Plate Tectonics

The theory that the Earth’s outermost layer is fragmented into a dozen or more large and

small plates that move relative to one another as they ride on top of hotter, more mobile

material.

What Are Tectonic Plates?

• A plate is a large, rigid slab of solid rock.

– Plates are formed from the lithosphere: the crust and the upper part of the mantle.

• The plates “float” on the slowly flowing asthenosphere: the lower part of the mantle.

• The plates include both the land and ocean floor.

• The Mohoriovicic discontinuity or Moho is the boundary between the crust and the mantle.

What Drives Plate Tectonics?

• The slow movement of hot,

softened mantle lies below

rigid plates.

• The hot, softened rock in the

mantle moves in a circular

manner in a convection flow

– the heated, molten rock

rises to the surface,

spreads, and begins to cool,

and then sinks back down to

be reheated and rises again.

Earth’s Plates

Movement of the

Plates Over Time

Permian

248 million years ago

Triassic

206 million years ago

Jurassic

206 million years ago Cretaceous

65 million years ago

Present Day

Progression of Indian

Land Mass

Plate Boundaries

Different Types of Boundaries

Different Types of Boundaries

• Convergent boundaries come together – Places where crust is destroyed as one plate

dives under another

• Divergent boundaries spread apart – Places where new crust is generated as the

plates pull away from each other – New crust is created from magma pushing

up from the mantle

• Transform boundaries slide against each other – Places where crust is neither produced nor

destroyed as the plates slide horizontally past each other

Oceanic-Continental Convergence

• The oceanic plate subducts under the continental plate because it has lower density.

• The oceanic Nazca Plate is being subducted under the continental part of the South American Plate.

• The South American Plate is being lifted up, creating the Andes mountains.

• Strong, destructive earthquakes and rapid uplift of mountain ranges are common in this region.

• These earthquakes are often accompanied by uplift of the land by as much as a few meters.

• Mount Saint Helens is along the subduction zone of the Juan de Fuca plate (an oceanic plate) and the North American plate (a continental plate).

Oceanic – Continental Convergence

Oceanic-Oceanic Convergence

• When two oceanic plates converge, one is usually subducted under the other.

• An older oceanic plate is colder, therefore more dense and less buoyant, and will subduct under a younger, hotter, less dense, and more buoyant oceanic plate.

• In the process, a trench is formed.

– The deepest trenches in the oceans are along oceanic-oceanic subduction zones (i.e., the Marianas Trench in the Pacific, which is deeper than Mt. Everest is high).

• Subduction in oceanic-oceanic plate convergence can result in the formation of volcanoes.

• Examples of oceanic-oceanic convergence are the arcuate chains of islands in the southwest Pacific, Japan, and the Aleutian Islands.

Oceanic – Oceanic Convergence

Continental-Continental Convergence

Continental – Continental Convergence

• When two continents meet head-on,

neither is subducted because the

continental rocks are relatively light and,

like two colliding icebergs, resist downward

motion.

• Instead, the crust tends to buckle and be

pushed upward or sideways.

The collision between the Indian and

Eurasian plates has pushed up the

Himalayas and the Tibetan Plateau.

Continental-Continental Convergence

• 50 million years ago, the Indian Plate collided into the Eurasian Plate.

• After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalaya and the Tibetan Plateau to their present heights.

• The Himalaya form the highest continental mountains in the world.

Divergence

• Divergent boundaries occur

along spreading centers where

plates are moving apart and new

crust is created by magma

pushing up from the mantle.

• The Mid-Atlantic Ridge is a

divergent boundary.

• Sea-floor spreading over the

past 100 to 200 million years has

caused the Atlantic Ocean to grow

from a tiny inlet of water between

the continents of Europe, Africa,

and the Americas into the ocean

that exists today.

Divergence • Iceland is splitting along the Mid-Atlantic Ridge between the North

American and Eurasian Plates, as North America moves westward relative to Eurasia.

• In East Africa, spreading processes have already torn Saudi Arabia away from the rest of the African continent, forming the Red Sea.

• The actively splitting African Plate and the Arabian Plate meet in what geologists call a triple junction, where the Red Sea meets the Gulf of Aden.

Transform

• The zone between two plates that

slide

past one another is called a

transform-fault boundary, or

transform boundary.

• These large faults connect two

spreading centers or connect

trenches.

• Most transform faults are found on

the ocean floor.

Transform

• The San Andreas Fault is one of the few transform faults exposed on land. – It connects the East

Pacific Rise, a divergent boundary to the south, with the Juan de Fuca Ridge, a divergent boundary to the north.

– Most earthquakes in California are caused by the accumulation and release of strain as the two plates slide past each other.

Consequences of Plate Tectonics

• Earthquakes and volcanic activity are linked to plate

tectonic processes.

• The Ring of Fire is the most seismically and volcanically

active zone in the world.

Consequences of Plate Tectonics

The San Andreas Fault – a

transform fault

Aerial view of the area around Thingvellir, Iceland,

showing a fissure zone (in shadow) that is an on-land

exposure of the Mid-Atlantic Ridge. Right of the fissure,

the North American Plate is pulling westward away from

the Eurasian Plate (left of fissure).

Consequences of Plate Tectonics

The Aleutian Islands, an island arc The 1980 eruption of Mount

Saint Helens

Consequences of Plate Tectonics

The convergence of the Nazca and South American Plates

has deformed and pushed up limestone strata to form

towering peaks of the Andes, as seen here in the

Pachapaqui mining area in Peru.

Helicopter view (in February 1994) of the active

lava lake within the summit crater of 'Erta 'Ale

(Ethiopia), one of the active volcanoes in the

East African Rift Zone.

References

• Understanding Plate Motions. USGS.

http://pubs.usgs.gov/publications/text/unde

rstanding.html

• Plate Movement. OptIPuter Outreach.

http://education.sdsc.edu/optiputer/teacher

s/platemovement.html

• Plate Tectonics. The Way the Earth

Works. LHS GEMS, 2002.