Earth’s Changing Crust
Vocabulary
fault
Cracks in the crust
Geologists
Scientists who study the Earth
Magma
Hot molten rock deep below the Earth’s
surface
Lava
Magma that reaches the Earth’s surface
Weathering Breaking down rocks into smaller pieces
Erosion
The picking up and carrying away of pieces
of rock
Deposition
The dropping off of bits of eroded rock
Meteorite
Rocks from space that strike a surface
What Makes the Crust Move
• http://videos.howstuffworks.com/howstuffw
orks/236-how-earthquakes-work-video.htm
Forces on and Under Earth
Shape Its Surface
• The Earth’s crust is constantly moving.
• Earthquakes are related to cracks in the
crust called faults.
• San Andreas Fault
• During an earthquake the crust on either side,
or both sides of a fault is in motion.
• Vibrations travel through the crust. The
farther away people are from the earthquake,
the harder it is to feel the vibrations.
• Seismographs record the motion at locations
all around the crust.
• Most of the time the crust moves VERY
slowly and people only notice when there is
a visible change.
• To measure crust movement, surveyors
measure elevation (how high a place is
above sea level).
• Geologists place sensitive devices all along
faults. They hope that records of tiny
movements can be used to predict an
earthquake.
Plate Tectonics
• The crust is the
Earth’s hard surface.
– It is very thin.
– About one-thousandth
of the Earth’s thickness
Plate Tectonics
• The Mantle
– Located under the crust
– Earth’s thickest layer
– Rock material here is solid, however
it can flow like a liquid. (Like putty
does when you squeeze it.)
– Rock material is always in motion
like heated water in a pot.
– It rises and pushes against the
bottom of the crust. This causes it to
break into pieces, or plates.
– Earthquakes and slow motions of the
crust result from moving plates.
Plate Tectonics
• Earth’s Core
– Located below the
mantle
– Two parts
• A liquid Outer Core
• A solid Inner Core
What Forces Act On The Crust
• http://videos.howstuffworks.com/howstuffw
orks/236-how-earthquakes-work-video.htm
What Forces Act on the Crust?
• As plates of the crust move they can collide, pull
away from each other, or slide past each other.
• These movements cause 3 kinds of force to act on
the crust.
– 1. Tension – stretches or pulls apart the crust
– 2. Compression – squeezes or pushes together the crust
– 3. Shear – twists, tears, or pushes one part of the crust
past another
• Forces in the crust cause a fault to form and
movement is caused along the faults.
• Compression can crumple rock layers into
wavy folds. When two pieces of crust crash
together, mountains are formed.
• Mountains made of folded and crumpled
layers are called fold mountains
(Appalachians, Alps, and Himalayas)
Volcanoes
• Tension and shear can also build up the
crust. Mountains can be formed as the crust
is pulled apart.
• Hot molten rock deep below the Earth’s
surface (magma) rises upward. If this
reaches the surface it may flow out as lava.
• Lava flows out when a volcano erupts.
• Volcanoes can also form islands.
Check This Out!
• Hawaii Big Island Volcano eruption
Fault-block
Mountains
• Tension and shear can cause great blocks of
crust to break apart cleanly and move along
faults. This forms fault-block mountains.
What Other Forces Shape the
Earth’s Surface?
• While movements of the crust are building
up the Earth’s surface, other forces are
breaking it down.
– Weathering
– Erosion
Weathering
• The breaking down of the materials of
Earth’s crust into smaller pieces.
• Occurs when the crust is exposed to water,
air, and changes in temperature.
• http://www.uky.edu/AS/Geology/howell/go
odies/elearning/module07swf.swf
How Does Weathering Occur?
• Water can dissolve some minerals right out
of the crust.
• Moving water can make pieces of rock bang
into each other.
– Small chips break off of the surface of the rock
causing the rock to get smaller and rounder.
– Churning waters of a stream can wear down big
pieces of rock into small, rounded pebbles.
How Does Weathering Occur?
• The wind blows sand and other bits of broken bits of
rock over Earth’s surface. These particles also wear
away rock.
• When temperatures are low enough, water freezes
and expands. The force of expanding water is so
great that it can split rock apart.
• Changes in temperature can also cause rock to
expand and contract. When parts of a rock expand
or contract more than others it can cause the rock to
break apart.
Chemical Weathering
• Air contains gases that react chemically to
form new substances.
• Oxygen in air reacts with iron to form rust.
• Carbon dioxide and sulfur dioxide in air
react with rain to form acids.
• These acids eat away at limestone rock.
Erosion
• The carrying away of pieces of weathered rock by
gravity, water, wind, and ice.
• Erosion can carry away a boulder, hill, or even a
mountain range piece by piece.
• Water is the greatest agent. Each drop of water
falling from the sky erodes the land.
• Water moving downhill picks up pieces of rock
and carries them downhill. The faster the water
the bigger the pieces of rock.
What Does This All Look Like?
• HowStuffWorks Videos "Basics of
Geology: Erosion and Weathering"
How Can Wind Erode Rock?
• Wind does not exert as hard a push as water
moving.
• Wind mostly erodes pieces of rock that are
the size of sand particles or smaller.
How Can Ice Erode Rock?
• Ice causes lots of erosion.
• When ice of a glacier freezes onto rock and then
the glacier moves downhill, the rock is torn right
out of the ground. It can carry chunks of ice
bigger than your house with ease.
• Glaciers also wear away the land. Rocks of all
sizes become frozen to the bottom of the glacier.
As the glacier moves the rock beneath it is
scratched and worn down.
Ice and Sand Experiment
• Describe in detail what you observed during
the experiment.
Where Do Eroded Rocks Go?
• They are dropped to the bottom of the stream,
lake, or ocean when the water stops moving.
(DEPOSITION)
• Deposition takes place when glaciers melt and
wind stops blowing.
• Layer by layer, pile after pile, bits and pieces of
rock deposited by the water, wind, and ice build
up on Earth’s surface.
• Deposition eventually fills up depressions in the
Earth’s surface. It can build up land along shore
lines.
What Forces Shape The Moon’s Surface?
• Without air and water, there can be very little
weathering or erosion.
• The only weathering and erosion is due to the
impact of rocks from space hitting the Moon’s
surface (meteorites).
• Some craters formed by meteorites are big enough
to be seem from Earth.
• Earth’s atmosphere protects its surface from such
impacts. Rocks from space “burn up” as they pass
through our atmosphere.