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Plate Tectonics
Earth’s Interior
Earth is made of layers
 Crust
 Upper Mantle (asthenosphere)
 Mantle
 Outer Core
 Inner Core
Scientists discovered these
layers using seismic waves
from Earthquakes.
Lithosphere- Crust
 Lithosphere is the outermost layer of the Earth
 The Lithosphere (crust) is broken into sections called
Tectonic Plates, these are sections (plates) of lithosphere
that move around on the upper mantle (asthenosphere)
Types of Crust:
1. Oceanic Crust- Basaltic Materials, Heavy and Dense
2. Continental Crust- Granitic Materials, light and less
dense than oceanic crust
The thickness of the crust ranges from 5-60km thick depending on
what type of crust.
Mantle and Asthenosphere
 Has an upper layer called the Asthenosphere which
is responsible for the movement of plates
 Composed of mostly silicon, oxygen, magnesium and
 Texture is plastic like which has the characteristic of
a solid but flows like a liquid when under pressure
 Approx 2885 km thick
 Has large convection currents
that drive the movement of the
Inner and Outer Core
Outer Core:
 Liquid rock composed of Iron, Sulfur and Nickel
 Approx 2270 km thick
Inner Core:
 Solid dense rock composed of Iron and Nickel
 Pressure from the upper layers cause it to be solid
Types of Plate Boundaries
1. Divergent- new crust is generated and the plates
pull AWAY from each other
2. Convergent- Crust is being destroyed or uplifted
and the plates move TOWARDS each other
3. Transform- where crust is neither created or
destroyed and the plates that slide horizontally past
each other
Divergent Boundaries
There are two types of Divergent Boundaries where
the crustal plates are pulling away from each other
1. Oceanic-Oceanic- Mid Ocean Ridges
This is responsible for Sea floor spreading where new
crust is being made as magma rises and cools in the mid
ocean ridges
Divergent Boundaries Continued
2. Continental- Continental Divergent
When a divergent boundary cuts through a piece of land, the
feature called a Rift Valley is formed.
Example is Iceland where the North American and Eurasian
Plates are pulling apart and the African Rift Valley
Volcanoes are created in Rift Valleys as magma plumes up
from the separation of the plates
Convergent Boundaries
 Convergent Boundaries-
Two plates move toward each other pushing together
Subduction Zone- zone where one plate subsides
(goes underneath) another plate
Responsible for active volcanism and deep subduction
There are 3 types of Convergent plate boundaries
Oceanic-Continental convergence- subduction
2. Oceanic-Oceanic convergence- subduction
3. Continental-Continental convergence- minimal or
ancient subduction and uplifting of large mountains
Oceanic to Continental Convergence
 When an oceanic plate collides with a continental
plate, there is a Subduction Zone
A volcanic arc is created inland 100-300km from the coast
Trenches are formed off the coast of a subduction zone
Oceanic Crust subducts under continental crust because it is
more dense
Oceanic-Oceanic Convergence
 When two oceanic plates converge there is a battle
between basaltic crusts until one subducts
Volcanic Island arcs are created 100-300km away from the
plate boundary – Example: Japan, Aleutian Islands
Trenches are created off the plate boundary
Cleveland Volcano Aleutian
Islands, Alaska
Continental-Continental Convergence
 When two continental
plates collide both are
very buoyant and want to
stay above each other.
They battle towards each
other but neither one
subducts under the other
Builds large mountain
chains with very tall
No volcanoes because
there is not any
subduction causing
melting of the plates and
rising magma
The ancient Himalayas
were volcanoes because
as India was pushing
towards Asia, there was
an ocean between them.
This caused a subduction
of ocean crust resulting
in volcanoes.
Transform Boundary
 Transform boundaries are
where two plates slide past each
Usually a part of a system of divergent or
convergent boundaries
Example is the San Andreas Fault in
Responsible for frequent shallow
Hot Spots
 There are several other volcanoes that are not created
near a plate boundary but instead in the middle of a plate
 They are created when a mantle plume is super heated by
the core and burns through the crust building a volcano
 In the case of an oceanic hotspot, the plate continues to
move and the volcano moves off the hotspot, and
becomes inactive and just an island. A new volcano will
begin to form on the ocean floor and as it peaks above the
ocean surface it will become the new active volcanic
island in the chain
 In the case of a continental hotspot, the caldera moves off
the hotspot and when there is a new volcanic explosion, a
new caldera forms in the chain
Examples of Hot Spots
Hawaiian Islands- The active and newest island is the big island of
Hawaii, and the oldest inactive island is Kauai. Since the inactive volcanoes
are no longer adding to the size of the island, the island will erode away over
millions of years until it becomes a seamount where is no longer above the
ocean surface.
Yellowstone- this is in the middle of a continent causing large scale
eruptions that form calderas or collapsed volcanoes. The continental crust is
too thick to produce tall standing volcanoes from a hotspot.
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