plants, burrowing animals, and lichen (a crusty, rubbery, light green organic material that grows in patches on rocks as well as on wood). Lichen is a combination of fungus and algae, living together in a symbiotic relationship. Lichens can live on bare rock, and they break down rocks by secreting acids and other chemicals. The fungal part of the association secretes the acids, which react to dissolve the minerals, which are then used by the algae. Later, water seeps into the crevices etched by the acid, and assists in the breakdown through freezing (frost-wedging) and chemical weathering.

TYPES OF PHYSICAL WEATHERING

  1. Frost wedging - water expands when it freezes, breaking rocks into angular fragments

    2. Talus slope, Lost River, West Virginia

    The above photograph illustrates a sediment source area. The bedrock is being broken down into sediment of a variety of sizes, primarily by physical weathering processes.

  2. Exfoliation - the bedrock breaks into flat sheets along joints which parallel the ground surface. This phenomenon is caused by the expansion of rock when the pressure of overlying rock is removed by erosion. It is sometiumes called unloading.

    Exfoliation of granite at Stone Mountain has produced a rounded mountain. At the time the granite body cooled, it is calculated that the land in this area stood about 10,000 ft higher than at present. Over the past 325 million years, this 10,000 ft of rock has been eroded away.


    Stone Mountain, Georgia. Stone Mountain is a granite body which is a sediment source area. The second image shows active exfoliation.

  3. Thermal expansion - heat causes expansion; cooling causes contraction. Different minerals expand and contract at different rates causing stresses along mineral boundaries. Repeated daily heating and cooling of rock causes the rock to break down.

TYPES OF CHEMICAL WEATHERING

  1. Dissolution alters rocks by removing soluble minerals. Minerals such as halite, gypsum, and calcite are soluble (dissolve) in water (especially water that is slightly acidic). When the minerals react with water, ions (such as Ca and Na) are released. The ions are carried as "dissolved load" by rivers flowing to lakes or to the sea. As lake or sea water evaporates, the dissolved minerals precipitate or crystallize out as solid minerals. (Examples: halite, gypsum, or calcite. These types of minerals that form from the evaporation of sea water are called evaporites.) Minerals may also precipitate or crystallize from ground water in and around springs (particularly hot springs), and in caves (example - travertine).

  2. Hydrolysis is the process by which feldspar (and some other aluminum-bearing silicate minerals) are weathered to form clay. For example, potassium feldspar weathers to form the mineral kaolinite.
2KAlSi3O8 + 2 (H+ + HCO3- ) + H20 => Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2
potassium
feldspar 		  carbonic acid   water   kaolinite
(clay formed through weathering) 		  potassium ion
(dissolved in water) 		  bicarbonate ion
(dissolved in water) 		  silica
(dissolved in water)

In humid climates (such as the southeastern United States), most of the feldspar in rocks such as granite will weather to form clay.

Nearly all of the minerals in the common rocks of the Earth's crust will weather to form clay (with the exception of quartz). Because of this, clays make up nearly half of the sedimentary rocks on Earth.

  • Oxidation is the process by which iron-bearing minerals weather to produce iron oxides (or "rust"). Iron-bearing silicate minerals which also contain aluminum (such as pyroxene, amphibole, and biotite) undergo both oxidation and hydrolysis, forming both iron oxides and clays. Iron-bearing alumino-silicate minerals weather to form the red clayey soils, such as are found in Georgia, as well as lateritic soils formed in more tropical areas.

    • MINERAL STABILITY IN THE WEATHERING ENVIRONMENT

    Some minerals weather more quickly than others. A few minerals are readily soluble in slightly acidic water, whereas others weather to produce clay, and still others are very resistant to weathering, and persist for a long time without alteration. One of the controls on the weathering of minerals is the temperature at which the minerals originally formed when they crystallized from magma or lava.

    Minerals which formed at high temperatures and pressures are least stable in the weathering environment, and weather most quickly. This is because they are farther from their "zone of stability", or the conditions under which they formed. On the other hand, minerals which formed at lower temperatures and pressures are most stable under weathering conditions.

    The order in which minerals tend to weather is related to the temperature at which they crystallized. You may remember Bowen's Reaction Series, which described the order in which minerals crystallize from magma. There is a similar ordering of minerals as related to their weathering rates, and it is called the Goldich Stability Series.

    The order of mineral stability in the weathering environment is the same order as Bowen's Reaction Series. 

    
Least stable (high temperature minerals)



        Olivine         Ca plagioclase feldspar

        Pyroxene                

        Amphibole

        Biotite         Na plagioclase feldspar

                    Potassium feldspar

                          Muscovite

                           Quartz

                

Most stable (low temperature minerals)

    What happens when granite is weathered?

    Unweathered granite contains these minerals:
    1. Na plagioclase feldspar (white)
    2. K feldspar (pink, but may be white in other granites)
    3. Quartz (gray)
    4. Small amounts of biotite and/or amphibole (black)
    5. and sometimes muscovite (not shown)

    Here is what will happen to each of the mineral constituents in a granite under warm, humid weathering conditions:

    1. The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ions

    2. The sodium and potassium ions will be removed through leaching and will be carried in solution in running water

    3. The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides.

    4. The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to weathering.

    Under warm, humid conditions, the granite bedrock will weather in place until the feldspars alter to soft clay. The weathered rock is called saprolite, a term meaning "rotten rock". In areas of the southeastern U.S. which are underlain by granite (and other igneous and metamorphic rocks), a thick soil zone of weathered rock or saprolite has developed. Where the bedock contained iron-bearing minerals (such as biotite, amphibole, or pyroxene) which weathered to iron oxides, the saprolite has been stained a deep red color. (This is the same principle by which one red sock in a load of laundry can stain all of the clothes red.) The red-stained clay in the saprolite has given rise to the famous Georgia Red Clay.

    What happens after the rock has been weathered to saprolite?

    1. The clays will be eroded and transported by running water to the sea. Clay is fine-grained and remains suspended in the water column. The clay may ultimately be deposited in deep quiet water far from shore.

    2. As the soft clay is removed, the unweathered, residual quartz grains will be released from the saprolite by erosion. The quartz in granite is sand-sized, and it becomes quartz sand. The quartz sand is ultimately transported to the sea, where it accumulates to form beaches.

    3. The dissolved ions (sodium and potassium) will be transported by rivers to the sea, and will become part of the salts in the sea.

    THE CHARACTERISTICS OF SEDIMENT

    Terrigenous sediment is derived from the weathering of pre-existing rocks. (Sometimes it is also called clastic or SILICICLASTIC OR detrital sediment). The grain size of sediment depends on the types of rocks in the source area from which the sediment was derived. The textures and mineralogy of the rocks in the source area control the grain size and composition of the resulting sediment.

    DESCRIBING THE TEXTURE OF SANDS

    Texture refers to the size and shape of the grains in a sediment.

    Sediment can be separated into four main groups based on grain size. These four size groups are gravel, sand, silt, and clay. Some of these groups (gravel and sand) can be further subdivided.

    The sediment grain size scale is known as the Wentworth Scale.

    Particle name

    Particle diameter

    Gravel

    Boulders

    > 256 mm

    Cobbles

    64 - 256 mm

    Pebbles

    2 - 64 mm

    Granules

    2 - 4 mm

    Sand

    Very coarse sand

    1 - 2 mm

    Coarse sand

    0.5 - 1 mm

    Medium sand

    0.25 - 0.5 mm

    Fine sand

    0.125 - 0.25 mm

    Very fine sand

    0.0625 - 0.125 mm

     Silt

    		 1/256 - 1/16 mm
    (or 0.004 - 0.0625 mm)

    Clay

    < 1/256 mm
    (or < 0.004 mm)

    Gravel forms through physical weathering of rock. A piece of gravel is usually a "rock fragment" composed of more than one mineral. Sometimes a piece of gravel is a single mineral, most commonly quartz. This is because quartz is sometimes present as veins, which may be several inches wide (or more), thus producing gravel-sized clasts.

    Sand forms through the breakdown and disintegration of rocks which have sand-sized (1/16 - 2mm) grains, such as granite and gneiss.

    In humid climates, quartz sand grains are released from granite after the feldspar grains alter to clay by chemical weathering (hydrolysis). In more arid areas, granite breaks down by physical weathering (such as frost wedging), releasing both feldspar and quartz grains.

    Silt originates from the chipping of coarser grains during sediment transport, or from the disintegration of fine-grained crystalline rocks (such as slates, phyllites, and schists).

    Clay originates primarily through chemical weathering of feldspars and other alumino-silicate minerals (those which contain aluminum and silicon). The term "clay" refers to a particular size of sediment particle, which could be a quartz grain or a clay mineral flake, or some other very small mineral fragment. The term "clay" is also used to refer to a group of minerals. There are a number of clay minerals, including kaolinite (the white clay mined in central Georgia and used for shiny coatings on paper, and additives to rubber), illite (which contains potassium), and montmorillonite or smectite (a group of clays which can take in large amounts of water, and as a result these clays are commonly referred to as "swelling clays").

    SORTING

    Sorting refers to the range in grain sizes in a sediment or sedimentary rock. Sediment (or rock) which is well sorted will have most of the grains roughly the same size. A poorly sorted sediment or rock has a wide range of grain sizes. Sorting can be estimated using a visual comparison chart.

    ROUNDNESS

    Roundness is a measure of the sharpness or roundness of the corners of a sedimentary particle. Roundness is determined by comparing the sand grains with a visual comparison chart.

    As sediment is transported, it undergoes abrasion by coming into contact with the stream bottom, sea-floor, or other grains of sediment. The abrasion tends to "round-off" the sharp edges or corners. Rounding is also related to the size of the grains. Boulders tend to round much more quickly than sand grains because they strike each other with much greater force.

    SPHERICITY

    Grains of sediment are three dimensional. Sphericity refers to "equal dimensions". Is the sediment particle elongated (one dimension longer than the other two), flattened or sheet-like (one dimension much smaller than the other two dimensions), or is it spherical (its three dimensions roughly the same length)? Sphericity can be described as high or low. According to this definition, a ball would have highly sphericity, but so would a cube (high sphericity, but low roundness). In contrast, a submarine sandwich would have low sphericity, but high roundness. A shoebox would have both low sphericity and low roundness. Sand grains may have high or low sphericity. Some minerals may produce elongated or flattened grains, depending primarily on original crystal shape and cleavage.

    Be careful not to confuse rounding with sphericity. A well-rounded grain may or may not resemble a sphere. And a spherical grain may or may not be well rounded.

    INTERPRETING THE TEXTURE OF SANDS

    Texture is an indicator of energy levels in the environment of deposition (the place where sediment accumulates, perhaps a beach, a riverbed, a lake, or a delta).

    Moving water (such as waves or currents) is considerd to be a high energy environment.

    Quiet water or still water (water without waves or currents) is considered to be a low energy environment. Deep water environments commonly have quiet water, because wave motion is restricted to the upper part of the water column.

    How do you determine energy levels in the depositional environment from looking at sediment?

    Grain size

    Coarse-grained sediments (sand, gravel) indicate high energy environments. A large amount of energy is required to transport gravel-sized clasts, and moving water is required to transport sand.

    Fine-grained sediments (clay or silt) indicate low energy environments. There is insufficient energy to bring larger clasts into the environment. Aso, if the water were moving, the clay would not be able to settle out and be deposited on the bottom.

    Sorting

    Well-sorted grains indicate that the sediment was probably transported for a long time in a fairly high energy environment (waves or currents). The finer grains were probably washed or winnowed away.

    Poorly sorted grains indicate that the sediment has not been transported very far from the source area. It also suggests fluctuating energy levels, and a fairly short time in the depositional environment.

    • Good sorting implies consistent energy (washing)
    • Poor sorting implies inconsistent energy (dumping)

     Grain shape

    A well rounded sand grain indicates that the sediment has been transported far from the original source area, and that it has been in the depositional environment for a long time.

    The environment of deposition is also a factor in sand grain roundness. Sands from desert environments tend to be more rounded than sands from beaches.

    Angular sand grains have probably only been transported for a short distance from the source area, or they have been in the depositional environment for a short time.

    TEXTURAL MATURITY

    Textural maturity is a concept which proposes that as sediments experience the input of mechanical energy (the abrasive and sorting action of waves and currents), they pass through a series of four stages.

    1. Stage 1- Immature - Sediment contains mud (clay and/or silt)
    2. Stage 2 - Submature - Poorly sorted sediment with no mud
    3. Stage 3- Mature - Well sorted sediment with no mud
    4. Stage 4 - Supermature - Well sorted and rounded sediment with no mud

    Three steps are involved:

    1. Winnowing or washing out of fines - makes an immature sediment become submature
    2. Sorting of grain sizes - makes a submature sediment become mature
    3. Rounding - makes a mature sediment become supermature

    DESCRIBING THE MINERALOGIC COMPOSITION OF SANDS

    The minerals in sands (and in sandstones) can be identified using a microscope (or a handlens if a microscope is not available). Identifying the minerals present is important because sandstones are classified based on the composition of their grains.

    Three components are considered when naming sandstones:

    1. Quartz grains
    2. Feldspar grains
    3. Fine-grained rock fragment grains. Possibilities include shale, slate, phyllite, basalt, rhyolite, andesite, chert, and possibly schist. Limestones would not be included usually because they dissolve so readily.

    The three major types of sandstone are:

    1. Quartz sandstone (also called quartz arenite) - which is dominated by quartz
    2. Arkose - which is dominated by feldspar
    3. Litharenite or lithic sandstone (commonly but imprecisely called graywacke) - which is dominated by rock fragment grains.

    Other minerals may also be present in sands and sandstones. In fact, in some areas, sands may be composed almost entirely of minerals other than quartz and feldspar. For example, at White Sands National Monument in New Mexico, the sands are composed of gypsum grains. There is a beach on the southern end of the Big Island of Hawaii that has green sand composed of olivine grains. There are beaches in tropical areas in many parts of the world that are composed almost entirely of the sand-sized shells and shell fragments of marine organisms (made of calcium carbonate - calcite or aragonite).

    It is important to keep in mind that "sand" is a texture term, not a composition term. A sand can be composed of any types of sand-sized mineral or rock-fragment grains.

    In addition to the major constituents in sand, there is often a suite of heavy minerals (those with high specific gravity - greater than 2.85), which may consist of less than 1% of the sand grains to perhaps several percent (or more). Examples of heavy minerals include rutile, tourmaline, zircon, garnet, kyanite, staurolite, apatite, olivine, pyroxene, amphibole, magnetite, ilmenite, hematite, pyrite, and others. The particular types of heavy minerals present depend on the composition of the rocks in the source area. For example, garnet, kyanite, and staurolite are metamorphic minerals, whereas olivine, pyroxene, and amphibole are constituents of mafic igneous rocks (gabbro and basalt). Heavy minerals are important indicators which can tell us the type of rocks that existed in the sediment source area.

    Heavy minerals make up the "black sands" present in layers along the coast of Georgia and adjacent Florida, and inland in the Atlantic Coastal Plain. One notable example is a deposit of heavy minerals along Trail Ridge, the sand barrier holding back the waters of the Okefenokee Swamp in southeastern Georgia. DuPont owns or leases 38,000 acres along Trail Ridge, and has expressed interest in beginning a 50-year project to mine these heavy mineral sands to obtain titanium ore. (Both ilmenite and rutile contain titanium, a white pigment used in paper, plastics, and coatings (paint). The mining proposal has been quite unpopular with the public, because there is concern that the Swamp hydrology might be irreparably damaged by mining activities.

    [Update - In August 2003, DuPont donated a 16,000 acre tract of land adjacent to the Okefenokee Swamp to the Conservation Fund, a non-profit land preservation group. This donation permanently protects the acreage from development and mining.]

    [Update: In April 2004, another company Iluka Resources/TE Consolidated (see p. 6 of linked article), opened a new strip mining for titanium just east of Nahunta on highway 82. ]

    NOTE: Sands from warm, shallow, tropical seas, far from continental sources of sediments (such as quartz), may be entirely or almost entirely made up of calcium carbonate (CaCO3) grains such as calcite and aragonite. (They have the same chamical formula but different crystal structures. You do not need to worry about how to tell them apart. We can use either term when describing the sediments and sands in the lab.) Calcium carbonate sediment may be made up entirely or almost entirely of microscopic shells (the remains of planktonic organisms), along with broken shells and coral. Other calcium carbonate grains may be spherical or highly rounded. These grains are known as oolites. (See Lab 4, Sedimentary Rocks for more information). Calcium carbonate sand grains are typically white to tan to pink, and are opaque (rather than transparent and glassy like quartz). Because calcite is a soft mineral (only 3 on Mohs Hardness Scale), calcite (and aragonite) grains round quickly. (Recently-broken shells, however, will be angular.) You are likely to see some carbonate sands in your lab.

    IDENTIFYING MINERALS IN SANDS

    The following is a handy dandy guide to identifying sand grains under the stereomicroscope or handlens.

    Grain type Identifying features
    Quartz Glassy, gray or white (may be covered by brownish iron oxide stain), lacks cleavage
    Feldspar Has cleavage (look for flat surfaces or square corners), usually white or pink in color
    Rock fragments fine-grained, commonly dark gray or black, may be coated with iron oxide stain
    Muscovite Silvery color, flat sheets, shiny, may look sub-metallic
    Magnetite or ilmenite Black, opaque. magnetite is magnetic.
    Rutile Deep red or yellow, may look opaque, generally elongate and well-rounded
    Tourmaline Elongated with triangular cross-section, dark color
    Zircon Colorless, elongated crystals
    Garnet Most commonly pale pink or red, no cleavage
    Staurolite Brown to yellow, elongate, may be filled with tiny inclusions to resemble swiss cheese
    Apatite Colorless, rounded or elongated
    Olivine Olive green, glassy, no cleavage, may be rounded
    Pyroxene Stubby, angular cleavage fragments, gray or greenish to colorless
    Amphibole Elongated to fibrous, greenish
    Biotite Brown, flat sheets, shiny
    Hematite Red
    Pyrite Brassy gold, metallic,

    INTERPRETING THE MINERALOGIC COMPOSITION OF SANDS

    Each type of sandstone implies something about depositional history:

    1. Quartz sandstone implies a long time in the depositional basin.
    2. Arkose implies a short time in the depositional basin (because feldspar typically weathers quickly to clay). Arkose also implies rapid erosion, arid climate, tectonic activity, steep slopes.
    3. Litharenite implies rapid erosion, temperate or arid (not humid) climate

    As noted above, the particular suite of heavy minerals present in sand also can tell a lot about the source area from which the sediment is derived.

    READING THE RECORD IN THE ROCKS:
    A SANDSTONE INTERPRETATION GUIDE

     One of the goals in Historical Geology is to try to interpret the depositional conditions of the sedimentary rocks that make up the geologic record.

    Sandstone textures and compositions may be used to interpret many things about the history of the sand, including source area lithology, paleoclimate, tectonic activity, processes acting in the depositional basin, and time duration in the basin. Remember that the source area is the land which is weathering and eroding to supply terrigenous debris to the depositional basin.

    SOURCE AREA LITHOLOGY

    Composition gives the key information (minerals or rock fragments present). Remember that quartz sandstone or quartz arenite is dominated by quartz grains; arkose is dominated by feldspar grains (usually potassium feldspar); and graywacke is dominated by rock fragment grains.

    • Sand-sized quartz grains could come from the weathering of source area rocks such as granite, gneiss, or other sandstones which contain quartz (recycled sandstones).

    • Sand-sized feldspar grains could come from the weathering of source area rocks such as granite or gneiss.

    • Sand-sized rock fragment grains come from the weathering of fine-grained source rocks. Possibilities include shale, slate, phyllite, basalt, rhyolite, andesite, chert, and possibly schist.

    PALEOCLIMATE

    Paleoclimate refers to the climate which existed in the source area. We are particularly concerned with weathering rates here. Remember that in humid climates, feldspar weathers to clay by hydrolysis. Other minerals also weather to clay (with associated iron oxides), such as olivine, pyroxene, and amphibole.

    Also remember the difference between weathering (BREAKDOWN of rock by hydrolysis, dissolution, oxidation, exfoliation, frost wedging, or freeze thaw), and erosion (TRANSPORTATION of particles).

    • If feldspar is present in your sand, it indicates that the climate was probably arid. (Or that erosion rates were very rapid, and that tectonic activity was extremely high - lots of uplift,and steep slopes.)

    • If quartz is the dominant mineral in the sand, the climate was probably humid (all of the feldspars weathered away to clay).

    • If rock fragments are present in your sand, it helps to know what lithology they are. If they are rock types which would weather rapidly (such as basalt or limestone fragments), the climate was probably arid. If they are rock types which would be relatively stable (shale, slate, or chert), the climate may have been temperate to humid. (remember Bowen's Reaction Series and the Goldich Stability Series to determine what is stable or unstable). If rock fragments are present and no rock types are given, a good compromise answer would be temperate climate.

     TECTONIC ACTIVITY IN THE SOURCE AREA

    We are basically classifying tectonic activity as "active" or "passive". For a good model, consider the west coast of the US as tectonically active - steep slopes, mountains close to the sea, lots of earthquakes, tectonic uplift, and volcanic activity. On the other hand, consider the east coast of the US as tectonically passive - broad, flat coastal plain, few or no earthquakes, no uplift, and no volcanic activity.

    • If a sand has a lot of feldspar or rock fragments, it probably indicates high tectonic activity.

    • If a sand has a lot of quartz, it probably indicates low tectonic activity - a passive setting.

    Tectonic activity also influences sorting, time duration in the depositional environment (and to some extent, compositional maturity). High tectonic activity might produce rapid dumping of sediments into the basin with little or no time for sorting. Low tectonic activity means little uplift, low erosion rates, and therefore little sediment supplied to the basin; what sediment that is there is likely to wash around for a long time and become well sorted and rounded, and grains other than quartz are likely to be destroyed (by abrasion or chemical weathering).

     PROCESSES ACTING IN THE DEPOSITIONAL BASIN

    This refers to energy levels ("high" vs. "low") and consistency of energy. Texture gives the key information.

    Grain size:

    • Coarse sediments generally indicate high energy, and fine sediments indicate low energy.

    Sorting:

    • Well sorted sediments indicate consistent, fairly high energy levels. (Winnowing and washing.)

    • Poorly sorted sediments indicate inconsistent energy levels - rapid dumping (which might involve short episodes of high energy), followed by low energy conditions.

    TIME DURATION IN THE DEPOSITIONAL ENVIRONMENT

    Both mineralogy and texture can be used to determine time in the depositional environment.

    A sand with abundant quartz grains suggests a long time in the depositional environment. Quartz is more resistant to abrasion than feldspar or rock fragments.

    A sand with abundant feldspar or rock fragment grains suggests a short time in the depositional environment.

    Textural maturity is also useful in interpreting time in the depositional environment. Immature or submature sediments probably spent only a short time in the basin before burial. Mature or supermature sediments were probably rolling around in the basin for a long time before burial. Roundness is a good clue to a long time in the depositional environment. Rounding of grains takes a long time; it is more likely in a tectonically passive situation. Desert sands are often well rounded because of the "sandblasting" process of wind transport. Hence, in an arid desert, it is possible to get a well-rounded (supermature) arkose.

    Go to Weathering Lab - Exercises

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    This page created by Pamela J. W. Gore
    Georgia Perimeter College

    July 6, 1998
    Modified June 11, 1999
    Updated December 1, 2003
    Updated October 15, 2004
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