6.15 Intraplate Activity

What would you think if you heard that all geological activity does NOT take place at plate boundaries?

These photos of fabulous geological activity are going to rock your world. Why? After all of these concepts in which you learned that volcanoes and earthquakes are located around plate boundaries, this last concept in Plate Tectonics doesn’t quite fit. These volcanoes are located away from plate boundaries. Two such locations are Hawaii and Yellowstone. Yellowstone is in the western U.S. and Hawaii is in the central Pacific.

Intraplate Activity

A small amount of geologic activity, known as intraplate activity, does not take place at plate boundaries but within a plate instead. Mantle plumes are pipes of hot rock that rise through the mantle. The release of pressure causes melting near the surface to form a hotspot. Eruptions at the hotspot create a volcano.

Hotspot volcanoes are found in a line (Figure below). Can you figure out why? Hint: The youngest volcano sits above the hotspot and volcanoes become older with distance from the hotspot.

Intraplate Activity in the Oceans

The first photo above is of a volcanic eruption in Hawaii. Hawaii is not in western North America, but is in the central Pacific ocean, near the middle of the Pacific Plate.

The Hawaiian Islands are a beautiful example of a hotspot chain in the Pacific Ocean. Kilauea volcano lies above the Hawaiian hotspot. Mauna Loa volcano is older than Kilauea and is still erupting, but at a slower rate. The islands get progressively older to the northwest because they are further from the hotspot. This is because the Pacific Plate is moving toward the northwest over the hotspot. Loihi, the youngest volcano, is still below the sea surface.

Since many hotspots are stationary in the mantle, geologists can use some hotspot chains to tell the direction and the speed a plate is moving (Figure below). The Hawaiian chain continues into the Emperor Seamounts. The bend in the chain was caused by a change in the direction of the Pacific Plate 43 million years ago. Using the age and distance of the bend, geologists can figure out the speed of the Pacific Plate over the hotspot.

Intraplate Activity on the Continents

The second photo in the introduction is of a geyser at Yellowstone National Park in Wyoming. Yellowstone is in the western U.S. but is inland from the plate boundaries offshore.

Hotspot magmas rarely penetrate through thick continental crust, so hotspot activity on continents is rare. One exception is the Yellowstone hotspot (Figure below). Volcanic activity above the Yellowstone hotspot on can be traced from 15 million years ago to its present location on the North American Plate.

Summary

• Not all geological activity is found at plate boundaries. Some volcanic activity, with accompanying earthquakes, is located within a plate. This is called intraplate activity.
• Intraplate activity occurs above mantle plumes that cause melting at a hotspot.
• Hotspots erupt mostly on oceanic crust. Hawaii is an example. A few hotspots, like Yellowstone, erupt on continental crust. The difference is due to the thickness of the crust.
• Hotspots can be used to tell the speed and direction that a plate is moving, since the hotspots are stationary within the mantle.

Review

1. What is a mantle plume and how is it related to a hotspot?
2. How do scientists use hotspot volcanism to tell the direction and speed of a plate?
3. Why are hotspot volcanoes much more common in the oceans than on continents?

Explore More

Use these resources to answer the questions that follow.

1. Where are the Hawaiian islands located in relation to plate boundaries?
2. What are hotspots?
3. The speaker falsely says that plumes of material come from the upper core. If that were true what would the Hawaiian volcanoes be made of?
4. Why do mantle plumes not move?
5. What evidence supports the theory that hotspots are stationary?
6. Why is Kauai older than the big island?
7. How does a volcano form on a moving plate?
8. Why is the big island bigger than Kauai?