Earth Dynamics Notes
Lesson 1: Forces That Shape Earth
A. Plate Motion
1. The theory of plate tectonics states that Earth’s surface is made up of
a number of rigid plates that move on top of the fluid, uppermost
layer of the mantle.
2. Mountains and valleys form where Earth’s plates collide, move
away from each other, or slide past each other.
B. Vertical Motion
1. You need to understand the forces that act on Earth’s plates to
understand how they can rise to form mountains.
2. Earth’s plates float on the surface of Earth’s mantle for similar
reasons as icebergs float on the surface of water.
a. Plates formed of continental crust are less dense than the fluid rock
that forms the mantle, and part of the continental crust floats above
the surface of the mantle.
b. The continental crust displaces some of the mantle below it until
an equilibrium is reached, which is called isostasy.
c. If the continental crust gets thicker, it will sink deeper into the
mantle, but the mantle will also push up on the thicker crust
until a(n) balance is reached.
d. Weathering and erosion remove the top part of a mountain. After
this happens, the crust rises to maintain isostasy.
e. The mountain stops moving when the mass of the mountain
equals the mass of the mantle it displaces.
3. Twenty thousand years ago, part of Earth’s crust was covered by ice,
which pushed down on the crust, forcing it to sink into the mantle in
a process called subsidence.
4. The melting of the glaciers and the draining of the water upset the
isostatic balance, and the crust started moving upward in response in
a process called uplift.
C. Horizontal Motion
1. A force acting on a surface is called stress.
a. A stress that squeezes against a surface is called compression.
b. Tension is a stress that pulls something apart.
c. Shear forces include forces that act parallel to one another
but in opposite directions.
2. Strain occurs when stress acting on rocks causes the rocks to change shape.
a. Elastic strain does not permanently change, or deform, rocks; the rocks return to
their original shapes.
b. Plastic strain creates a permanent change in a rock’s shape and affects hot or
weak rocks.
3. In the upper crust, the rocks are cold, and forces cause the rocks here to break
rather than to change shape.
a. When strain breaks rocks rather than just changing their shape, it is called failure.
b. Fractures, or faults, form when rocks fail.
D. Plate Tectonics and the Rock Cycle
1. Rocks are always moving through the rock cycle, both vertically and horizontally.
2. Together, plate motion, uplift, and subsidence keep rocks moving through the rock
cycle.
3. Uplift brings metamorphic and igneous rocks from deep in the crust up to the
surface, where erosion breaks down rocks into sediment.
4. Buried sediment becomes sedimentary rocks, which with pressure and temperature
eventually become metamorphic rocks.
5. Subduction takes all types of rocks deep into Earth, where they can melt and
become new igneous or metamorphic rocks.
Discussion Question
Compare how compression, tension, and shear affect layers of rock. Imagine you see three
similar outcrops of layered sedimentary rock, each under one of these stresses. Compare
and contrast how each would look.
Rock layers that are under compression bulge upward and look like they have been folded
in pleats. Rock layers that are under tension sink downward in the middle and look like
they have been stretched in long, thin layers. Rocks that are subject to shear stresses have
layers that are offset from each other or are turned horizontally relative to each other.
Lesson 2: Landforms at Plate Boundaries
A. Landforms Created by Plate Motion
1. Massive, slow-moving tectonic plates have so much force that they
can form tall mountains and deep valleys.
2. Three types of stresses—tension, compression, and shear—each
produce a different type of landform.
B. Landforms Created by Compression
1. The largest landforms on Earth are produced by compression at
convergent plate boundaries.
2. If two continental plates collide, tall mountains, like the Himalayas, can form.
a. The resulting mountain ranges form in stages, slowly, over millions of years.
b. Although plates move horizontally, the collision causes the
crust to move vertically as well.
3. When two plates collide, one can go under the other and be forced
into the mantle in a process called subduction.
a. A deep trench forms where the two plates meet.
b. Ocean trenches are deep, underwater troughs created by one
plate subducting under another plate at a convergent plate
boundary.
4. Volcanic mountains can form in the ocean where plates converge
and one plate subducts under another one.
a. The volcanoes form islands about 100 kilometers in distance from
where the two plates meet.
b. A(n) volcanic arc is the curved line of volcanic islands that
forms parallel to a plate boundary.
C. Landforms Created by Tension
1. Where plates move apart, tension stresses stretch Earth’s crust.
2. At divergent boundaries, oceanic plates move apart, and there are
tension stresses that cause crust to spread apart.
a. As tension stresses cause oceanic crust to spread apart, hot rock
from the mantle rises.
b. The hot mantle rises and pushes the seafloor upward, making a
long, tall ridge on the bottom of the ocean, called a(n) mid-ocean
ridge.
3. When divergent boundaries occur within a(n) continent, they can
form continental rifts, or enormous splits in Earth’s crust.
a. Tension stresses in the cold upper part of the crust create faults.
b. At these faults, large blocks of crust move downward, creating a rift valley
between two ridges.
D. Landforms Created by Shear Stresses
1. Shear stresses at transform boundaries produce faults where plates slide past one
another horizontally.
2. Faults that form where tectonic plates slide horizontally past each other are called
transform faults.
a. Segments of mid-ocean ridges are sometimes separated by transform faults.
b. The transform faults are perpendicular to the mid-ocean ridges, and they get
longer as the plates move.
c. As old(er) transform faults move farther away from the mid-ocean ridge, new
transform faults form.
3. A(n) transform fault that can be seen at Earth’s surface is the San Andreas Fault in
California.
a. Many transform faults that are part of this fault system cannot be seen on the
surface of Earth, but instead are underground.
b. A(n) fault zone is an area of many fractured pieces of crust that lie along a large fault.
Discussion Question
Name some high mountain ranges in the United States and other places. Describe how you
think they formed. What kinds of stresses contributed to their formation?
High mountains in the United States include the Rocky Mountains (and smaller mountain
ranges within the Rocky Mountains); the Sierra Nevada mountains are another high mountain
range in the United States. Students might also be familiar with other high mountains around
the world, including the Himalayas, the Andes, the Alps, the Urals, and the Caucasus. Many of
these mountain ranges have formed due to collisions of continental plates and, therefore, have
been created by compression forces.
Lesson 3: Mountain Building
A. The Mountain-Building Cycle
1. Mountain ranges form slowly, and they change slowly, over millions of years.
a. Because many different collisions occur to form the mountains,
mountains are made up of many different kinds of rocks.
b. The processes of weathering and erosion (either order) can carry
away all or parts of mountains.
2. When plates collide at a(n) plate boundary, a combination of
folding, faulting, and uplift form mountains.
a. The forces that originally brought the plates together can become
inactive after many millions of years.
b. When the plate boundary is no longer active, one new continent is
formed from the two old continents.
c. With no compression at a(n) convergent plant boundary, the
mountains stop growing.
3. The movement of Earth’s tectonic plates causes the continents to always be changing.
a. A(n) divergent plate boundary that forms on a continent often
forms close to the place where two plates first collided.
b. First a large split, or rift, forms; as it grows, water flows into it, forming a(n) ocean.
4. Weathering has rounded the peaks and lowered the elevations of
the Appalachian Mountains.
5. As a mountain erodes, the root under it must rise to restore the
balance between what is left of the mountain and how it floats
on the mantle.
a. Rocks deep under continents rise slowly toward Earth’s surface.
b. In old mountain ranges, metamorphic rocks that formed deep
below the surface are exposed on the top of mountains.
B. Types of Mountains
1. Plate movements can change the positions of rocks within a mountain range.
2. Folded mountains are made of layers of rock that are folded.
a. When erosion removes the upper part of the crust, folds are exposed on the surface.
b. The folds are perpendicular to the direction of the force of
compression that created them.
3. Parallel ridges that form where blocks of crust move up along faults are called
fault-block mountains.
a. If the tension that caused the mountains to form pulled in an east-west direction,
the mountains will form ridges oriented in the north-south direction.
b. Fault-block mountains have a high ridge next to a(n) valley; between these two
landforms is a(n) fault where the movement that caused the landforms occurred.
4. Uplifted mountains form when large regions of land rise with very little
deformation of Earth’s crust.
a. One idea of how uplifted mountains form involves sinking mantle pulling the
crust downward, which causes the crust near the surface to become compressed
and thicker.
b. As the crust thickens, the upper part of the crust rises to maintain isostasy;
sometimes the crust rises high enough to create tall mountains.
5. Volcanoes are special kinds of mountains.
a. Volcanic mountains form when molten rock erupts onto Earth’s surface and hardens.
b. Volcanic mountains that are dormant have not erupted in a long time, but they
might erupt again someday.
Discussion Question
How are folded mountains and fault-block mountains similar to one another? How are they
different from one another?
Both form in the direction that is perpendicular to the original forces that created them.
Folded mountains form at the edges of two continents, where plates collide and the crust is
hot enough to fold. Fault-block mountains form within a continent, where tension pulls the
crust apart, causing faults to form.
Lesson 4: Continent Building
A. The Structure of Continents
1. In most continents, the highest elevations are located close to the edges of the
continent.
2. The lower, flat parts are located close to the middle of the continent.
a. There are few mountains in the interior of continents.
b. The interior parts of continents are formed of old igneous and metamorphic
rocks and are the oldest parts of continents.
c. The middle parts of the continents are flat because millions or billions of years of
erosion have worn the rocks smooth and level.
B. How Continents Grow
1. Continents can grow when volcanoes erupt, adding igneous rock to the surface of
the land.
2. Continents can also grow when tectonic plates carry island arcs, whole continents,
or continent fragments with them and add them to existing continents.
3. When the moving plate that is carrying the piece of land reaches a(n) continent at
a(n) convergent boundary, the less dense piece of land gets pushed onto the edge of
the continent, causing the continent to grow.
4. Continents can change size and shape due to rifting and become smaller due to
weathering and erosion.
C. Continental Interiors
1. Rocks near the center or in the interior of continents are usually flat, stable, very
old, and very strong.
2. An extensive area of level or rolling land is called a(n) plain.
a. In North America, most of the central area is called the Interior Plains.
b. The rock that form the Interior Plains came from smaller plates that collided
about 1 billion years ago.
c. Shallow seas once covered these plains, and over millions of years, weathering
and erosion have worn the plains so they are nearly flat.
3. Plate motion and isostasy can cause places to sink, or subside.
a. Basins are areas of subsidence and regions with low elevation.
b. When sediments erode off of mountains, they often collect in basins.
c. Animal and plant remains can also get buried in basins and converted by pressure
and heat over millions of years into oil, natural gas, and coal.
d. Therefore, basins are often important places, economically, because they
contain valuable energy resources.
4. Plateaus are flat regions with high elevations.
a. Uplift causes some plateaus, such as the Colorado Plateau, to form.
b. Eruptions of lava can also form large plateaus.
D. Dynamic Landforms
1. Earth’s surface is constantly changing.
2. Mountains form and erode away.
3. Continents grow, shift, and shrink.
Discussion Question
Look at a map of Africa. Where would you expect the oldest part of the continent to
be? What would you expect it to be like? Explain your answer.
The oldest part of the continent of Africa is likely to be at the center of the continent.
Students should expect it to be a nearly flat region that has old igneous and
metamorphic rocks.
Students should expect Africa to be this way because, like most continents, the youngest
parts of the continent would be along the edges, where pieces get added as continents
collide; the oldest parts would be in the interior of the continent. In fact, most of Africa
is a plateau with two mountainous regions: the Atlas Mountains in northwestern Africa
and the Ahaggar Mountains in the Sahara. Interested students might research the East
African Rift System.