Geology

Geology - The study of rocks and the solid part of the Earth. 

A good website for info and can be found here.

Supporting files for this material can be found here

Continental Drift

1912 - Alfred Wegener

Based on three observations:

  1. The shape of the continents seem to fit together like puzzle pieces.
  2. There are similar plants and animals on different continents.
  3. There is evidence of glaciers in warm places (like Australia) and warm environments in cold places (like Antarctica).

The Theory of Continental Drift has three parts...

  • All continents were once joined together in a supercontinent called "Pangaea".
  • The continents have drifted to their current positions over millions of years.
  • The continents are still moving.

Plate Tectonics

1965 - John Tuzo Wilson

The Theory of Plate Tectonics states...

  • The Earth's crust is divided into pieces called tectonic plates.
  • As the plates move, they carry the continents with them.

The are three main types of plate boundaries:

  1. Divergent Plate Boundaries
    • Plates move apart.
    • Magma from the mantle comes up, is cooled by ocean water, turns to rock, and pushes the plates apart.  (New plate is formed here.)
    • ex. - Mid-Ocean Ridge (Mid-Atlantic Ridge)
  2. Convergent Plate Boundaries
    • Plates come together to form mountains.
      • Happens when two continental plates collide.
      • ex. - Himalaya Mountains (India & Asia)
    • One plate goes under another: subduction.
      • Ocean plate goes under a continental plate.
      • ex. - Japan, Andes Mountains, Alaska
  3. Transform Plate Boundaries
    • Plates move along side each other.
    • ex. - San Andreas Fault (California).

Earthquakes
Fault - A break in the Earth's crust where the rocks on either side move.
Focus - The origin of an earthquake.  This is the place inside the crust where the rocks actually break and move.
Epicenter - The place on the surface that is directly above the focus.

Seismic Waves are vibrations in the ground that are caused by an earthquake.  There are three types:
  a. Primary (P) Waves
      These are the fastest type of seismic wave.  P-waves represent the back and forth motion of the rock.
      (On this site, look at the longitudinal wave.)
  b. Secondary (S) Waves
      S-waves represent the up and down motion of the rock.
      (On this site, look at the transverse wave.)
  c. Surface (L) Waves
      These are the slowest, but most destructive type of seismic wave.  L-waves cause the ground to move like the surface of the ocean.
      (On this site, look at the Rayleigh surface waves.)

A seismograph is a device that measures the strength of earthquake waves.  A diagram and explanation of a simple seismograph are attached here.  A short video on how a seismograph works can be found here.

A site listing the strongest earthquakes since 1900 can be found here.

An explanation of the Richter Scale - and other earthquake scales - can be found here.



Volcanoes
A volcano is a break in the Earth's crust where lava comes out.  There are three main parts to a volcano:

  • Vent - This is the crack in the crust where magma comes up from the mantle.
  • Crater - This is the opening, at the top of the vent, where the lava comes out.
  • Cone - This is the "mountain" that is built up around the volcano's crater.

Magma - Molten rock below the surface of the Earth.
Lava - Molten rock on the surface of the Earth.

Volcanic projectiles are objects that are thrown through the air by a volcano.  There are three main types.  Ash and dust are the smallest type, only a few millimeters across.  Cinders are larger - up to several centimeters across.  Lava bombs are the largest.  They can be up to a meter across, and whistle as they fall through the air.

There are three major types of volcanoes:
  1. Cinder Cones are volcanoes which tend to erupt violently, throwing out a lot of ash and cinders.  This builds up a steep cone of material.
  2. Shield Volcanoes tend to erupt more gently, with lava flowing out without throwing projectiles.  This builds up a gently sloping cone, made of layer upon layer of hardened lava rock.
  3. Composite Volcanoes alternate between erupting violently and quietly seeping.  As a result, their cones are made of alternating layers of ash/cinders and lava rock.

Other Forces that Shape the Crust
Diastrophism - The movement of the solid rock in the Earth's crust.
Isostasy - The balance of how the crust floats on the mantle.

A good way to think about isostasy is to consider how a boat floats on water.  When the boat is empty, it sits pretty high in the water.  When people start getting into the boat, it rides lower and lower in the water.  The crust is the same way.  The thinner oceanic crust does not sink as far into the mantle as the thicker continental crust.

Fault - A break in the crust where the rock on either side moves.
Joint - A break in the crust where the rock does not move.
Fold - A place in the crust where the rock bends, but does not break.


Building Mountains
  1. Fault-Block Mountains are formed when large "blocks" of the crust get tipped over, forming steep and jagged mountains.
  2. Syncline & Anticline are places where the crust is folded, making more gently sloping mountains.  (Syncline folds down; anticline folds up.)
  3. Dome Mountains are formed when magma from the mantle pushes up the rock layers to form a dome-shaped mountain.

Plateaus
Plateaus are large, raised areas of flat land.  There are three main ways that they are formed...
  • when water carves away the land, plateaus are left behind.
    • (i.e. - the area around the Grand Canyon.)
  • when lava builds up layers of rock across large areas.
    • (i.e. - the Columbian Plateau.)
  • when the surrounding land sinks.
    • (i.e. - Death Valley.)


 

Weathering & Erosion
Weathering is the breaking of rock into smaller bits.  There are two main types of weathering:
   - Physical Weathering is the breaking of rock into smaller
      pieces of the same material.
   - Chemical Weathering is the breaking of rock into different
      (often simpler) materials.
Erosion occurs when the bits of rock are carried away.

Examples of Physical Weathering
1. Frost
    - Water gets into cracks in the rock, freezes and expands,
      pushing the cracks open wider.
2. Exfoliation
    - This is when the outer layers of rock peel or flake off.
3. Heat
    - When rock is heated, the minerals in it expand at different
       speeds.  This puts stress on the rock, which can cause it to
       crack.
4. Plants
    - As plant roots grow in cracks in the rock, they push them
       open wider and wider.

Examples of Chemical Weathering
1. Water
    - Some minerals dissolve in water, changing the composition of
       the rock.
2. Oxygen
    - Oxygen can combine with some materials to make new
       substances.  This is called oxidation.
    - An example of this is rust: a compound that is made up of
       both iron and oxygen.
3. CarbonDioxide
    - Carbon dioxide can be absorbed by rain to make carbonic
       acid - a weak acid that can dissolve some minerals.
4. Plants
    - Some plants secrete acids through their roots, which dissolves
       certain minerals and weakens the rock.

Formation of Soil

Horizon - any separate layer of soil.

  C-Horizon - partly weathered rock.

  A-Horizon - topsoil - made of humus (decayed plants and animals).

  B-Horizon - subsoil - formed when water washes down tiny particles and dissolved minerals from the topsoil.


Erosion can be caused by...

  • water
  • wind
  • gravity
  • animals
Sediments - Particles dropped off by water or wind.
  • Silt - Smallest
  • Sand
  • Gravel - Largest
The faster the water moves, the more/larger the sediment it can carry.
Runoff Water - Water flowing over the surface of the land.
  • Tributaries - Smaller streams/rivers that flow into larger streams/rivers.
  • Watershed - The area of land that drains into a river/lake.
  • Delta - A deposit formed at the mouth of a river.
    • Sediment in a delta is sorted by size as the water slows down.

Examples of Erosion Caused by Gravity

  • A landslide is the sudden downhill movement of large amounts of loose rock and soil. 
    • Talus - pile of broken rock at the bottom of a cliff or slope. 
  • A slump is the sudden downhill movement of a block of rock and soil.  
  • The slow downhill movement of rock and soil is called creep.
Water Cycle - Precipitation, groundwater and runoff can cause erosion.


Rocks & Minerals
Parts of the Earth

 - Atmosphere - The layer of air that surrounds the Earth.
 - Hydrosphere - All of the water on the surface of the Earth.
 - Lithosphere - The solid part of the Earth's crust (rock).
 - Biosphere - All of the living things on the surface of the Earth.
 - Cryosphere - All of the ice on the surface of the Earth.

Minerals are solid, naturally-forming substances that are found in the Earth and always have the same properties.  See the attached PowerPoint on minerals below.

Rocks are solid pieces of the Earth's crust.  They are made up of minerals.  There are 3 main types of rock:
      Igneous Rock  -  Sedimentary Rock  -  Metamorphic Rock

Igneous Rock is formed when magma/lava cools and hardens.  The word "igneous" means "coming from fire."  There are 2 types of igneous rock:
   1. Intrusive Rock is formed from magma inside the Earth's
       crust.  It cools slowly and tends to form larger crystals.  An
       example is granite.
   2. Extrusive Rock is formed from lava on the Earth's surface.
       It cools quicker than intrusive rock and tends to form smaller
       crystals (or no crystals).  Examples are obsidian and pumice.
As a general rule, the slower the lava/magma cools, the bigger the crystals will be.  Intrusive igneous rock can form structures inside the crust.  A dike is a layer of igneous rock that cuts across other rock layers.  A sill is a layer of igneous rock that runs parallel to other rock layers.  A batholith is a huge area of underground igneous rock.  Batholiths often form the cores of mountains.

 

 

Sedimentary Rock is made of sediments (particles dropped off by water or wind).  There are 3 types of sedimentary rock:
   1. Clastic (Cemented/Compacted) Rock forms
       when sediments get crushed into rock.  The type of rock that
       forms depends on the type of sediment that gets crushed.
            - Silt forms
shale.
            - Sand forms
sandstone.
            - Gravel forms
conglomerate.
   2. Chemical Sediments form when water evaporates,
       leaving minerals behind.  (This is like when you swim in the
       ocean - afterwards, the water evaporates and leaves a layer
       of salt on you.)  Examples of chemical sediments include
       gypsum and rock salt.
   3. Organic Sediments form from the remains of living things.
       For example, peat moss that is buried for thousands of years
       can form coal.  The shells of ocean animals can gradually be
       crushed into chalk.
As a general rule, the faster water moves, the bigger the particles that it can carry.  This means that the fastest streams and rivers will carry away the sand and silt, leaving rocks and gravel on their bottoms.  As the water slows down, it will drop off the sand - this creates a sandy bottom.  Really slow water will drop off silt, creating a silty/muddy bottom.

Metamorphic Rock forms when other types of rocks are changed by heat and pressure.  Examples include:
   - gneiss (changed from granite)
   - schist (changed from mica)
   - slate (changed from shale)
   - quartzite (changed from sandstone)
   - marble (changed from limestone)

The Rock Cycle is the endless process by which rocks are formed, destroyed, and formed again.  This cycle begins with magma, which cools into igneous rock.
   Igneous Rock can...
     - melt back into magma,
     - weather into sediments, which form sedimentary rock,
     - change, with heat and pressure, into metamorphic rock.
   Sedimentary Rock can...
     - melt into magma,
     - weather into sediments, which form other sedimentary rock,
     - change, with heat and pressure, into metamorphic rock.
   Metamorphic Rock can...
     - melt into magma,
     - weather into sediments, which form sedimentary rock.

The Rock Record
The Earth is about 4.6 billion years old.  To think about this, imagine that we stretch a piece of string from the windows in the middle school Resource Room to the windows in Mr. Byrne's classroom. This string represents the Earth's history from the very beginning to today.  Each centimeter (about the length of the word "each" at the beginning of this sentence) would be equal to about 1 million years.  All of human history would be within about the last 3 millimeters.

Uniformitarianism is the idea that the same processes are happening on the Earth now as have happened in the past.  Some examples of processes that have happened throughout Earth's history include: weathering, erosion, the Water Cycle, and the Rock Cycle.

Absolute Age means how old something is in years.  Relative Age means how old something is compared to something else (older or younger).  Geologists look for certain clues to figure out a rock layer's age.

The Law of Superposition states that, if you're looking at a set of rock layers, the layers on top will tend to be younger than the layers on the bottom.  This is because the layers on the bottom formed first, with later layers forming on top of them.

Crosscutting is the idea that a fault or magma-flow that cuts across layers of rock must be younger than the rock they cut across.

An unconformity is the boundary between younger layers of rock and older layers that have been eroded. 

Fossils
A fossil is aany preserved part, trace, or entire remains of an organism that lived long ago.  There are several ways in which fossils may be formed:
  1. Ice forms fossils by freezing the organism, so that it does not decay.
  2. Amber is fossilized tree sap.  Most of the insect fossils that we
      have today are from amber.
  3. Tar is a type of oily sludge.  Animals that wander into tar pits
      can get stuck and pulled down into the tar, where their bones
      get preserved.
  4. Burial means that the organism is buried in the ground.
      Because oxygen can't get to it, the process of decay is slowed
      down or even halted.  The hard parts of the organism, like the
      bones or shells, get preserved.
  5. Petrification happens when an organism is buried.  Water
      that is in the ground can dissolve the organic bits of the
      organism and replace them with minerals.  Over time, the
      entire organism is "turned to stone" in this way.
  6. Molds and Casts also form when an organism is buried.  The
      mud that the organism is buried in gets compressed into rock,
      and then the organism decays and is washed out by
      groundwater.  This leaves an imprint, which is called a mold.
      If the mold gets filled in with minerals, this model of the
      organism is called a cast.

The Age of Rocks
A key bed is a rock layer that is easily recognized and is found over a large area.  Once geologists identify a key bed, they can use it as a guide to study the layers above and below it.

Index fossils are the remains of plants or animals that only lived during a short part of the Earth's history.  If you know when a certain critter lived, then you know how old the rocks are that contain its fossilized remains.

Key beds and index fossils are used to create a geologic column.  This is a model that shows the order that rock layers formed in.

Radioactive decay is there process by which elements break down into simpler elements over a long period of time.  The time it takes for half of a sample of a given element to break down is called its half-life.  (See the attached PowerPoint.)  Some examples of elements' half-lives are:
  - Uranium decays into Lead.  Half-life: 4.5 billion years.
  - Potassium decays into Argon.  Half-life: 1.3 billion years.
  - Carbon-14 decays into Carbon-12.  Half-life: 5,800 years.

Earth History

The Earth is about 4.6 billion years old.  To study its history, we divide this time into sections.

  • Eras
    • Long time periods in the Earth's history.
    • Separated by major changes in the crust and in living things.
  • Periods
    • Eras are divided into periods.
    • Based on changes in the fossil record.
  • Epochs
    • Periods are divided into epochs.
    • Based on smaller changes in the fossil record.

We used to separate Earth's history into three eras: the Paleozoic, the Mesozoic, and the Cenozoic.  Everything that happened before the Paleozoic Era was lumped together in what we call "Precambrian Time", because we did not have the knowledge of what the Earth was like during this period to be able to divide it into eras.  (Precambrian Time is much longer than the three eras combined.)

 

Now that we are able to separate Precambrian Time into Eras, we sometimes refer to the combined Paleozoic/Mesozoic/Cenozoic time as the Phanerozoic Eon.



Resources


Supporting files for this material can be found here

Subpages (1): Geology Files