Tectonic Earth

4.3 Tectonic Plates and Volcanoes

In this section, you will investigate how the properties of volcanoes are influenced by the crust and tectonic plate boundaries. By the end of this section, you should be able to:

  • Describe how magma is generated at tectonic plate boundaries and hot spots.
  • Determine how melted crust involved in the creation of the volcano influenced volcano properties.

Is the earth’s crust thick or thin?

Step 1: Go to the ArcGIS Online map, Plate Type Effect on Volcanoes, and explore the map.

Step 2: Click the pin on the Pacific Coast.

Step 3: Click the image in the pop-up to see more detail.

Geographic Questions
  • What is this ocean’s average depth?
  • What is the continent’s average elevation?

On the map, it appears that there are generally two types of the earth’s crust. That is because there is the oceanic crust and continental crust. Oceanic crust is thinner, but denser than continental crust. Continental crust is thicker, but less dense. So when the two collide, the heavier, oceanic crust subducts below the continental crust.

How else are the earth’s crust’s different?

Volcanoes form when melted crust (magma) cools at the surface (lava). Stickier, viscous, or slower-flowing lave piles up steeply before solidifying. More easily flowing, less viscous lava spreads out broadly before cooling.

Step 4: Click the button, Bookmarks, and select Mount Rainier. Click the arrow icon.

Mount Rainier’s shape implies that the magma has a high viscosity (thick), which causes the volcano to grow high with a steep peak. If the magma were less viscous (thinner), the volcano would have shallow slopes.

Geographic Question
  • The oceanic crust consists of different denser material than continental crust, which sinks deeper into the Earth’s crust. (T/F)

Step 5: Click the button, Bookmarks, and select Mauna Loa. Next, click the arrow.

Geographic Question
  • What does the volcano’s shape imply about the viscosity of the lava?

How are other sticky solutions like lava?

Think of lava-like a sugar solution. Higher sugar content is more viscous and flows slowly. Consider an analogy between the shape of a freshly poured puddle of juice compared to a puddle of syrup or honey. Juice is thin, and runny, making a broad puddle. The syrup is thicker and makes a small peak where poured. Honey is thickest, with a steeper, higher peak when first poured. In a volcano, the amount of silicate acts like a sugar in solution. Ocean plates have little silicate, while continental crust is rich in silicate. Like honey, continental volcanoes are steeper and more localized.

Does each tectonic plate type create its own distinct pattern of volcanoes?

Step 6: Click the Default Extent button (between the Zoom In and Zoom Out buttons, the upper-left corner of the map.

Step 7: With the Details button underlined, click the button, Show Contents of Map (Content).

Step 8: Turn on the two layers, Plate Boundaries and Global Volcanoes.

Step 9: Click different tectonic plate boundaries to see a summary of graphs of the types of volcanoes that occur within 250 miles.

Geographic Questions
  • Which boundary type has the most volcanoes?
  • Which has the highest percentage of shield volcanoes (the portion in white)?
  • Which boundary type has a higher percentage of cinder cone formation?

Why do many oceans have their ridges?

Geographic Question
  • Volcanoes that form along convergent boundaries form off to one side of a tectonic boundary. The crust on the other side of the boundary must run deep enough under a volcano’s tectonic plate to melt. (Zoom in for more detail.)

Step 10: Pan and zoom to a divergent boundary (e.g., Chile or Aleutian Islands).

Geographic Question
  • Volcanoes along divergent boundaries occur directly over rising hot convection currents from the earth’s core, melting the crust into volcanoes (points) and fissures (longer lines). (T/F)
  • When a continent is over a divergent boundary, it has the potential to split the continent, and after enough time, could create a sea or ocean. (T/F)

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