MINERALS AND ROCKS

ENGINEERING GEOLOGY LESSON 2

Minerals and Rocks

Minerals are the building blocks of the earth. A mineral is a combination of elements that forms
an inorganic, naturally occurring solid of a definite chemical structure. For example, SiO2 is
always the mineral quartz. A rock is a solid material that is composed of various minerals.

Minerals can have a variety of crystalline shapes. The shape of the crystal is dependenton the
sizes of the atoms of the elements, the chemical bonds that hold the elements together to form
the mineral, and the pressure and temperature at which the mineral formed.

Most minerals are built around silica tetrahedrons—four oxygen atoms connected to asmaller,
central silicon atom. Different arrangements of silica tetrahedrons create distinctive atomic
structures in minerals, such as sheet silicates (the mica and clay mineral groups), chain silicates
(the pyroxene mineral group), or framework
silicates (the quartz and feldspar mineral groups).

Only several hundred of the thousands of known minerals are important rock‐forming minerals.
As one might guess, their chemical compositions contain mostly the eight most common
elements in the crust—oxygen, silicon, aluminum, iron, calcium, sodium,potassium, and
magnesium. The important rock‐forming mineral groups are quartz, feldspars, amphiboles,
pyroxenes, clays, micas, and carbonates.

A rock's color is determined by its mineral components: quartz, feldspars, carbonates, and some
micas are generally light‐colored, tan, or pinkish; pyroxenes, amphiboles, andsome micas are dark
green to blackish because of their high iron and magnesium content.
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Extrusive Rock Types

The kind of rock an extrusive lava makes is largely dependent on the chemistry of theventing
magma (Figure 1).

Figure 1 Intrusive and Extrusive Rock Classification

Basalt, andesite, and rhyolite. Basalt, usually the first lava to form, contains a high percentage
of ferromagnesian minerals and about 25 to 50 percent silica, making it darkgreen, gray, or
black. Andesite is a lighter greenish‐gray and has more silica and plagioclase feldspar and less
ferromagnesian minerals than basalt. Rhyolite is the mostsilicious of the extrusive rocks,
containing at least 65 percent silica (mostly in feldspar minerals and quartz) and few
ferromagnesian minerals. This chemistry gives it a tan, pink, or cream color. Dacite has a
composition that falls between those of andesite and rhyolite— it has slightly less potassium
feldspar and quartz and slightly more ferromagnesian minerals than rhyolite. Dacite is generally
a light grayish‐green and often difficult to distinguish from rhyolite in the field.

Mafic, felsic, and intermediate extrusive rocks. More general terms for these rocks are mafic,
felsic, and intermediate. Mafic rocks have about 50 percent silica and high amounts of iron,
magnesium, and calcium and are dark in color. A common mafic rock is basalt. Felsic rocks are
rich in silica, potassium, sodium, and aluminum and contain only small amounts of iron,
magnesium, and calcium. Typical felsic rocks are dacite andrhyolite. Felsic magmas are the most
viscous because of their high silica
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content. Intermediate rocks, such as andesite, fall between the mafic and felsicclassifications.

Ultramafic extrusive rocks. A less common group are the ultramafic rocks, which consist
almost entirely of ferromagnesian minerals and have no feldspars or quartz. They contain less
than 45 percent silica, and are believed to originate from the mantle. These are some of the least
viscous lavas because of their low silica content. A komatiite is a typical ultramafic extrusive
rock that is mostly olivine and pyroxene, with lesser feldspar.

Rock Textures

The origin of a rock can often be detected from its texture—the sizes and orientations of its
mineral or rock fragment components. Most extrusive rocks are fine grained, meaning their
mineral components (grains) are less than 1 millimeter in diameter. Lavaflow rocks typically
have a chilled margin that is very fine grained, or aphanitic. Grainsize then increases
progressively toward the center of the flow. Thicker flows can havemedium‐ to coarse‐grained
centers.

A porphyritic rock contains coarser‐grained crystals (phenocrysts) that are supportedin a

matrix (groundmass) of finergrained minerals. The larger minerals had already crystallized
and were extruded with the magma, which then rapidly cooled to form the groundmass.
Obsidian (volcanic glass) is a hard, supercooled, very fine‐grained volcanic rock composed of
silica.

Basalt flows that have a ropy surface are called pahoehoe flows and form when the lava's
exterior quickly solidifies into rock. An aa (pronounced ah‐ah) flow develops a partially
solidified surface as it moves forward. Continued advance breaks the solidifiedflow's top and
sides into a rough, rubbly mass.

Magmas often contain dissolved gas because of higher pressures deep underground. When the
magma is suddenly released and vents at the surface, the gas “bubbles” out of the magma,
creating numerous holes, cavities, or voids called vesicles. Pumice is avolcanic rock that has so
much internal void space from gas bubbles that it floats in water. Scoria is a very vesicular
basalt that contains more gas space than rock and hasa very rough, irregular, and pocked
exterior.

The lithification of ejected rock fragments and other pyroclastic material creates a variety of
fragmental textures. Dust and ash are the finest‐grained particles, followed by cinders (pea
sized), lapilli (walnut sized), and bombs or blocks, which can be up toa meter across or larger.
Blocks are ejected pieces of hardened lavas; bombs are semimolten pieces of lava that solidify as
they fall. Small crystals (generally feldspars) that had been formed in the magma before it was
ejected are also deposited with the other pyroclastics. A tuff is composed of fine‐grained
pyroclastic material and is named by the most distinctive component, such as an ash tuff or
crystal tuff. A welded tuff isa rock that consists of ash particles and glass shards that were hot
enough to fuse
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together when it was deposited. The rocks that contain the larger bombs are called tuffbreccias
or agglomerates.

Other distinctive extrusive rock textures occur in flood basalts and submarine lava flows.Flood
basalts cool and contract to form vertical, parallel, generally six‐sided columns called columnar
structures or columnar jointing (Figure ). As a submarine lava flow cools, blobs of lava may
break through the exterior and harden immediately in the cold water, forming small rounded
shapes called pillow structures. These are especially useful to the geologist for determining that
the rock was formed on the ocean floor and for indicating the base of the flow (Figure ).

Figure 1

Columnar Jointing

Figure 2

Pillow Structures

Intrusive Rock Types

Intrusive rocks crystallize from magmas that have been intruded into the earth's crust atdepths far
below the surface. These intrusions are then usually exposed millions or billions of years later
through the processes of uplift, mountain‐building, and erosion.
Other intrusive rocks are discovered through deep‐drilling programs. Country rock is the
surrounding rock that the magma invades. A contact then separates the cooled intrusive rock
from the country rock. Contacts are rarely straight lines, are quite irregular, and mark the
change in rock type. The edge of the intrusive rock is usually very fine grained because it is here
where the most rapid cooling took place. This edgeof the intrusion is called the chill zone. The
grain size in the intrusion increases away from the chill zone toward the center, where it
remained the hottest for the longest time.The intrusive rock often contains xenoliths—fragments
of the country rock that were
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torn away during the emplacement of the magma and that are generally most abundantnear the
contact with the country rock.

Plutonic rocks. Intrusive rocks that were formed deep in the earth's crust are
called plutonic rocks and are generally coarse grained (mineral grains greater than 1
millimeter in diameter), large, and often associated with mountain‐building.

Mafic, felsic, and intermediate intrusive rocks. Intrusive rocks are classified the same way
extrusive rocks are—according to the relative amounts of feldspars, quartz, and ferromagnesian
minerals. Gabbro is a mafic rock and has the same chemistry andmineralogy as basalt; diorite
is an intermediate rock equivalent to andesite;
and granite is a felsic rock equivalent to rhyolite. For example, a magma that would form
rhyolite if it vented at the surface would crystallize into a granite in a subterraneanchamber
kilometers below the surface. Granite is the most common intrusive rock on the continents;
gabbro is the most common intrusive rock in oceanic crust.

Ultramafic intrusive rocks. Ultramafic intrusions are almost completely composed of

ferromagnesian minerals, mostly olivine and pyroxene. They contain less than 45 percent silica
and are thought to be derived from the mantle. A typical ultramafic intrusion is called a
peridotite.

Intrusive Structures

Intrusions are also classified according to size, shape, depth of formation, and geometrical
relationship to the country rock. Intrusions that formed at depths of less than 2 kilometers are
considered to be shallow intrusions, which tend to be smaller andfiner grained than deeper
intrusions.

Dikes.

A dike is an intrusive rock that generally occupies a discordant, or cross‐cutting,crack or

fracture that crosses the trend of layering in the country rock. Dikes are called pegmatites
when they contain very coarse‐grained crystals—a single such crystal can range in size
from a few centimeters to 10 meters in diameter.

Sills.

Sills are formed from magmas that entered the country rock parallel to the bedding (layering)
and are thus concordant with the country rock. Sills can sometimes look likevolcanic flows
that were interbedded with sedimentary units.

Laccoliths.

A laccolith resembles a sill but formed between sedimentary layers from a more viscous
magma that created a lensshaped mass that arched the overlying strataupward.
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Volcanic necks.

A volcanic neck is the rock that formed in the vent of a volcano at the end of its eruptive life
and remains “standing” after the flanks of the volcano have eroded away.

Plutons.

Plutons are discordant intrusive rocks that formed at great depths. They tend to be large, coarse
grained, and irregular in shape. If the intrusion occupies less than 100 square kilometers (60
square miles) at the earth's surface it is called a stock; if it is larger than 100 square kilometers, it
is termed a batholith. Batholiths are usually composed of granite. They have formed over long
periods through the accumulation of smaller magma blobs called diapirs, which result from
localized melting of the crust; thediapirs then slowly move upward toward the surface and
coalesce into a larger mass.
Granitic batholiths usually form the cores of mountain complexes and are a result ofplate
tectonic action.

How Different Magmas Form

Both extrusive and intrusive igneous rocks are derived from magmas. The temperatureand
pressure conditions in the crust and upper mantle influence the melting temperatures of the
minerals in the rocks.

Temperature and pressure increase with depth from the surface and eventually reach apoint at
which rocks melt. The geothermal gradient is the rate at which temperature increases with
depth. In the upper crust, the geothermal gradient is about 2.5 degrees centigrade for every 100
meters (330 feet). Geothermal gradients are higher in volcanicregions. Mantle plumes are “hot
spots” in the crust where mantle material has ascended along deep penetrating cracks in the
crust and contributes heat for higher‐ level melting. Country rock can also be melted from the
heat of adjacent intrusions.

Friction is a source of heat in areas where large rock masses are grinding against one another—
for example, during mountain‐building and plate tectonic activity. Heat is also released through
the radioactive decay of elements such as uranium, a less important process that only
marginally raises the geothermal gradient.

Because of higher pressures, temperatures, changes in density, and gases in solution, magmas
tend to rise toward the surface through deep cracks and faults. Being more viscous, felsic
magmas rise more slowly than mafic magmas. As magma moves upwardit begins to cool, and
minerals begin to differentiate.

A very hot magma assimilates the country rock it is moving through—that is, the country
rock in contact with the magma melts and becomes part of the magma. If amagma
assimilates a large amount of country rock, the chemistry of the magma
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changes. Different extrusive and intrusive rock types form from magmas according to the
chemistry of the magma and the differentiation reactions that precipitate the variousminerals
that make up the igneous rock.

Partial melting is the process by which a portion of the magma that is forming from a melting
mass of rock separates and rises as a distinct magma. As a rock is being heated, the first liquid
that forms contains a high proportion of the minerals that have lower melting temperatures. A
good example is basaltic magma, which is thought to bethe result of partial melting in the
mantle; the remaining magma in the mantle is then ultramafic in composition. If the entire rock
melts, and no magmatic phases escape, theearlier‐forming and later‐forming liquids mix to form
a magma that has the same composition as the original rock.

Igneous Rocks and Plate Tectonics

Igneous rocks form from magmas, and most magmas are associated with plate tectonics. Mafic
(basaltic) and ultramafic magmas form along the divergent midoceanic ridges and are major
components of new oceanic crust. More felsic magmas, such as andesites and rhyolites, are
associated with the edges of continental crust at subductionzones along converging plate
boundaries. Whether a magma is intermediate or felsic may depend on the relative amounts of
oceanic crust and continental crust in the subduction zone that melt to form the magma. The
great abundance of granitic intrusions in continental crust is thought to be related to the partial
melting of the lower continental crust.

Intraplate igneous activity occurs in the interior of a single continental plate and is thought to
be related to mantle plumes (such as the eruptions at Yellowstone National Park) or flood
basalts. Intraplate activity is not associated with moving plate boundariessuch as subduction
zones.

Magmatic Differentiation

The defining characteristic of igneous rocks is that at one time they were molten and part of
magmas or lavas. A magma is a body of molten rock that occurs below the surface of the earth.
When magma rises along a deep fault and pours out on the earth'ssurface, it is termed lava. This
material then cooled to form a variety of intrusive and extrusive igneous rocks. Extrusive rocks
crystallized from liquid magmas that reachedthe surface and were generally vented as volcanic
lavas. Intrusive rocks crystallized from magmas that did not reach the surface but moved upward
into cracks and voids deep in the crust.

When a magma cools, chemical reactions occur that create a series of different minerals. This
process of differentiation occurs along two branches: discontinuous andcontinuous.
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The discontinuous branch.

The minerals that form in the discontinuous branch are all ferromagnesian—that is,they
contain high percentages of iron and magnesium, which impart a dark green to black color. The
branch is called discontinuous because the minerals form at discrete temperatures and not
continuously during cooling. The first mineral to crystallize
is olivine, followed by pyroxene, amphibole, and biotite.The

continuous branch.

The continuous branch is made up of the plagioclase feldspars. The calcium/sodiumratio in

this mineral type changes continuously as the magma cools. The first feldspars to form contain
the highest amounts of calcium; subsequent feldspars have progressively less calcium and more
sodium. These minerals tend to be pink, tan, brown, or whitish.

Any magma left over after all these reactions have been completed crystallizes at thelowest
temperature as quartz.

These theories were first proven in the laboratory by N. L. Bowen in the early 1900s and are
also known as Bowen's reaction series. The progression in the series explains why the first
lavas from a volcanic vent are rich in iron, magnesium, and calcium, are low in quartz, and are
dark green to black and why the later lavas are lighter colored and contain more quartz.

Volcanoes and Lavas

Volcanism, or volcanic activity, is the venting of liquid magma at the surface of the earth.
Occasionally explosive, the process is important in producing continental and oceanic crust.
Volcanoes are hills or mountains that form around the vent and consistof cooled magma, rock
fragments, and dust from the eruptions.

Pieces of rock that are blown out of a volcano are called pyroclasts or pyroclastic debris.
Pyroclasts may also be beads of liquid magma that supercool in the air during descent to form
glassy shards of rock. Pyroclastic flows are dense, cloudlike mixturesof hot gas and pyroclastic
debris that flow down a volcano's sides like an avalanche.
These flows can be especially deadly—for example, 30,000 people were killed by ascalding
pyroclastic flow on the Caribbean island of Martinique in 1902.

Craters and calderas.

The crater is the circular depression at the top of the volcano. A caldera is a larger depression
at least 1 kilometer in diameter that forms at the top of the volcano when thesummit is destroyed
during an eruption or when the crater floor collapses into the magma chamber below.
 MINERALS AND ROCKS
There are three kinds of volcanoes: composite, shield, and cinder cone.

Composite volcanoes (stratovolcanoes) have been the sources of some of the more famous and
destructive eruptions, such as those of Mount St. Helens, Vesuvius, and Krakatoa. Built up over
millions of years, they consist of alternating layers of lava and pyroclastic debris that can
approach slopes as steep as 45 degrees. They are characterized by long periods of dormancy, or
inactivity, that can last for up to hundreds of thousands of years. How violent an eruption is
depends on the temperatureof the lava and the amounts of silica and dissolved gas in the lava.

Composite volcanoes are located along the circum‐Pacific belt and

the Mediterranean belt, which mark the boundaries of colliding crustal plates. The circum‐
Pacific belt, also known as the “Ring of Fire,” runs along the west coasts of South and North
America, through the Aleutian Islands south of Alaska, and along theeast coasts of Asia and
Indonesia.

Shield volcanoes are broad, cone‐shaped hills or mountains made from cooled lava flows. The
sides are very gently dipping and rarely exceed 10 degrees from the horizontal because the lavas
have a low viscosity and spread quickly after eruption. (Viscosity is defined as resistance to
flow; a lava with high viscosity flows sluggishly.)
A spatter cone is a smaller feature that usually develops on a cooling lava flow from a shield
volcano. Gas and lava are ejected through a small vent, building up a steep‐sidedcone that
resembles an appendage.

A cinder cone (pyroclastic cone) is composed of pyroclastic material (not lavas)ejected from a
vent and commonly has slopes of about 30 degrees.

Volcanic domes.

If a magma is thick and viscous and does not easily flow, it may form a volcanic dome.
Volcanic domes are steep sided or rounded and form near the volcanic vent,creating a plug that
can trap gases, build up internal pressures, and lead to violent explosions.

Lava floods.
Nonvolcanic lavas called lava floods or plateau basalts are often associated with deep cracksin
the continental crust. Although volcanoes don't form, huge amounts of very nonviscous, “runny”
lavas pour from the rift and spread for hundreds of square kilometers. Repeated outpourings of
lava have reached thicknesses of 2 kilometers or more in the geologic past.
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Clastic Sedimentary Rocks

Clastic sedimentary rocks are classified according to the grain size of the sediment and the
kinds of rock fragments that make up the sediment (Table ). Grain size is largely a function of
the distance the particle was transported. In general, the greater thedistance traveled, the smaller
and more rounded the sediment particles will be. This smoothing of rock fragments during
transportation is called rounding.

Large, coarse, angular pieces of sediment will be deposited near the source area; well‐ rounded
sand grains will have been transported a considerable distance before being deposited; silt, mud,
and clay have been carried even farther. This process is
called sorting. Coarse‐

grained rocks.

Sedimentary breccia contains an abundance of coarse, angular fragments of gravel that were
deposited very near the source area. A conglomerate is formed from coarse‐ grained, rounded
pieces of gravel. Sandstone is a medium‐grained rock that contains rock particles (mostly
quartz) about the size of sand. The grains in a quartz
sandstone are at least 90 percent quartz. Graywacke is a type of “dirty” sandstone thatconsists
of more than 15 percent silt‐sized or clay‐sized (finer‐grained) material that imparts a darker or
speckled appearance. If a coarse‐grained sandstone consists of over 25 percent feldspar grains it
is called an arkose.

Finer‐grained rocks.
The finer‐grained clastic sedimentary rocks are called shale, siltstone, and mudstone. Shale is a
smooth, thinly layered rock that is made up of fine‐grained silt and clay particles. Shale is
considered a fissile rock because it splits very naturally along its layers. A siltstone contains
mostly silt grains and looks very similar to shale but is not as fissile. Mudstone, the finest‐
grained clastic rock, is not well layered, and contains more clay than does shale or siltstone.
Most shales, siltstones, and mudstones are tan, brown, gray, or black.
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Chemical Sedimentary Rocks

Limestones.

The most common chemical sedimentary rock is limestone. Composed mostly of themineral
calcite (CaCO3), limestones are usually formed by biochemical processes in shallow seawater.
Coral and algae are especially important limestone builders. Oolitic limestones form in ocean
shallows from the accumulation of oolites, sand‐sized spheres of chemically precipitated calcite
that develop in the tidal zone.

Other variations of limestone result from the deposition and cementation of calcium‐rich shells,
shell fragments, corals, algae, and the remains of tiny marine
organisms. Coquina is formed from the cementation of large pieces of broken
shells. Bioclastic and skeletal limestones are fine‐ to coarse‐grained accumulations of a wider
variety of shell fragments and fossils. Chalk is a very fine‐grained bioclastic limestone
composed of accumulations of skeletal debris from tiny marine organisms
that drifted down to the ocean floor. All of these “redeposited” limestones could be
considered clastic sedimentary rocks, as well as organic sedimentary rocks.

Dolomites.

Limestones are frequently converted into dolomites, or dolostones, during the earlystages of
compaction, dewatering, and lithification of the limestone sediment. The process of
dolomitization involves the removal of calcium from the limestone by magnesium‐rich solutions
and its replacement in the rock by magnesium. Dolomite's chemical formula is CaMg(CO3)2.

Cherts.

Chert (varieties of which are flint, agate, and jasper) is a hard, glassy sedimentaryrock
composed of silica that precipitated from water. Chert nodules, also known
as geodes, are commonly found in limestones and less so in clastic sedimentary rocks.They form
in pockets or voids that might have once been occupied by gas or organic material that has since
been removed or decomposed. Cherts can also occur as continuous layers in sedimentary rocks.
Chert usually composes at least half of a spectacular layered rock called iron formation, which
crystallized in shallow seas around the world and is an important source of iron.

Evaporites.
Evaporites are rocks that are composed of minerals that precipitated from evaporating seawater
or saline lakes. Common evaporites are halite (rock salt), gypsum, borates, potassiumsalts, and
magnesium salts.
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Organic Sedimentary Rocks

Organic sedimentary rocks form from the accumulation and lithification of organic debris,
such as leaves, roots, and other plant or animal material. Rocks that were once swampy
sediments or peat beds contain carbon and are black, soft, and fossiliferous. Rich enough in
carbon to burn, coal is an organic sedimentary rock that is a widespreadand important fuel
source. Coquina, bioclastic limestone, and skeletal limestone are also technically organic
sedimentary rocks but are usually grouped with the other limestones as being chemically
precipitated.

Sedimentary Features

Features that were part of the sediments when they were deposited are often preservedwhen the
sediments become lithified. These features are very useful in reconstructing how the sediment
grains were transported, where they came from, the age relationshipsof different layers, and what
the environment was like when the sediments were deposited.

Bedding.

Bedding is often the most obvious feature of a sedimentary rock and consists of lines called
bedding planes, which mark the boundaries of different layers of sediment. Mostsediments were
deposited along a flat surface that was roughly parallel with the depositional surface. An
exception is cross‐bedding, where sediments are carried overan edge or slope by a strong surge
of water or wind, forming steeper layers. Cross‐ bedding tends to occur locally within a larger
block of rock, and is overlain and underlainby flat‐lying beds. Herringbone cross‐bedding is a
distinctive pattern of alternating cross‐bedding directions that is reflective of a rhythmic, high‐
energy environment, such as a tidal zone.

Graded beds are common when a sediment is being deposited by a slow‐moving current. The
base of the bed consists of coarser material, which settles to the bottom first. The subsequent
beds grade upward through sand and silt, to the finest clay sizesat the top. This pattern is typical
in submarine turbidity flows, where sediments are dislodged and tumble down an ocean floor
slope.

Fossils.

Fossils are the remains of plants or animals buried in sediments that were later lithifiedinto rock.
They can be extremely useful in determining the depositional environment andthe age of the
rock. The most obvious fossils are those parts of an organism that have been preserved by being
replaced by calcite or silica during lithification. A fossil can also be a cast that formed when the
organic remains dissolved, leaving an opening,
or mold, shaped like the organism and later filled with calcite or silica. Other types offossils
include tracks, worm trails, feces, and burrows.
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Desiccation cracks and ripple marks.

Common structures preserved in sedimentary rocks can be seen forming today alongbeaches
and rivers. Desiccation cracks, or mud cracks, develop when a muddy sediment is exposed to
air and begins to dry out, creating a polygonal pattern of
cracks. Ripple marks are gentle repeated ridges, usually in sand or silt, that are formed
perpendicular to the flow of wind or water.

Sedimentary Environments

Sedimentary rocks give us important information about what the world was like millionsof
years ago, such as the location of the source, or provenance area, from which the sediment
originated, the kinds of source rocks, and the paleocurrents (the direction offlow that
deposited the sedimentary grains and how the direction changed with time).

Rock types and structures allow the geologist to determine if the sediments were deposited by
glaciers, rivers, lakes, deltas, beaches, sand dunes, wind, lagoons, continental shelf currents,
reefs, or deeper ocean waters. High‐energy environments such as steep river channels usually
deposit coarse arkosic sandstones or conglomerates. Beaches and barrier islands consist of well‐
rounded quartz sandstone. Lower‐energy environments like lake beds, deltas, lagoons, and the
deep ocean can beidentified by the finer‐grained rocks such as shale and siltstone. Limestones
usually identify marine reef environments.

An integration of this information over a large region leads to the reconstruction of the
depositional environment—what the region was like in the geologic past. This three‐
dimensional reconstruction, over what can be thousands of square kilometers, can be detailed
enough to identify such events as flooding and fluctuations in sea level that happened hundreds
of millions of years ago

How Sedimentary Rocks Form

Sedimentary rocks cover about three‐fourths of the surface of the continents. There are three
kinds of sedimentary rocks: clastic, chemical, and organic. Clastic sedimentary rocks form
from the consolidation of material such as gravel, sand, or clay (sediment) derived from the
weathering and breakdown of rocks. Chemical sedimentary rocks result from biological or
chemical processes, generally under water,that crystallize minerals that accumulate on the sea
floor. Organic sedimentary rocks, such as coal, have as their major component accumulations
of organic remains from plants or animals that make the rock distinctive.

As the sediments become buried under other sediment layers, pressures and temperatures
increase. The sediment hardens into a sedimentary rock, or lithifies, afterit has gone through the
stages of compaction, dewatering, and cementation.
During compaction, the grains of sediment are packed more tightly together. With
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increasing pressure some of the water between the sediment particles is squeezed out,
dewatering the sediment. This process reduces the pore space, or open spacesbetween the
grains. At this point, pressure and temperature conditions are such that
certain minerals, usually calcite or quartz, fill some or all of the pore spaces and adhereto the
sediment fragments, cementing them into a sedimentary rock.

A rock formation is an occurrence of rock with a set of characteristics that distinguishes it from
the rocks above or below it. A formation can then be broken down into smaller rock layers
called members. A sedimentary contact is the boundary surface between two different kinds of
rocks and is usually a straight line that represents the original surface where one sediment type
was deposited on another.