Plate Tectonics

Migrating Continents and Oceans

Continents migrate over the Earth's surface because they are parts of the moving lithospheric plates. In the span of a human life, plate motion is slow. However, in the 65 million years since the extinction of dinosaurs, North America has migrated about 1620 km. As the Atlantic Ocean widens, the Pacific shrinks. Thus, as continents move, ocean basins open and close over geologic time.

Mantle Convection

Continually stirs the entire mantle as old, cold plates sink and hot rock rises to Earth's surface.

Convergence of Two Plates Carrying Continents

If there are two converging plates that are carrying continents, neither can sink into the mantle because of their low densities. So, the two continents collide and crumple against each other, forming a huge mountain chain (The Himalayas formed like this).

The Asthenosphere

It is "plastic," with a high temperature that is hot enough that 1-2% of it is molten. It also, because of its high temperature, has solid rock that is mechanically weak and plastic. Increasing the temperature causes the transition from the hard, strong rock of the lithosphere to weak, plastic rock of the asthenosphere.

The Mantle

It lies directly below the crust and is almost 2900 km thick and makes up 80% of Earth's volume. Earth's temperature and pressure increase with depth, which causes the strength of the mantle rock to vary with depth, and thus creates a layering within the mantle (upper mantle has two layers).

Mountain Building

Several process combine in order to form a mountain chain at a subduction zone. The magma rises into the crust heats and adds material to the crust. Additional crustal thickening may occur where two plates converge and finally the volcanic eruptions build chains of volcanoes. This thick crust then floats upward on the plastic asthenosphere to form a mountain chain.

The Structure of a Tectonic Plate: #7

Tectonic plates move at rates that vary from less than 1-16 cm per year.

Hot Spot Example:

The Hawaiian Island Chain is an example of a volcanic center at a hotspot. It erupts in the middle of the Pacific tectonic plate because the plume originates deep in the mantle, away from the plate boundary.

Isostasy

The concept that the lithosphere is in a floating equilibrium on the asthenosphere.

Effects on the Hydrosphere, Atmosphere, and Biosphere

The distribution of continents, oceans, and mountain ranges strongly affects global wind patterns and transfers of warm and cold air masses between low and high latitudes. The positions and movement of continents also affects ocean currents and the ways in which they transfer heat around the globe. Climatic changes alter the habitats and distribution of plants and animals. The shapes of the ocean basins change as tectonic plates migrate, altering currents and regional climates. Additionally, a large sequence of volcanic eruptions emirs great quantities of car.

The Core

The innermost of the Earth's layers, composed of iron and nickel. The outer core is molten because of the high temperature in that region. Near the center of the core, its temperature is as hot as the Sun's surface, with very high pressure. This extreme pressure compresses the inner core to a solid despite the fact that is even hotter than the molten outer core.

Plate Movements

The movements of tectonic plates generate volcanic eruptions and earthquakes. They also build mountain ranges and change global distributions of continents and oceans.

Divergent Plate Boundary Example #2: Mid-Oceanic Ridge (rifting in the oceans)

The new lithosphere at an oceanic spreading center is hot and therefore of low density. Consequently, it floats to a high level and forms an undersea mountain chain called the mid-oceanic ridge.

The Crust

The outermost and thinnest layer which consists of hard, strong rock because the crust is relatively cool

The Lithosphere

The uppermost mantle is relatively cool and is therefore a hard, strong rock.

Isostatic Adjustment

The vertical movement in response to a changing burden.

Earthquakes

They are common at all three types of plate boundaries, but uncommon within the interior of a tectonic plate. Quakes concentrate at plate boundaries because those boundaries are zones where one plate slips past another. An earthquake is a vibration of rock caused by abrupt movements.

Continental Crust

Thickness of 20-40 km and is composed of primarily light-colored, less dense granite.

Oceanic Crust

Thickness of 4-7 km and is composed of mostly dark, dense basalt.

Transform Plate Boundaries

This forms where two plates slide horizontally past one another as they move in opposite directions.

Convergent Plate Boundaries

Two lithospheric plates move toward each other and as the two plates converge, one may sink into the mantle beneath the other.

Divergent Plate Boundary Example #1: Spreading Center/Rift Zone

Two lithospheric plates spread apart from one another. The underlying asthenosphere then oozes upward to fill the gap between the separating plates. As the asthenosphere rises between separating plates, some of it melts to form magma. This magma then rises to Earth's surface, where it cools to form new crust.

Divergent Plate Boundaries

Two plates move apart from each other

Volcanoes

Volcanic eruptions occur where hot magma rises to Earth's surface; they are common at both divergent and convergent plate boundaries. Hot rocks can melt from high temperatures, pressure decreases, or if water is added to them.

Vertical Movement

When a large mass is added to the lithosphere, it settles and the underlying asthenosphere flows laterally away from that region to make space for the settling lithosphere.

Convergence of Oceanic Crust with Continental Crust

When an oceanic plate converges with a continental plate, the denser oceanic plate sinks into the mantle beneath the edge of the continent. As a result many subduction zones are located at continental margins.

Supercontinents

When tectonic plates move together. These last for a few hundred million years and then break into fragments and the fragments separated on their own tectonic plates.

Convergence of Two Plates Carrying Oceanic Crust

When two oceanic crusts converge, the denser one sinking into the mantle. Oceanic subduction zones are commonly found in the Pacific Ocean. Also the density of the oceanic lithosphere increases with age because the newly formed oceanic lithosphere, which is hot, thin, and low density, spreads away from the mid-oceanic ridge and becomes older, cooler, thicker, and denser.

Microcontinents

An isolated fragment of continental crust forming part of a small crust plate.

Hot Spot

As pressure decreases in a rising plume, magma forms and rises to erupt from volcanoes at a hotspot.

Why Do Plates Move?

1. The seafloor continually spreads outward from the mid-atlantic ridge. 2. After subduction begins, a tectonic plate sinks all the way to the core-mantle boundary to 2900 km. At the same time, equal volumes of hot rock rise from the deep mantle to the surface beneath the spreading center, forming new lithosphere to replace that lost to subduction. 3. Hot mantle rock flows upward from the core boundary. As the upper portion of the rising mantle nears the surface, it cools to become new lithosphere at a spreading center. The new lithosphere then glides over the asthenosphere away from the spreading center; while the older portion of the same plate sinks deeply into the mantle and replaces the rock rising beneath the spreading center. Thus, the mantle convects in huge elliptical cells.

Divergent Plate Boundary Example #3: Continental Rifting

A divergent plate boundary can rip a continent in half through continental rifting. A rift valley develops a continental rift zone because continental crust stretches, fractures, and sinks as it is pulled apart (ie: Eastern Africa will separate from the main portion of the continent and the new ocean basin will open between the separating portions of Africa).

Plate Boundaries

A fracture that separates one plate from another. Neighboring plates can move relative to another in three different ways.

The Structure of a Tectonic Plate: #5

A plate behaves like a slab of ice floating on a pond; it may slightly flex which will allow vertical movements. In general, each plate moves as a large tact sheet of rock.

The Structure of a Tectonic Plate: #4

A plate floats on the underlying hot, plastic asthenosphere and glides horizontally over it.

The Structure of a Tectonic Plate: #1

A plate is a segment of the lithosphere; thus, it includes, the uppermost mantle and all of the overlying crust.

The Structure of a Tectonic Plate: #3

A plate is composed of hard mechanically strong rock.

The Structure of a Tectonic Plate: #6

A plate margin is tectonically active. Earthquakes and volcanoes are common at plate boundaries, however; the interior of a lithospheric plate is normally tectonically stable.

Mantle Plume

A relatively small column of hot, plastic mantle rock. Many plumes rise from great depths in the mantle because rock near the core-mantle boundary becomes hotter and more buoyant than the surrounding regions of the mantle.

The Structure of a Tectonic Plate: #2

A single plate can carry both oceanic crust and continental crust. The average thickness of lithosphere is 75 km, whereas the lithosphere covered by continent is 125 km.

Plate Tectonic Theory

A single, unifying theory that explains earthquakes, volcanic eruptions, mountain building, moving continents, and many other events. It also allows scientists to identify many natural hazards before they affect humans.