Download Chapter 7 Notes - Wachter Middle School

Earth Science Notes
Chapter Seven-Plate Tectonics
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SECTION 1-Inside the Earth
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1. The Earth is made of three basic compositional layers--the crust, the mantle, and the core.
a. Continents and ocean basins (lands beneath the oceans) are part of a rocky "skin" that surrounds the main
body of the Earth. This skin is called the Earth's crust.
1. The average thickness: about 5 miles (8 kilometers) under the oceans; about 25 miles (40 kilometers)
under the continents.
b. Mantle is the layer of rock between Earth's crust and core.
1. The mantle makes up approximately 83 percent of Earth's volume and about 67 percent of its mass
(amount of matter).
2. Movements within the mantle cause volcanic eruptions, most earthquakes, and continental drift (the
slow movement of land masses).
c. Core-extends from the bottom of the mantle to the center of the Earth; divided into two parts.
2. The Earth is made of five main structural layers--lithosphere, asthenosphere, mesosphere, outer core, and inner core.
a. Lithosphere-the crust and part of the upper mantle; is divided into pieces called tectonic plates
that move around on top of the asthenosphere.
b. Asthenosphere-the plastic like layer [like silly putty] below the lithosphere (middle mantle); flows
very slowly--at about the same rate your fingernail grows.
c. Mesosphere-the strong, lower part of the mantle; extends from the bottom of the asthenosphere
down to the Earth’s core.
d. Outer core-composed of liquid iron and nickel; very dense material; 1200 miles thick. Very hot--3000+
degrees Celsius.
e. Inner core-composed of solid iron and nickel; extremely dense material.
3. Knowledge about the structure of the Earth comes from the study of seismic waves caused by earthquakes. See Fig. 6
on page 172.
a. Seismic waves are vibrations that travel through the Earth.
b. The speed of seismic waves depends on the density of the material they travel through. The denser the
material, the faster seismic waves move. Scientists use the time travel measurements to calculate the density and
thickness of each layer of the Earth.
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SECTION 2-Restless Continents
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1. Continental Drift and Evidence for Continental Drift
a. Theory of Continental Drift = the idea that the continents have moved horizontally to their current locations.
The theory of continental drift was inspired in part by the puzzle-like fit of the continents.
1. This theory was developed by Alfred Wegener.
2. Wegener believed that all of the continents were connected as one large land mass (he called this
Pangaea) about 245 million years ago.
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b. Evidence to prove Continental Drift:
1. Fossils of Mesosaurs have been found in South America and Africa.
2. Glacial deposits and grooved bedrock were found in southern areas of South America, Africa, India,
and Australia.
3. Parts of the Appalachian mountains in the eastern US are similar to those found in Greenland and
western Europe.
c. How Could Continents Drift?
1. Rock, fossil, and climate clues were the main evidence for continental drift during Wegener’s lifetime.
2. Wegener’s theory was often rejected because no one could explain how the continents moved.
2. Sea-floor Spreading--Clues on the Ocean Floor
a. Echo-sounding devices, developed in the early 1950’s, allowed scientists to construct accurate maps of the
ocean floor; they found underwater mountains, valleys, and ridges.
b. Theory of sea-floor spreading = the idea that hot, less dense material in the mantle is forced upward to the
surface at a mid-ocean ridge; then it turns and flows sideways, carrying the sea floor away from the ridge in
both directions.
1. mid-ocean ridge = are underwater mountain chains that run through Earths ocean basins. This is
where sea-floor spreading occurs. Ex. Mid-Atlantic Ridge
c. The theory of sea floor spreading, developed by Harry Hess in the early 1960’s, was shown to be correct due to
the following pieces of evidence:
1. Age of ocean floor rocks: Scientists aboard the research ship Glomar Challenger drilled the sea floor
for rocks, which were determined to be about 180 million years old (continental rocks are over four billion
years old)
2. Magnetic Reversals: Iron minerals in basalt rock align themselves according to the magnetic field
orientation at the time that they form; rocks on the ocean floor showed many magnetic reversals.
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SECTION 3-The Theory of Plate Tectonics
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1. Theory of Plate Tectonics-the idea that the Earth’s crust and upper mantle are broken into sections called tectonic
plates that move around on the mantle.
2. What “drives” the plates? Possible answers include...[See Figure 13 page 177]
a. Convection current = the driving force of plate tectonics in which hot, plastic-like material from the mantle
rises to the lithosphere, moves horizontally, cools, and sinks back to the mantle
1. The convection currents provide enough energy to move the plates in the lithosphere
b. Ridge Push-process by wh/ an oceanic plate slides down the lithosphere-asthenosphere boundary.
c. Slab Pull-because oceanic lithosphere is denser than the asthenosphere, the edge of the oceanic plate sinks and
pulls the rest of the tectonic plate with it.
3. Plate Boundaries
a. divergent boundaries = the boundary between two plates that are diverging, or moving away from each other.
1. Examples include...Mid-Atlantic Ridge and The Great Rift Valley in Eastern Africa
b. convergent boundaries = the boundary between two plates that are converging, or moving towards each other.
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Three types of Convergent Boundaries:
(1) OCEAN PLATE collides with CONTINENTAL PLATE
a. Continental plate less dense than ocean plate.
b. Result-when both plates collide, the more dense oceanic plate is forced under the less dense
continental plate .
c. This forms a deep trench.
d. The area where one plate is forced under another plate is known as an subduction zone.
e. The oceanic plate that is being pushed down into the subduction zone melts, causing volcanic
mountains to form.
f. The Andes Mts. of South America contain many volcanoes that formed this way.
(2) OCEAN PLATE collides with another OCEAN PLATE
a. Plate edges are bent downward into a deep trench--a subduction zone.
b. Part of the subducted plate melts; molten rock rises to sfc. along the trench to from volcanic
islands called island arcs.
c. Example--Islands of Japan.
(3) CONTINENTAL PLATE collides with another CONTINENTAL PLATE
a. Borders of both plates pile up into high mountains. (Folding)
b. Densities of both plates are similar so neither plate subducts (sinks).
c. Instead, rocks buckle & rise upward.
d. Little volcanic activity occurs because the rocks don’t sink into to the mantle where melting
occurs.
e. Earthquakes common as rocks break under the forces and strains.
f. Himalayan Mts. were formed as a result of the Indian plate colliding with the Eurasian plate.
c. transform fault boundaries = the boundary between two plates that are sliding horizontally past one other.
a. Earthquake activity is strong along these boundaries.
b. Little volcanic activity.
c. San Andreas Fault is an example--Pacific Plate is sliding past the North American Plate.
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SECTION 4-Deforming the Earth’s Crust
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1. As tectonic plates move next to and into each other, a great amount of stress is placed on the rocks at the boundary.
a. Compressional-is a force or stress that squeezes rocks together.
b. Tensional-is a force or stress that causes stretching or elongation of rocks.
c. Shearing-is a force that causes slippage and rocks on either side of the fault move past each other.
2. Folding occurs when rock layers bend due to compressional stress.
a. Depending on how the rock layers deform, different types of folds are made--anticlines-an up-fold,
syncline-a down fold.
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3. Faulting occurs when rock layers break due to either compressional or tensional stress and then move on
either side of the break.
a. Rock layers on either side of the fault are called fault blocks. The position of a fault block
determines whether it is a hanging wall or a footwall.
b. When rocks are pulled apart due to tension, normal faults often result. [See Fig. 20, page 183]
c. When rocks are pushed together by compression, reverse faults often result. [See Fig. 21, page 183]
d. Strike-slip fault is a fault in which the two fault blocks move past each other horizontally.
4. Effects of Plate Tectonics
a. Landforms caused by plate tectonics:
1. rift valleys-a valley that forms in a rift zone between to tectonic plates that are separating.
2. mountain ranges-formed when tectonic plates undergo compression or tension. [See Page 185186-Formation of the Appalachian Mountains]
3. volcanoes-a mountain that forms when molten rock is forced to the Earth’s surface. Develop
along convergent boundaries.
4. faults-a break in the crust along wh/ blocks of crust slide relative to one another due to
tectonic forces.
QUESTIONS TO CONSIDER:
1. What is the difference between continental crust and oceanic crust?
2. How is the lithosphere different from the asthenosphere?
3. How do scientists know about the structure of the Earth’s interior?
4. Describe Wegener’s theory of continental drift, and explain why it was not accepted at first.
5. Explain how sea-floor spreading provides a way for continents to move.
6. Describe how new oceanic crust forms at mid-ocean ridges.
7. Explain how magnetic reversals provide evidence for sea-floor spreading.
8. Describe the three forces thought to move tectonic plates.
9. List the three types of tectonic plate boundaries.
10. Why are there several categories of convergent plate boundaries?
11. Explain the process of subduction.
12. Describe the two major types of folds.
13. What is the difference between a normal fault and a reverse fault?
14. How is folding different from faulting?
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