Download Sedimentary Rocks

The land around you, no matter where you live, is
made of rock. If you live in a place that has good rich
soil, the soil itself is finely broken down or weathered
rock. People that live in a desert region can easily find
rocks on the surface. These rocks lay on a surface of
clay that is also a product of weathering rock.
Weathering is the process of breaking down rocks
and minerals into smaller pieces by water, wind, and
ice.
Sedimentary rocks are formed from the breaking apart
of other rocks (igneous, metamorphic, or sedimentary
rocks) and the cementation, compaction and
recrystallization of these broken pieces of rock.
Sedimentary rocks are formed from broken pieces of rocks. These
broken pieces of rock are called sediments. Sedimentary rocks are
usually formed in water. Streams and rivers carry sediments in their
current. When the current slows or the river empties into a lake, or
ocean, or another river the sediments fall out because of gravity.
The larger sediments fall out first and the lightest sediments
fall out last.
In the spring the lake receives an influx of water from the
mountain snow melt. This snow melt carries with it a large
amount of sediment that becomes suspended in the lake water.
The heaviest and largest particles settle out first and the
lightest sediments such as silts and clays settle out last.
The number 1 shows sediment that would have been laid
down during 1994, number 2 in 1995, and number 3 would
have been laid down in 1996. The gray area above the 3
would be the latest layer being laid down at the present
time.
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This laying down of rock-forming material by a natural
agent is called deposition. Natural agents of deposition are
water, ice, gravity, and wind.
* Sediment is deposited in flat, horizontal layers with
the oldest layers on the bottom and the younger layers
laying on and over the older layers. Geologists use
this knowledge to read layers of sedimentary rock like
the pages in a book. They can date layers by the
fossils that are found in them. If a layer has a fossil in
it that is known to be 50 million years old the layer
itself must be at least 50 million years old and the
layers below it have to be older than 50 million years.
* The size of sediment is defined by the size of the particles that make
up the sediment. The largest sediment size is called a boulder. Boulders
have a diameter that is larger than 256 millimeters (about 10 inches).
*
Cobbles are the next largest sediment, followed by Pebbles
The next sizes of sediments are very small, granules are 2-4
mm, sand 1/16-2mm, silt 1/256-1/16 mm, and the smallest
sediment size is clay which is less than 1/256 of a
millimeter in diameter.
Sedimentary rocks are formed in three ways from these
different sized sediments.
A sedimentary rock is a layered rock that is formed
from the compaction, cementation, and the
recrystallization of sediments.
Compaction is the squeezing together of layers of
sediment due to the great weight of overlying layers of
rock. This squeezing of the layer results in reducing the
thickness of the original layer. When the layers are
reduced in thickness the pore spaces around the
sediments are also reduced, which leads to a tighter
packing of the layers.
Cementation is the changing of sediment into rock by
filling spaces around the sediments with chemical
precipitates of minerals binding the sediments, and forming
solid rock. Calcite and silica are common minerals that
cement the sediments together.
Recrystallization is the third way that sedimentary rocks
are formed. Recrystallization is the formation of new
mineral grains that are larger than the original grains. As
the sediments recrystallize they arrange themselves in a
series of interlocking crystals that connect the other grains
together into a solid rock.
Sedimentary rocks form a thin layer of rock over 75 per cent of the
Earth's surface. They are the site of very important resources such
as ground water, coal, oil, and soil. Shale, sandstone, and limestone
are the most common types of sedimentary rocks. They are formed
by the most common mineral that is found on or near the surface of
the Earth. The mineral that forms these sedimentary rocks is
feldspar.
Running water, such as the mountain stream above, sorts and
transports more sediment than any other agent of deposition.
Clastic sedimentary rocks: are made of pieces of rock or
mineral grains that have been broken from preexisting rock.
These particles and grains have become solid rock by the
processes of compaction or cementation of sediments.
Some clastic rocks are conglomerate, shale, breccia, gray
and red sandstone.
Non-clastic sedimentary rocks form from the precipitation
(Precipitation is the separating of a solid from a
solution) of minerals from ocean water or from the
breakdown of the shells and bones of sea creatures. Sea
animals such as coral produce calcium carbonate solutions
that harden to form rock.
*
As the chemicals, that comes from the mineral or
biological precipitation, mix with sediments on the floor of
the ocean or lake they crystallize and grow in the spaces
around the sediment. When these crystals grow large
enough to fill the spaces they harden and form a solid rock. *
Some non-clastic rocks are limestone, chert,
dolostone, gypsum, halite (rock salt), diatomite, and chalk.
Organic sedimentary rocks form from the build up and
decay of plant and animal material. This usually forms in
swamp regions in which there is an abundant supply of
growing vegetation and low amounts of oxygen. The
vegetation builds so quickly that new layers of vegetation
bury the dead and decaying material very quickly. The
bacteria that decay the vegetation need oxygen to survive.
Because these decaying layers are buried so fast the
bacteria use up what oxygen there is available and can not
finish the decomposition of the vegetation.
Organic Sedimentary rocks
The overlaying layers become so heavy that they
squeeze out the water and other compounds that aid in
decay.
This compressed vegetation forms coal. The longer and
deeper that coal is buried makes it of higher quality. Peat
is the first stage of coal formation. Lignite is the next
grade of coal followed by bituminous and the highest
grade, anthracite. Anthracite is actually a metamorphic
rock. It forms during mountain building when compaction
and friction are extremely high. This form of coal burns
very hot and almost smokeless. It is used in the
production of high grade steel.
Shale is one of the most common sedimentary rocks. It is
composed of silt or clay that has been compacted or
squeezed together to form a solid rock. Shale is usually found
in thin layers. The silt or clay that composes shale is made of
very small pieces of weathered rock. The pieces are from
1/16 to 1/256 of a millimeter in diameter. The color of a
sample of shale is that of the clay or silt that it was formed
from.
Limestone is the most abundant of the non-clastic
sedimentary rocks.
Limestone is produced from the mineral calcite (calcium
carbonate) and sediment. The main source of limestone
is the limy ooze formed in the ocean. The calcium
carbonate can be precipitated from ocean water or it can
be formed from sea creatures that secrete lime such as
algae and coral.
Chalk is another type of limestone that is made up of
very small single-celled organisms. Chalk is usually
white or gray in color.
Limestone can easily be dissolved by acids. If you
drop vinegar on limestone it will fizz. Put a limestone
rock into a plastic jar and cover it with vinegar. Cover
the jar and watch the bubbling of the calcium
carbonate and also the disintegration of the rock over
a few days.
Limestone caves are an interesting geological feature. They
form because the limestone deposits located under the ground
are chemically dissolved by moving ground water. The ground
water contains minerals that make the water slightly acidic.
When an acid comes into contact with a rock that is composed
of calcium carbonate a chemical reaction takes place. The acid
"eats" the limestone. The calcium carbonate then goes into the
ground water which moves down farther into the cave.
The water will find its way into small crack and
crevasses. The dripping water will create formations
called stalactites and stalagmites.
Stalactites (they grow from the ceiling) and stalagmites
(they grow from the floor) are not technically limestone.
They form in caves because as the limestone is
dissolved calcium carbonate is put into solution in the
ground water. This solution drips through crack and
slowly forms stalactites and stalagmites.
Conglomerate is a clastic sedimentary rock that forms the
cementing of rounded cobble and pebble sized rock
fragments. Conglomerate is formed by river movement or
ocean wave action. The cementing agents that fill the spaces
to form the solid rock conglomerate are silica, calcite, or iron
oxides.
Notice in the photo above the rounded rock particles in the
conglomerate. These rounded particles make conglomerate
different from the next rock you are about to study, breccia.
Breccia is formed in a very similar fashion to conglomerate.
The difference between the two rocks is that breccia's rock
fragments are very sharp and angular.
These rock fragments have not been transported by water,
wind, or glaciers long enough to be rounded and smoothed like
in the conglomerate. The cementing agents silica, calcite
(CaCO3), and iron oxides are the same as in conglomerate.