6th Study Guide: Earth Structure
Layers of the Earth
Layer
description
crust
Solid rock; thinnest layer; dry land and ocean floor
Lithosphere
Layer made of upper mantle and the crust; solid rock
Asthenosphere
Plastic-like layer of the mantle; the convection currents that cause
tectonic plate movement occur in this layer
Lower mantle
Solid rock
Outer core
Made of liquid iron and nickel; creates Earth’s magnetic field
Inner core
Solid iron and nickel; solid due to intense pressure
Temperature and pressure increase as we go deeper into the Earth.
Convection Currents that cause the tectonic plates to move:
1. Convection currents flow in the asthenosphere.
2. The heat source for these currents is heat from Earth’s core and from the mantle itself.
3. In the asthenosphere, hot rock rises because it is less dense than cooler rock.
4. The cooler rock is pushed aside and down by the rising hot rock.
5. The sinking cool rock is heated by the lower mantle and core so it begins to rise
6. This is a continuous cycle that keeps the tectonic plates in motion
Sea-Floor Spreading and Subduction Cycle
Seafloor spreading:
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Mid-ocean ridges occur where new oceanic crust is formed between tectonic plates that are moving
away from each other.
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Magma wells up into the gap between the plates, hardens, and forms new crust.
•
As the plates continue to separate, these newly formed pieces of oceanic crust separate and gradually
move outward away from the mid-ocean ridge at the rate of 1-20 cm/year.
•
The ridge is itself is a chain of underwater mountains, hills and volcanoes. A segment of the ridge is
above water and is one of the volcanoes in Iceland.
•
New seafloor emerges at the mid-ocean ridge and old seafloor subducts where it collides with the
continental plates.
Subduction zones
Seafloor spreading at the mid-ocean ridge
Theory of plate tectonics – pieces of Earth’s lithosphere are in slow, constant motion, driven by convection
currents in the mantle. Gravity pulls one edge of a plate down into the mantle; this causes the plates to move.
Convection currents in the mantle – the cycle of hot rock rising and cooler rock sinking creates currents of
molten rock in the mantle.
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As plates move, they collide, pull apart from each other or grind past each other – causing changes to
Earth’s surface.
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Oceanic plates are denser than continental plates so oceanic subducts under continental.
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Where plates meet is called plate boundaries.
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Most earthquakes, volcanoes and mountains occur at plate boundaries.
3 Types of boundaries:
1. Divergent – plates moving apart
2. Convergent – plates colliding
3. Transform – plates sliding/grinding past each other
KNOW THE PLATE BOUNDARIES AND WHAT’S HAPPENING/BEING FORMED AT THE
BOUNDARY FROM THE WEBQUEST! Web-quest: http://www.amnh.org/ology/features/plates/
Earthquakes – see your note’s page that I gave you or print this copy:
Earthquakes!!!
Movement of Earth’s plates creates enormous forces that squeeze or pull the rock in the crust as if it were a candy bar.
These forces (a push or a pull) are examples of STRESS.
Stress – a force that acts on rock to change its shape and volume.
How does stress in the crust change Earth’s surface? Stress pushes, pulls or twists the rocks in Earth’s Crust
Types of Stress
Types of stress
How affects crust
Type of boundary
Landform created
Tension
Pulls on the crust
Where plates are separating
Compression
Squeezes rock until it folds or
breaks
Where continental plates are
colliding
Valleys on land (example is
Rift Valley);
Mid-ocean ridges in the
oceans
Mountains – example is
Himalayas that are still
growing
Where oceanic plate subducting
under continental plate
shearing
Pushes masses of rock in
opposite directions; causes
plates to “slide” past each
other
Causes masses or rock to break
and slip apart
Creates underwater
trenches – deepest places in
Earth’s oceans; can also
create volcanic mountain
chains
Strike-slip fault at transform
boundaries between plates.
Example is San Andreas
fault in California
Fault – when enough stress builds up in the rock, the rock breaks, creating a fault.
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A fault is a break in the rock of the crust where rock surfaces slip past each other.
The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these
faults.
Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when
the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.
Earthquake - what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is
called the fault.
Hypocenter - location below the earth’s surface where the earthquake starts
Epicenter - location directly above the hypocenter on the surface of the earth
Seismic Waves – vibrations that travel through the Earth, away from the hypocenter, carrying the energy released when
rocks “unstick” and slip.
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The energy radiates outward from the fault in all directions, like ripples on a pond.
The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they
shake the ground and anything on it.