Plate Tectonics, Earthquakes, and Volcanoes

Earth's Pattern of Volcanoes and Earthquakes

Make sure you READ the text before answering any questions.

Vocabulary Words

On your vocabulary paper, add the following words. As you read the text, add definitions and sketches:

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lithospheric plate volcano hot spot rift valley

plate tectonics magma seamount

lithosphere​ lava mid-ocean ridge

Subject | Subjec

Earth's Patterns of Volcanoes and Earthquakes

Plate Tectonics

EarthComm 3rd Edition

​The figure to the right shows Earth without oceans. This allows us to see that Earth's surface is broken into gigantic slabs. These gigantic slabs are called lithospheric plates or tectonic plates. These plates move in relation to one another. The theory that explains this large-scale movement of Earth's lithosphere is called plate tectonics. Geologists use patterns in the locations of volcanoes and earthquakes to define the margins (boundaries) of lithospheric plates. The patterns are also used as evidence to support their explanations about why these gigantic slabs of crust move.

Earth's Volcanoes and Their Locations

A volcano is a vent or fissure in Earth's crust that allows magma, gases, and ash to escape from below the surface. Magma is the molten rock material generated within Earth. When the molten rock comes out of a volcano, it is called lava. Geologists have known for a long time htat there are many volcanoes along the edges of certain continents.

Plate Tectonics

EarthComm 3rd Edition

Earth's Volcanoes and Their Locations

A volcano is a vent or fissure in Earth's crust that allows magma, gases, and ash to escape from below the surface. Magma is the molten rock material generated within Earth. When the molten rock comes out of a volcano, it is called lava. Geologists have known for a long time that there are many volcanoes along the edges of certain continents.

Plate Tectonics

EarthComm 3rd Edition

Earth's Volcanoes and Their Locations

Volcanoes can erupt under the ocean or on land. Volcanoes under the ocean form in the following way. All of Earth's ocean basins have a continuous mountain range extending through them. This range is called a mid-ocean ridge. These ridges are broad rises in the ocean floor. They are usually in water depths of 1000 or 2000 m. They are 50,000 km long in total. At the crest of the ridge there is a steep-sided rift valley. Note that these topographic features are formed by TWO tectonic plates and they are moving away from each other.

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Magma from deep in Earth rises up into the rift valley to form volcanoes under the sea. Volcanic rocks on the floors of all the ocean basins are evidence that there are more volcanoes under water than on land.​

Plate Tectonics

EarthComm 3rd Edition

​Cross-section of a mid-ocean ridge

Earth's Volcanoes and Their Locations

Plate Tectonics

EarthComm 3rd Edition

​Cross-section of a mid-ocean ridge

​Do now: Add labels to your mid-ocean ridge cross-section using this figure

Earth's Volcanoes and Their Locations

Volcanoes that erupt on land are much more dangerous than volcanoes beneath the ocean. Eruptions along the western edge of the United States have formed the Cascades volcanic mountain range. They can also form island chains, such as the Aleutians in Alaska. Volcanoes like these are common in a narrow belt all around the Pacific Ocean.

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Geologists call this the "Ring of Fire." A famous example of an eruption along the Ring of Fire was the dramatic eruption of Mount Saint Helens in Washington in 1980. A small percentage of volcanoes occur in the interior of a plate. The Hawaiian islands, shown in the figure to the right, are an example. Studies of volcanic rock show that the islands get older the further northwest they are located. Only the youngest island, the "Big Island" of Hawaii, has active volcanoes. You can see it in the image - it is the biggest island!

Plate Tectonics

EarthComm 3rd Edition

​The Ring of Fire follows the edges of the Pacific Plate, named for the ocean it sits under.

Earth's Volcanoes and Their Locations

How do geologists explain the pattern of the Hawaiian Islands? Deep beneath Hawaii, there is a fixed source of abundant (lots of) rising magma, called a hot spot. As a plate moves across this area, it passes over the fixed hot spot. Magma from the hot spot forces its way through the moving plate to form a chain of islands. The sharp bend in the chain was formed when the direction of movement of the plate changed abruptly at a certain time in the past. Far to the northwest, the chain consists of seamounts. This is an elevated part of the seafloor, over a 1000 m.

Plate Tectonics

EarthComm 3rd Edition

​Map of the Pacific ocean floor. Note the extension of the Hawaiian islands to the Emperor Seamount Chain almost to Japan.

Multiple Choice

Which of the following is true:

• Magma and lava mean the same thing and can be used interchangeably

• Magma is molten rock that has erupted to the surface while lava is still beneath the surface

• Lava is molten rock that has erupted to the surface while lava is still beneath the surface

• Lava is molten rock that describes a more liquid consistency while magma tends to be thicker

Multiple Choice

There are more volcanoes...

• On Earth's surface

• Beneath the oceans

• on the moon

• on the interior of continents

Multiple Choice

Evidence to support the locations of the margins (boundaries) of tectonic plates and their movements comes from patterns in...

• earthquakes and volcanoes

• sedimentary rocks

• meteorites

• the shapes of river channels and deltas

Multiple Choice

The "Ring of Fire" surrounds the edges of the

• Cocos Plate

• Pacific Plate

• Antarctic Plate

• Eurasian Plate

Multiple Choice

Hawaii is the island chain in the bottom right corner. It extends into an underwater chain called the Emperor Seamounts which takes a more northern track. Which of these was used in the reading to explain this?

• the hot spot took a different path

• there is no explanation

• the currents change in this area

• the Pacific Plate changed its movement over the hot spot

Earthquake Patterns and Plate Tectonics

Vocabulary Words

​earthquake

seismic wave

focus

epicenter​

Subject | Subject

Some text here about the topic of discussion

Earthquake Patterns and Plate Tectonics

Earthquakes are another geologic process that takes place on Earth's crust. An earthquake is a sudden motion or shaking of Earth as rocks break along an extensive surface within Earth. The concentration of earthquakes along plate boundaries is very high. As you read earlier, Earth's plates move relative to one another at their boundaries. In some places, two plates slide past one another. In other places, plates move away from each other or toward each other. These motions cause forces in the rocks near the plate boundaries. When the forces build up to be greater than the strength of the rocks, the rocks break, causing an earthquake. The sudden release of energy as rocks rupture causes intense vibrations called seismic waves that extend in all directions.

Earthcomm 3rd Edition

Earthquake Patterns and Plate Tectonics

Earthquakes usually occur at some depth below the surface of Earth. The place in Earth where rupture (the break) occurs is called the earthquake focus (plural: foci). The epicenter is the geographic point on Earth's surface directly above the focus. The depths of earthquake foci match the types of boundaries where the earthquakes occur. Earthquakes and volcanoes are closely related to the movement of Earth’s crust. However, they do not always happen in the same places. During explosive volcanic activity, the crust can be fractured. This breaking of the crust is often accompanied by the release of seismic waves. Other kinds of volcanism happen without any great shaking of the ground. Movements of the crust that release earthquakes may occur without any volcanoes. For example, the movement of Earth crust along the San Andreas Fault in California does not involve volcanoes.

Earthcomm 3rd Edition

Aerial photograph of the San Andreas Fault

Open Ended

From the reading, what evidence do geologists have that volcanoes occur on the ocean floor?

Open Ended

What is the Ring of Fire and where is it located?

Open Ended

Where do most volcanoes on land form?

Open Ended

What are hot spots? Give one example.

Open Ended

How are the locations of earthquakes and volcanoes on Earth's surface similar?