Plate Tectonics

Plate Tectonics

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Whats happening?

When you blow into a balloon, the balloon expands. Its surface area also increases. As more air is added to the balloon, the balloon gets larger. Similarly, if ocean crust continually forms at mid-ocean ridges and is never destroyed, Earth’s surface should be expanding. But measurements of the planet show that Earth is not getting larger. How can this be explained?

The Theory

Geologists proposed a more complete theory in the late 1960s. It was called plate tectonics theory. The theory of plate tectonics states that Earth’s surface is divided into late plates of rock.

These plates move with respect to each other, or in relation to each other. This new theory suggested that Earth’s surface, the lithosphere, is divided into large pieces of rock. These pieces are called plates. Each plate moves slowly over Earth’s hot and semiplastic mantle. 

Multiple Choice

What is another name for our Earths surface

• rock

• lithosphere

What can happen?

The word tectonic describes the forces that shape Earth’s surface and the rock structures that form as a result. Plate tectonics explains why earthquakes occur and volcanoes erupt.

When plates separate on the seafloor, earthquakes result and a mid-ocean ridge forms.

When plates come together, one plate can dive under the other. This causes earthquakes and a chain of volcanoes

When plates slide past each other, earthquakes can result.

Multiple Choice

When plates separate on the seafloor, __________result and a mid-ocean ridge forms.

• mountains

• rifts

• earthquakes

• tornadoes

Earth's Plates

Earth’s surface is divided into rigid plates that move relative to one another. Look at the map. It shows Earth’s major plates and their boundaries. Notice how some boundaries are in the middle of the oceans. Many of these boundaries are located at mid-ocean ridges. The Pacific Plate is the largest plate. The Juan de Fuca is one of the smallest plates.

Multiple Choice

Juan De Fuca is one of the _______ plates

• biggest

• most famous

• smallest

• most rigid

The lithosphere

Earth’s outermost layers are cold and rigid compared to the layers within Earth’s interior. The cold and rigid outermost rock layer is called the lithosphere. The crust and the solid, uppermost part of the mantle form the lithosphere

The lithosphere varies in thickness. It is thin below oceans. It is thick below continents. Earth’s tectonic plates are large pieces of lithosphere. These plates fit together like the pieces of a giant jigsaw puzzle

Multiple Choice

The crust and the solid, uppermost part of the ______ form the lithosphere

• inner core

• mantle

• outer core

• crust

The Asthenosphere

Directly below the lithosphere is a very hot part of the mantle called the asthenosphere. Even though it is solid, the asthenosphere is so hot, it flows like plastic. The asthenosphere flows below Earth’s plates. Plates of lithosphere move because they rest on the flowing asthenosphere. THIS IS WHAT CAUSES OUR PLATES TO MOVE.

Multiple Choice

What state of matter is the asthenosphere?

• solid

• liquid

• gas

• plasticlike

Plate Boundaries

Imagine placing two books side by side. Imagine that each book is a tectonic plate. The place where the edges of the books meet represents a boundary, just like the place where two plates meet is called a plate boundary.

How many ways can you move the books along a set of boundaries? You can pull the books away from each other. You can push the books together. You can slide the books past each other. Earth’s tectonic plates move in much the same way as you can move these books. 

Multiple Choice

The location where two things meet is called a ____________

• location

• boundary

• hotspot

Divergent Plate Boundary

When two plates move away from eachother a divergent plate boundary forms. Divergent means “moving apart.” Mid-ocean ridges are located along divergent plate boundaries in the ocean. When the seafloor spreads at a mid-ocean ridge, lava erupts. As the lava cools and hardens, it forms new oceanic crust. As this process continues, the plates move away from each other.

Divergent plate boundaries can also exist in the middle of a continent. If divergent plate boundaries separate parts of a continent, rift valleys form. The East African Rift is one example of a continental rift.

Multiple Choice

A divergent plate boundary are when two plates move

• together

• away

• side by side

• they dont

Transform Boundaries

The San Andreas Fault in California is a transform plate boundary. When two plates slide by each other, a transform boundary forms.

As they move past each other, the plates might get stuck and stop moving. Stress builds up where the plates are stuck. When this stress is too great, the rocks break and suddenly move apart. The result is a rapid release of energy in this type of movement causes earthquakes.

Convergent Boundary

When two plates collide, a convergent boundary form. When plates collide, the denser plate sinks below the less dense plate in a process called subduction.

A subduction zone is the area where a denser plate dives into Earth along a convergent plate boundary. The two types of convergent plate boundaries are ocean-to-continent and continent-to-continent. 

Multiple Choice

when two plates collide, the LESS dense plate, sinks below the denser plate

• true

• false

Ocean-Continent and Ocean-Ocean Convergent Boundaries

When a dense(heavy) oceanic plate and a less-dense continental plate collide, the oceanic plate subducts, or sinks, under the edge of the continental plate. This creates a deep ocean trench. Near the trench, a line of volcanoes form.

This process can also occur when two oceanic plates collide. The older and denser oceanic plate will subduct or sink beneath the younger oceanic plate. A deep ocean trench forms and a line of volcanoes form.

Multiple Choice

When two oceanic plates collide, which one sinks?

• the younger

• the less dense

• the older

Continental-Continental Convergent Boundary

Convergent plate boundaries also form when two continental plates collide. When this happens, neither plate is sinks, they crumble up, and mountains form. 

Multiple Choice

When two continental plates collide, what forms?

• trench

• mid ocean ridge

• mountains

• lava

Evidence for Plate Tectonics

When Wegener proposed the continental drift hypothesis, technology was not available to measure how fast the continents moved. Remember that continents move apart or come together at speeds of only a few centimeters per year. This is about the length of a small paper clip.

Today, scientists now use satellites orbiting Earth to monitor plate motion

The theory of plate tectonics explains why earthquakes and volcanoes occur in certain locations.