Earth Science Unit 3 Lesson 6 Plate Tectonics

The earth's crust is broken up into giant pieces that float around on the asthenosphere. It's kind of like the earth's crust is a giant puzzle that slides around. The puzzle pieces of the earth can move into each other, next to each other, or away from each other.

The idea of constantly moving plates dividng Earth is called the theory of plate tectonics. This theory helps us explain why the plates of the Earth's surface move and what happens as they interact with each other.

asthenosphere the outer layer of the mantle; made up of plastic or semi-molten rock

continental crust the part of Earth's crust that forms landmasses

convection process of heat transfer by the circulation or movement of a gas, liquid, or plastic material

convergent boundary

the boundary created by two plates moving toward each other

divergent boundary

the boundary created by two plates moving away from each other

lithosphere earth's crust and rigid portion of uppermost mantle

magma the result of an oceanic plate as it begins to melt while conforming to a subduction zone

mid-ocean ridge a mountain range that is formed by seams that run along the floor of the ocean

oceanic crust the part of Earth's crust located beneath the oceans and seas

plate tectonics the theory that the lithosphere is broken into pieces that float on the asthenosphere

seafloor spreading

the process that occurs when new oceanic crust is formed through volcanic activity

sonar a device that uses sound waves to measure distance to an object

subductionthe process in which a deep part of the ocean is formed by the intersection of an oceanic plate and continental plate

transform boundary

the boundary created by two plates moving alongside each other

Plate Tectonics

Plate tectonics is the geological study of the movements of the Earth's crust or plates.

The Theory of Plate Tectonics has helped us understand Alfred Wegener's Theory of Continental Drift. Wegner proposed that the earth used to have one supercontinent called Pangaea that has since separated into seven different continents.

The concepts of plate tectonics rely on an understanding of the Earth's layers and movements. Remember that there are three main layers of the Earth: the crust, mantle, and core. The crust is the outermost layer of the earth, resembling the shell of an egg. It is thin and brittle, and can fracture or break. There are two types of crust; continental crust forms landmasses, and the oceanic crust lies beneath oceans and seas.

The second layer is the mantle. It is the middle layer between the core and crust, and is similar to the white of a boiled egg. It is semi-solid rock and is very hot and dense.

The third layer is the core. It is the center of the Earth, and similar to the yolk of an egg. The core is twice as dense as the mantle due to its metallic composition. It is divided into two parts: inner core and outer core.The inner core is solid, radioactive, and has a metallic composition. It causes the outer core and mantle material to move around due to its intense heat. The outer core is a liquid, metallic substance surrounding the inner core. It spins with the rotation of Earth, creating Earth’s magnetic field.

The Earth's crust and upper mantle consist of two spheres, the lithosphere and asthenosphere. The lithosphere is the crust and upper-most rigid portion of the mantle. The top of the upper mantle is rigid because the temperature is lower. The asthenosphere layer is a layer with hotter and more flexible, or plastic, rocks directly below the lithosphere. The lithosphere "floats" on top of the asthenosphere.

The boiling water example is an action similar to the release of the earth's internal heat, which we can more appropriately call mantle convection.

Even though the mantle is mostly solid rock, if hot enough, the rock will expand, become buoyant, and move upward.

When it reaches the top, it cools, moves off to the side, and sinks. Like the boiling water example, a convection cell or circular flow occurs, as shown in the mantle convection animation above (although the animation greatly exaggerates the speed of mantle convection).

Geologists think the movement of rock in the asthenosphere, due to convection cells within it, causes the movement of the tectonic plates that rest upon it. Remember, the movement is very slow—like growing fingernails!

Continental Drift

In the early 20th century, Alfred Wegener, a German scientist hypothesized that at one time, all of the continents fit together in one large land mass, and over time, something caused them to move apart. Wegener's hypothesis became known as continental drift. The continental drift hypothesis was supported with evidence collected by Wegener through the following sources:

rock distributions, mountain ranges, fossil distributions, glacial evidence; and geodetic surveys.

He called the land mass a supercontinent and gave it the official name of Pangaea. In 1915, Wegener published a book called The Origin of Continents and Oceans as evidence of his theory about Pangaea.

The Earth's Oceans A mid-ocean ridge is a mountain range that is formed by seams that run along

the ocean floors. Mid-ocean ridges lie under thousands of meters of water and form long lines of

mountains that begin on the ocean floor. In the mid-1900s, scientists used sonar, a device that uses sound waves to measure distance to an object, to map the mid-ocean ridges. The result of the mappings showed that mid-ocean ridges extend to all of the Earth's oceans.

A crack forms along the oceanic crust causing seafloor spreading to start at a mid-ocean ridge. Inside the Earth, new molten material rises, erupts, cools, and hardens forming a solid strip of rock.

The result of seafloor spreading is additional crust on the ocean floor. As the new strip of rock is forming, older strips move outward on each side of the ridge.

This action forces the ocean floor to plunge into deep underwater canyons called deep-ocean trenches. The oceanic crust bends downward at a deep-ocean trench and part of the ocean floor then sinks back into the mantle. This process takes tens of millions of year to complete.

Subduction

The new oceanic crust that forms through seafloor spreading is hot. As the crust moves away from the mid-ocean ridge, the temperature cools. The cooling

process increases the density of the crust. It is possible that when the crust eventually moves, it might collide with the edge of a continent. The older oceanic crust is pulled by gravity down beneath the trench and back into the mantle.

It goes from the mid-ocean ridge toward a deep-ocean trench. Seafloor spreading and subduction often work together by moving the ocean floor as if it were on a giant conveyor belt.

The sizes of the Earth's oceans are dependent upon how fast new crust is created at mid-ocean ridges and how fast old crust is being swallowed up at deep-ocean trenches. If an ocean is surrounded by several trenches, then it may shrink. If an ocean is surrounded by very few trenches, the ocean may grow larger. For example, the Atlantic Ocean has very few short trenches, thus the spreading ocean floor has nowhere to go, so the Atlantic Ocean is expanding. The oceanic crust of the Atlantic Ocean floor is attached to the continental crust of the continents around the ocean. As a result, as the ocean floor spreads, the continents along its edges also move. This causes the whole ocean to get wider.

Plate Movement

Earth's mantle experiences convection currents that cause the plates to move. As the plates move, they collide, pull apart, or grind past each other. The result of these movements is changes in Earth's surface and ocean floor causing mountain ranges, deep-ocean trenches, and the formation and volcanoes.

How do Plates move?

A relationship exists between the Earth's layers and the plates that make up the crust. What happens when you place a thin, cool, hard shell on top of moving, hot, pliable material? The shell is most likely to crack into pieces, or plates. This

is what happened to the lithosphere as it rested upon the hot surface of the asthenosphere.

When Alfred Wegener first introduced continental drift; he did not have the evidence he needed to prove his ideas.

However, in the 1960s, geologists found new evidence that helped explain how the continents move. The new evidence that the ocean floor was spreading helped solidify the theory of plate tectonics.

Geologists base the theory of plate tectonics on the idea that the lithosphere floats on the underlying layer, the asthenosphere. But how can these giant plates move around on top of the asthenosphere? Plate movement happens because of a process of heat transfer called convection.

Convection is the movement produced when a gas, liquid, or plastic is unevenly heated.

For example, when water is boiled, the water at the bottom of the pot heats first. It is hotter than the rest of the water, so it becomes less dense, and rises to the top in the form of bubbling, boiling water. After reaching the top, the water cools and thickens. The rising hot water pushes the cooler water to the side, where it then sinks. The circular motion of the water creates a convection cell.

Even though the mantle is mostly solid rock, if hot enough, the rock will expand, become buoyant, and move upward. When it reaches the top, it cools, moves off to the side, and sinks. The movement of rocks in the asthenosphere, due to convection cells within it, causes plate tectonics to move.

Plate Boundaries

The edges of Earth's plates meet at plate boundaries. Breaks in the Earth's crust where rocks have slipped past each other are called faults. Faults form along the three main types of plate boundaries - divergent boundaries, convergent boundaries, and transform boundaries.

Plates move away from each other at a divergent boundary. For the most part, a divergent boundary occurs along a mid-ocean ridge where new crust is added during seafloor spreading. If pieces of Earth's crust diverge on land, a rift valley is formed. When this happens, the crust is slowly pulling apart over a wide area.

A boundary where two plates collide is called a convergent boundary. When the two plates collide, oceanic crust becomes cooler and denser as it spreads away from the mid-ocean ridge. At a trench, the plate that has greater density sinks under the less dense plate.

A plate carrying oceanic crust can also collide with a plate carrying continental crust. The oceanic crust has a greater density and pushes up the continental crust. This process slowly forms thousands of miles of mountains such as the Andes Mountains along the west coast of South America. Meanwhile, the oceanic crust sinks as subduction occurs. Water leaves the sinking crust and rises into the wedge of the mantle above it. The melting point of the mantle in the wedge is lowered by the water and as a result, the mantle partially melts and rises up as magma. Volcanoes on Earth's surface are formed by the erupting magma.

Two plates carrying continental crust can also collide. Neither piece of crust is dense enough to sink far into the mantle. Instead, the two plates squeeze the crust into high mountain ranges.

The place in which two plates slip past each other, moving in opposite directions, is called a transform boundary. Below the surface, the sides of the plates are rocky and jagged. The two plates grab onto each other and lock in place. The two plates unlock only if a force inside the crust is strong enough. When the plates suddenly slip along their boundary, earthquakes often occur. Even though the earthquakes cause rapid

changes in the Earth's surface, crust is neither created nor destroyed at a transform boundary.