Plate Tectonics

What is the Theory of Plate Tectonics?

– The theory of plate tectonics states that Earth’s lithosphere is broken into enormous slabs called plates.

– Some of these plates are very small while others are made of whole continents. These plates move between ½ and ¼ inches a year. The movement of these plates explains how mountains, earthquakes and volcanoes occur.

View of Earth’s Plates

How do the plates move?

Types of Plate Boundaries

– Divergent Boundary – plates move away from each other

– Convergent Boundary – plates move toward each other

– Transform Boundary – plates slide past one another

Types of Divergent Boundaries

– Most divergent boundaries are found on the seafloor, or an Oceanic-Oceanic Divergent Boundary. The landform created here is an ocean ridge. New ocean crust is created at the ridges. This accounts for high heat flow, volcanism and earthquakes at these boundaries. The best example? Iceland and the rest of The Mid-Atlantic Ridge.

– Other divergent boundaries are found on land, or at Continental-Continental Divergent Boundaries. The landform created here is a rift valley. This could eventually lead to the formation of a new ocean basin. A specific example would be the rifting earth in Ethiopia.

– Along divergent boundaries new crust is formed at ridges.

Oceanic-Oceanic Divergent Boundary

Type of landform createdOcean Ridges

Iceland is part of the Mid-Atlantic Ridge. This ridge is still continuing to separate everyday. This occurs where the North American Plate and the Eurasian Plate are separating.

Continental-Continental Divergent Boundary

Rift Valley

Type of landform created

Three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian)

Rifting earth of Ethiopia

Types of Convergent Boundaries

– At Oceanic-Oceanic Convergent Boundaries one plate moves under the other. This is called subduction. This creates a deep-sea trench and an arc of volcanic islands. (Example: Philippine Islands)

– At Oceanic-Continental Convergent Boundaries there is still subducting of the heavier plate, made of oceanic crust. This creates a trench and a mountain range with many volcanoes. (Example: Peru-Chile Trench and Andes Mtns.)

– At Continental-Continental Convergent Boundaries the ocean basin is completely subducted and the continents collide. This creates a mountain range. (Example: Himalayas)

– Along convergent boundaries old crust is destroyed by subduction.

Oceanic-Oceanic Convergent Boundary

Trench and Island Arc of Volcanoes

**NOTE: When one plates goes under another it is called SUBDUCTION.

Type of landform created

Oceanic to oceanic convergence-located near the edge of the Pacific Ring of Fire

Philippine Islands-

Oceanic-Continental Convergent Boundary

Trench and Volcanic Mountain Range

**NOTE: When one plates goes under another it is called SUBDUCTION.

Type of landform created

The main cause of the rise of the Andes is the compression of western rim of the South American Plate due to the subduction of the Nazca Plate and the Antarctic Plate.

Andes Mountains (and Peru-Chile Trench)

Continental-Continental Convergent Boundary

Mountain Range

Type of landform created

Formed by the collision of the Eurasian Plate and the Indian Plate.

Himalayan Mountains

Transform Boundaries

– At Transform Boundaries the type of crust is not significant because the plates slide past one another.

– Transform boundaries can have jagged edges that get caught on each other. When this happens massive pressure builds up which releases in long faults and shallow earthquakes.

– (Example: San Andreas Fault – CA)

Transform Boundary

Long faults and shallow earthquakes

Type of landform created

San Andreas FaultBorder of the Pacific & North American plate, 810 miles long.

Arial photo of the San Andreas Fault

Causes of Plate Motions****you already know this!

– Convection currents in the mantle are thought to be the driving mechanism of plate movements. The upper and lower mantle are solid, but part of it, the asthenosphere, can flow like a soft, pliable plastic. The currents in this part of the mantle are set in motion by the transfer of energy between Earth’s hotter core and its cooler mantle.

– **Remember what we’ve already said about convection: warm, less dense (air) rises, and cools, then that cool, dense (air) sinks and warms up again to restart the process. The same is true even in liquids and other substances like the asthenosphere in the mantle.

There is always an exception…

–So, most faults/earthquakes/volcanoes form at PLATE BOUNDARIES.

–The Exception (not found at plate boundaries) is :

–HOT SPOTS – fixed in position as a plate moves over it (where magma rises up through the interior of a plate)

–Produce chains of volcanoes or seamounts

–One example is the chain of Hawaiian Islands.

HOT SPOTS

Hot Spots in the Continental US

A Key Part of Landform Creation

–ISOSTASY: the equilibrium of the earth’s crust, a condition in which the forces tending to elevate balance those tending to depress. dictionary.com–PT states that the lithosphere floats and moves on the asthenosphere, so….–If weight is added to the lithosphere, through deposition, the lithosphere slowly sinks deeper into the asthenosphere. –If weight is removed from the lithosphere, through erosion, the lithosphere slowly floats higher in the asthenosphere.

Layers of Earth through Plate Tectonics – Test 11/23 (A), 11/24 (B)

– It is important to understand how Earth’s layers and especially the type of crust affect plate tectonics. You will need to know how temperature and density affect the plates.

– You will need to know the correct order and composition of Earth’s layers, why the plates move, the various ways the plates move in relation to each other and, based on the type of crust involved, what landforms (general and specific) are created for each.

– You will need to know the theories of Continental Drift, Seafloor Spreading and Plate Tectonics, what they state and who proposed them, and what Magnetic Reversals are.

– You will need to know what Isostasy is and how it affects landforms.

– You will need to study; this one will NOT be an open-note test.