Different Rock Types
Rocks are categorized into three distinct types based on their
method of formation. The three types are igneous,
sedimentary, and metamorphic.
Anyone who wishes to collect rocks should become familiar
with the characteristics of these three rock groups. Knowing how a
geologist classifies rocks is important if you want to transform a
random group of rock specimens into a true collection.
Igneous Rock
An igneous rock is simply a rock that has solidified from magma or
lava upon cooling such as granite or pumice.
Igneous rocks are called fire rocks and are formed either
underground or above ground. Underground, they are formed when the
melted rock, called magma, deep within the earth becomes trapped in
small pockets. As these pockets of magma cool slowly underground,
the magma becomes igneous rocks.
Igneous rocks are also formed when volcanoes erupt, causing the
magma to rise above the earth's surface. When magma appears above
the earth, it is called lava. Igneous rocks are formed as the lava
cools above ground.
Sedimentary Rock
A sedimentary rock is one that is formed by the accumulation of
small to large sediment particles derived from all three types of
rock and in some cases organic material, and undergoes compaction,
cementation, or evaporation from/precipitation from a saturated
mineral solution.
Sedimentary rock is classified as organic, (derived from
organisms), clastic (formed from any size particle of preexisting
rock), or non-clastic (also referred to as chemical), where the
sedimentary rock is formed from the evaporation of a solution that
is saturated with mineral compounds.
Examples of organic sedimentary rocks are coal and limestone.
Examples of clastic sedimentary rocks are conglomerate and
shale.
Examples of non-clastic or chemical sedimentary rocks are rock
gypsum and rock salt.
Metamorphic Rock
A metamorphic rock is an igneous, sedimentary, or another
metamorphic rock that has either been squeezed by incredible
pressures deep underground and/or has been exposed to very high
temperatures, altering its structure, mineral alignment, or
chemical composition.
Examples of metamorphic rock are slate, quartzite and marble.
Weathering And Erosion
There are 2 types of weathering and erosion - mechanical and
chemical. Within the scope of this Earthcache, mechanical
weathering and erosion will contain the primary focus.
Mechanical Weathering
Weathering and/or erosion causes rock to break into smaller
particles. Mechanical is physical disintegrating. Smaller pieces
are formed; cliffs break off into pebbles and the pebbles into
sand. Only the size changes, not the chemical composition.
Frost, wetting and drying, plants and animals, and roots are part
of mechanical erosion. When water freezes, it has an unusual
property. Most materials expand when heated and contract when
cooled. This is true of water as well, except when it is cooled
from 4*C to 0*C. At these temperatures, water expands. Water
expands most when it solidifies into ice. When water becomes ice,
it takes up 9%-10% more space. This exerts incredible pressure on
rocks and then splits apart rocks. When water gets into tiny pores,
it is called ice wedging or frost action. Ice wedging occurs in
porous rocks and in rocks with cracks. This causes pot holes.
Wetting and drying is very effective at breaking up clay. It swells
and shrinks and then falls apart.
Plants, such as lichens, mosses, and tiny roots, wedge their way
into pores and crevices, and cracks. The seeds grow and expand,
causing cracking. Animals, such as earthworms and small mammals,
act as transportation for particles that get broken down.
Abrasion is where rocks rub against each other. This occurs mostly
in streams where fragments bounce off each other and round out like
a drum creating gems. This also occurs in the stream bed as
well.
The wind is a form of abrasion. This can be compared to sand
blasting.
Exfoliation is the peeling off of sheets of rock. This usually
occurs in rocks that contain feldspar. Clay is formed where water
gets deep into the feldspar. The clay then has a greater volume and
peels off in layers when it expands. Pressure unloading is when
pressure is removed and the granite expands and leads to joints, or
sheet joining. This can also occur when glaciers recede. All these
are examples of mechanical erosion.
Erosion
After the material is all weathered, it is usually moved by a
process called erosion. Sediments are moved from one place to
another. Waves that crash on the beach, streams, gravity, wind, and
glaciers are all means of moving weathered material. There are many
agents of erosion.
Mass movements and mass wasting is the downhill movement under the
direct influence of gravity. Gravity acts in components. Normal
force perpendicular to the surface that the sediment is on, and the
other force acts parallel to the surface, As the slope gets
steeper, the more force there is pulling the parallel.
The Angle of repose is the steepest angle at which a particular
remains stable. This depends on the size of the particles as well
as the shape and density. Sand, gravel, and clay all have different
angles of repose. Once the angle is exceeded, the sediments move
downhill and mass wasting occurs.
Talus slopes are called "fallen rocks zones" This is a clear
example of gravity.
Rock slides, or landslides, have a less steep slope which are often
triggered by rains. As the rain decreases, the friction between the
surfaces also decreases and chunks fall off.
A slump is huge amounts that slide in a large piece along the plane
of weakness. This is common where there are streams or ocean waves
undercutting a bank. Mud flow is the rapid movement of water
saturated mass of silt and clay.
Gradual slopes include earth flow and creep. Earth flow is the
shallow layer of earth, vegetation, and rock that gradually moves.
This takes about several hours to move. Creep is a very, very, slow
movement. You can notice fence posts leaning downhill after some
years of being there. Water in the soil can lubricate and add to
the movement.
Erosion can also occur by the movement of water.
Rounded and smoothed stones, if rounded and smoothed on all sides,
are usually the product of tumbling. Tumbling is usually
accomplished by transportation of the stone by water and contact
with other rocks. The rocks shown here, in the figure above, are
the actual little gems of Schoolhouse Beach. So, by visual
deduction, we can safely say that the stones of Schoolhouse Beach
were tumbled by water.
The weathering is the physical abrasion of rocks hitting one
another or rubbing against one another. The erosion, or transport,
is the constant tumbling (rolling, falling) of the stones within he
churning waters of Lake Michigan’s waves.
The figure above shows a very good example of the stones of
Schoolhouse Beach being pummeled by waves during a recent blow. For
centuries, these stones have been tumbled, washed and rubbed
together by the forces of the large lake finally evolving into the
smooth little gems that they are today.
To obtain credit for visiting and completing this
Earthcache:
a.) Email your answers to the four questions below, using the link
in my profile only.
Do not post your answers in your log or I will be forced to
delete your entry.
b.) Submit with your log to geocaching.com: A photograph of either
yourself and/or your GPSr taken at Schoolhouse Beach with the
western or eastern shores of Washington Harbor in the
background.
Multiple choice questions. Please select the single, most
correct answer.
1. What is the main feature that makes Boyer’s Bluff and
Niagara Falls very closely related?
a. both reside in a quadrant of the same
latitude.
b. both are composed of the very same rock
formation.
c. both structures were created by a scientific
process called “driftoral lift”.
d. magnetic deviation is identical at each
structure’s epicenter.
2. The polished rocks of Schoolhouse Beach were actually once
part of
a. a vast shallow inland sea.
b. a glacier covering the upper Midwest.
c. large chunks of limestone broken off
Boyer’s Bluff.
d. the southern shore of Rock Island
3. Littoral drift is the scientific process where
a. rocks are churned and tumbled within the water
in a certain direction.
b. chunks of limestone fall to the beach from an
adjoining bluff.
c. underground lake currents are steadily altered
in direction.
d. underground lake currents are steadily
decreased.
4. Which statement below is most true concerning Washington
Harbor and Schoolhouse Beach?
a. Washington Harbor’s basin is made mostly
of sand.
b. Only a very few beaches in the world are made
up of smooth stones such as these found on Schoolhouse Beach.
c. Schoolhouse Beach was named after the first
Island school built on this shore about the year 1850.
d. all of the above.
Glossary
Magma - molten rock that is found beneath the surface of the
Earth.
Silurian age - a time on the Earth which occurred 443 to 417
million years ago.
Talus slope - an accumulation of rock debris at the base of a cliff
or steep mountain slope.
Credits/References
http://www.fi.edu/fellows/fellow1/oct98/create/igneous.htm
http://www.uwgb.edu/DutchS/GEOLWISC/niagesc.htm
http://www.ucmp.berkeley.edu/silurian/silurian.html
http://www.scarborough.k12.me.us/high/projects/geoscience4/sbergg/erosion.htm
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