Transcript ppsx

An understanding of some of the basic characteristics of sedimentary rocks is
required to comprehend West Virginia’s geologic history. This presentation is not
intended to be anything other than a very brief introduction to sedimentary rocks.
Enter
{
When you see this symbol, click on it and it
will return you to this page.
Back
We have no record of the 500 million years following Earth’s formation. We speculate that it was a time of
intense meteoroid bombardment when the mantle and the crust that surrounds the molten iron core were
forming. Basaltic magmas rose to the surface and spread across the surface of the underlying mantle in a
relatively thin layer that was eventually to become the oceanic crust. The surface of the crust during this primeval
time was being formed and re-assimilated as underlying convection currents churned the outermost layer of
Earth. The heat from the molten core, heat generated by the breakdown of radioactive elements, and the intense
bombardment by meteoroids kept the temperature at Earth’s surface very high. Partial melting of the basaltic
crustal rocks eventually began to generate masses of molten granitic rock. In time, these granitic masses would
coalesce to form small continents. These micro-continents eventually converged and sutured together to form
larger continents. Over time, the thermal activity within the crust began to slow as Earth cooled, the abundance
of radioactive elements dwindled and the frequency of meteoroid impacts diminished.
Estimates indicate that by 2.5 billion years ago the present mass of continental crust had formed. At this point
the processes of weathering and erosion assumed their roles in creating sediments that would eventually
become sedimentary rock.
Back
Test Your Term Knowledge
What do the following terms mean?
Click on the terms and see if you’re right.
Beginning about 2.5 billion years ago Earth’s surface had
cooled sufficiently to allow precipitation. Water vapor,
erupted by the world-wide volcanism and stored in the
atmosphere, condensed and began to fall as rain. Once
the rains started, the water filled the basaltic low area
creating the oceans that cover 70% of Earth’s surface.
The granitic high area above sea level became land.
The presence of precipitation initiated weathering and
erosion. These processes produced sediment which was
moved by wind and water and ice to areas of deposition
where the sediment was lithified.
Weathering
The destruction of rock into sediment
due to the actions of wind, water, or
ice. (Making smaller rocks out of bigger
rocks!)
Sediments
Varying sized pieces of rocks produced
by the agents of weathering.
Erosion
Deposition
Lithification
Transport of sediment to new locations
by wind, water, or ice.
Accumulation of transported sediment
in a variety of different environments
and geographic settings.
Rock making process whereby
deposited individual sediment grains
are cemented together.
Back
Particle Size
 Sediment is weathered (broken) pieces of older rock.
 Sediment is classified by size ranging from clay to
boulder.
 Clay-sized sediment produces the rock shale. 70% of
West Virginia sedimentary rocks are shale!
 Sand-sized sediment becomes sandstone.
 Limestone forms in its own unique way.
Back
Shale is a fissile rock composed of composed of oriented clay-sized
particles. Fissile refers to the fact that a shale can be separated into layers.
Click on Steps below
to view step
descriptions:
Step 1:
Randomly arranged clay-size sediment is deposited as mud. Much
water is located between individual particles.
Step 2:
Continuing deposition buries sediment. Weight
of overlaying material begins to compress clay
particles together. Water is squeezed out as
particles become less randomly oriented.
Step 3:
Continuation of compression removes more water and
produces highly aligned clay particles.
Step 4:
Majority of water removed and sediment particles are aligned
producing the fissile rock we call shale.
Back
Sand is a size term referring to sediment particles
ranging from 1/16 to 2 millimeters in diameter.
When the sized particles cement together into the
rock called sandstone a entire suite of fine-grained,
medium-grained, or coarse-grained sandstones
may result.
As in shales, once the sand sediment is deposited,
the removal of water initiates the rock forming
sequence.
Question:
Without something to hold the individual sand
grains together, the rock would not exist. It would
just be a pile of sand. What process is required to
form the rock called sandstone that we have not yet
talked about?
Click here to find out.
Back
Answer = CEMENT
Sand-sized particles are cemented together when iron (Fe2O3), silica (SiO2), or calcium carbonate (CaCO3) precipitate
from ground water moving through or being forced out of the sediment particles as they are being compacted.
Water between
sand-sized
sediment grains
contains
cementing agents
that are dissolved
in water.
Some of the iron,
silica, or calcium
carbonate precipitates
and is left behind (brown
areas). This material
becomes the cement
holding individual grains
together to form the rock
called sandstone.
The open spaces between the sand grains are called pores. Pores provide
places for the accumulation of water, oil, and natural gas to collect.
Back
Limestone is predominately calcium carbonate (CaCO3).
Calcium carbonate is dissolved in water and can be removed
and turned into a carbonate-rich mud either chemically or
biologically. In either case, the CaCO3 is both the
accumulating sediment and the cementing agent holding the
sediment together to form the rock called limestone.
Aqueous environments where carbonate-rich sediment
accumulates over a large area is sometimes referred to as a
“carbonate shelf.”
Back
The two basic depositional environments are
terrestrial and marine.
Terrestrial environments are located on land
with the most important being those associated
with streams; such deposits are referred to as
fluvial deposits.
Marine deposits are those that accumulate in
ocean/sea environments. Unique nearshore
brackish areas such as estuaries, lagoons, and
tidal basins are referred to as non-marine.
Deposition occurs simultaneously at different
locations and, therefore, the elements of time
and location must be considered. Geologists call
this concept “facies.”
Back
Back
A typical facies scenario would occur where three types of sediment are being simultaneously deposited in bands parallel
to a shoreline. In this scenario sand-sized particles accumulate in the near-shore environment and beach area while claysized materials accumulate in slightly deeper water because they are separated from the sand by ocean currents and then
carried farther offshore. At the same time, if the water is relatively warm, limestone (also called carbonate) would
accumulate in still deeper ocean depths even further offshore.
Upon lithification, a single layer of sedimentary rock would be created that would, in various locations, be composed of
three different rock types: sandstone, shale, and limestone. These rocks would be seen to change into each other laterally.
Each rock type would represent a particular depositional environment and would be referred to as a facies of the overall
unit. The lateral change from one facies to another is called a facies change. Note that within an individual layer, the
order of facies change points either landward (limestone to shale to sandstone) or seaward (sandstone to shale to
limestone).
Back
The basic characteristic of all sedimentary rocks is that they are
bedded, that is, they are made up of layers called beds. Bedding is
the result of the original sediments having been deposited in
essentially horizontal layers, be it on an ocean floor, a lake bottom,
or a floodplain, a relationship summarized during the early days of
geology in the law of original horizontality.
The significance of the law of original horizontality is this…
Should you see sedimentary beds in anything other than a horizontal
position it means that the rocks were subjected to some degree of
deformation after they formed.
Back
The way any material responds to an applied force depends on the kind of force,
the ability of the material to withstand external forces and the physical nature of
the material. The general term for all external forces is stress. The ability of a
material to resist external stress is called strength. The way a material
responds to stress is referred to as strain or, more commonly, deformation with
deformation being defined as any change in either shape or size.
Click on a button to learn more
about each of these topics
STRESS
STRAIN
STRENGTH
Back
In compression, the forces are directed toward
each other. Compressional force can be oriented
either directly opposite each other (nonrotational compression) or along parallel paths
(rotational compression). This is a “PUSH
TOGETHER” or “SLIDING PAST” motion.
Tensional forces are always oriented
directly opposite each other. This is
a “PULL APART” motion.
Back
Once strength is exceeded, materials deform in one of three ways: 1) elastic, 2) plastic, or 3) brittle.
The type of strain can be determined using the decision tree.
START HERE
Did the material return to its original
shape when the stress was removed?
Click on terms below for more:
TIP: It is important to know that a material may
begin to deform plastically and then break. The
reason being that only so much energy can be
absorbed and internally consumed (plastic
deformation). The remaining energy must be
released by breaking (brittle deformation).
Back
Every material has an inherent strength which is simply the ability of a material to absorb stress without
deformation. Strength can be looked upon as an invisible wall that stands between stress (applied force) and
strain (deformation). Before any material deforms under stress, the strength of the material must be
exceeded. Once strength is exceeded, a material will deform.
Rocks are very weak under tension but very strong under compression. The best example involves a bit
of ancient history, in particular, the architecture of ancient Greece and of Rome demonstrates basic
engineering methods for dealing with the inherent strength of rocks.
Back
Compressive and tensile
forces in columns of the
Greek Parthenon.
The architecture of ancient Greece is characterized by the use
of many columns as is beautifully exemplified by the
Parthenon. The reason why the Greeks used so many
columns is because they were never able to overcome the
inherent weakness of rocks under tension. The ancient Greeks
spanned the distance between columns with rock slabs called
lintels. Whenever the distance between columns became too
great, the lintels would fail by collapse. The reason for the
failure was that with the lintels supported from their ends and
loaded from above, the forces generated within the lintel were
tensional. When the span between adjacent columns became
too great, the rock failed under the tensional forces. This, of
course, necessitated the use of many columns.
Back
The Romans spanned the space between
columns with an arch. They discovered
that by placing a keystone at the apex of
an arch, all of the forces within the
structure experienced non-rotational
compression. Under this situation rocks
are very strong. The result of their
discovery resulted in all of the vaulted
ceilings and domes
of all the cathedrals as well as all of the
arched structures such as the bridge over
the New River gorge.
Back
Hitting a baseball with a bat is a great example of
elastic deformation. At the moment of impact the
bat bends and the ball flattens) on one side. Both
bat and ball are deformed. However, both return to
their original shape. This is the basic definition of
elastic deformation. Kicking a football or soccer
ball also demonstrates elastic deformation
because both your foot and the ball return to their
original shape.
Back
Brittle failure of rocks under compression and tension creates faults.
Faults are breaks in Earth’s crust along which there is movement. In
both compression and tension, the break occurs at an angle to the
direction of stress; the difference being in the relative movement of the
rock mass on opposite sides of the fault.
Historically, the mass of rock above the fault is called the hanging
wall while the mass below the fault is called the foot wall. These are
terms originated by miners following beds of ore that encountered
faults in the mine. Faults formed under tensional forces are called
normal faults and are described as a fault where the hanging wall
has moved down relative to the foot wall. Faults formed under nonrotational compression are called thrust or reverse faults, where, in
both cases, the hanging wall has moved up relative to the foot wall.
Back
Plastic deformation of rocks does not break the rocks but folds them. Folds do not form at or near Earth’s surface where
rocks are most brittle but rather at depth where increased temperatures and pressures result in a more plastic response.
There are three types of folds: 1) monoclines, 2) anticlines, and 3) synclines.
A monocline is a regional steepening
of an otherwise uniform dip
Anticlines and synclines usually occur together and are
the result of compressive forces. Anticlines are upwarps
in Earth’s crust while synclines are downwarps.
The limb (side) of an anticline is also
the limb of the adjacent syncline!
Back