TECTONIC PLATESCh 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 nickel4. 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) (less common)

IN OCEAN....“Spreading Ridge”

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 apartConvergent 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 Boundaries3. 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