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TECTONIC PLATES
Ch 12.2
A Cross-Section of Earth
The Layers of Earth
• Earth is over 1200 km thick and has four
distinct layers:
1. Crust – outer solid rock layer (granite on land,
basalt in oceans)
2. Mantle – thickest layer, mostly solid except
for upper mantle being able to flow like “thick
toothpaste”
3. Outer core – composed of liquid iron and
nickel
4. Inner core – mostly solid iron, at tremendous
temperature and pressure
The Upper Mantle....
• We know the
“Lithosphere”
contains the Earth’s
crust and the
uppermost part of the
upper mantle.
• Just below this is the
“Asthenosphere”
– A partly MOLTEN layer
of the upper mantle
Plate Motion
• The plates move because of convection
currents arising in the hot magma of the
upper mantle in the Asthenosphere....
2 Q’s:
What makes this layer HOT?
What causes the “currents”?
What Makes the Layer HOT!
• Partly the upper mantle is hot because
of Earth’s hot core....
• The temp of the upper mantle varies
throughout
• Large amount of RADIOACTIVE
elements (ex: Uranium) occur in certain
spots
– When these decay...they release HEAT!
Radioactive URANIUM
What Causes the “Currents”
Just like in air masses.....HEAT RISES!
Hot (less dense) material rises…cools….sinks….reheats
“Mantle Convection”
The Driving Force...
• This “mantle convection” is the driving force
behind the movement of tectonic plates!
Tectonic Plates can PUSH or PULL
“Spreading Centers” are areas of the Earth’s
surface where magma rises up....
(common)
IN OCEAN....
“Spreading Ridge”
(less common)
On LAND....
“Rift Valley”
“Out with the OLD, In with the NEW”
• When magma rises and cools it forms new rock.
• This new rock PUSHES old rock ASIDE
“RIDGE PUSH”
SUBDUCTION
• When plates “bump” into each other, they
may subduct....
Consider a “heavy, dense” oceanic plate
bumping into a lighter continental plate....
SUBDUCTION
• The HEAVIER plate slides under the lighter plate.
This is called “SUBDUCTION”
SLAB PULL
• When a plate subducts deep into the
mantle....it PULLS the rest of the plate too
“SLAB PULL”
Action in the Subduction Zones!
• SUBDUCTION
ZONES experience
more
– EARTHQUAKES
–
VOLCANIC
ERRUPTIONS!
• YouTube - Continental Drift & Plate Tectonics
(clip)
Plate Interactions
• A plate boundary is an area where two plates are in
contact.
Divergent plate boundaries – areas where plates
are spreading apart
Convergent plate boundaries – areas where
plates meet
• Oceanic-continental
• Oceanic-oceanic
• Continental-continental
Transform plate boundaries – areas where plates
move past each other
Divergent Plate Boundaries
1. Divergent plate boundaries are areas where plates are
spreading apart.
– Ocean ridges and continental rifts are examples.
– The Mid-Atlantic Ridge is the longest mountain range on
Earth.
Convergent Plate Boundaries
2. Convergent plate boundaries are areas where plates collide.
A. Oceanic-continental plate convergence
• The oceanic plate subducts under the continental
plate, forming a trench.
• Cone-shaped volcanoes can form from magma seeping
to the surface.
• This is how the volcanic belt of North America’s west
coast has formed.
• Mountain ranges like the Coast Mountain
range also formed from the collision.
• Earthquakes can occur when subduction,
ridge push, and slab pull stall.
Oceanic-Continental Convergence
Oceanic-Oceanic Convergence
B. Oceanic-oceanic plate convergence
– The cooler, denser plate will subduct
under the less dense plate.
– Convergence may produce a volcanic island arc,
such as those found in Japan, Indonesia, and
Alaska’s Aleutian islands.
Oceanic-Oceanic Convergence
Continental-Continental Convergence
C. Continental-continental plate convergence
– Since both are continental plates,
their densities are similar.
– As they collide, their edges fold and
crumple, forming mountain ranges.
– The Himalayas are the world’s youngest (and tallest)
mountain range, formed as Asia and Africa plates collided
40 million years ago. They are still growing taller today.
Continental-Continental Convergence
Transform Plate Boundaries
3. Transform plate boundaries are where plates move past
each other.
– Usually are found near ocean ridges
– Since rock slides past rock, no mountains
or volcanoes form.
– Earthquakes and faults are very common.
Assignment