Earth’s Changing Surface - Geologists now know that Wegener was right when he said that the continents had once been joined into the supercontinent Pangaea and are now moving apart. Most of the geologic activity that we see on the planet today is due to the interactions of the moving plates. Where plates come apart at a divergent boundary, there is volcanic activity and small earthquakes.
If the plates meet at a convergent boundary, and at least one is oceanic, there is a chain of volcanoes and many earthquakes. If both plates at a convergent boundary are continental, mountain ranges grow. If the plates meet at a transform boundary, there is a transform fault. These faults do not have volcanic activity but they have massive earthquakes.
If you look at a map showing the locations of volcanoes and earthquakes in North America, you will see that the plate boundaries are now along the western edge. This geologically active area makes up part of the Pacific Ring of Fire. California, with its volcanoes and earthquakes, is an important part of this region. The eastern edge of North America is currently mostly quiet, although mountain ranges line the area. If there is no plate boundary there today, where did those mountains come from?
Remember that Wegener used the similarity of the mountains in eastern North America, on the west side of the Atlantic, and the mountains in Great Britain, on the eastern side of the Atlantic, as evidence for his continental drift hypothesis. These mountains were formed at a convergent plate boundary as the continents that made up Pangaea came together. So about 200 million years ago these mountains were similar to the Himalaya today.
Before the continents collided they were separated by an ocean, just as the continents rimming the Pacific are now. That ocean crust had to subduct beneath the continents just as the oceanic crust around the Pacific is being subducted today. Subduction along the eastern margin of North America produced continental arc volcanoes. Ancient lava from those volcanoes can be found in the region. Currently, Earth’s most geologically active area is around the Pacific.
The Pacific is shrinking at the same time the Atlantic is growing. But hundreds of millions of years ago, that was reversed: the Atlantic was shrinking as the Pacific was growing. What we’ve just identified is a cycle, known as the supercontinent cycle, which is responsible for most of the geologic
features that we see and many more that are long gone. Scientists think that the creation and breakup of a supercontinent takes place about every 500 million years.
Intraplate Activity
While it is true that most geological activity takes place along plate boundaries, some is found away from the edges of plates. This is known as intraplate activity. The most common intraplate volcanoes are above hotspots that lie beneath oceanic plates. Hotspot volcanoes arise because plumes of hot material that come from deep in the mantle rise through the overlying mantle and crust.
When the magma reaches the plate above, it erupts, forming a volcano. Since the hotspot is stable, when the oceanic plate moves over it, and it erupts again, another volcano is created in line with the first. With time, there is a line of volcanoes; the youngest is directly above the hot spot and the oldest is furthest away. Recent research suggests that hotspots are not as stable as scientists once thought, but some larger ones still appear to be.
The Hawaiian Islands are a beautiful example of a chain of hotspot volcanoes. Kilauea volcano on the south side of the Big Island of Hawaii lies above the Hawaiian hot spot. The Big Island is on the southeastern end of the Hawaiian chain. Mauna Loa volcano, to the northwest, is older than Kilauea and is still erupting, but at a lower rate. Hawaii is the youngest island in the chain. As you follow the chain to the west, the islands get progressively older because they are further from the hotspot.
The chain continues into the Emperor Seamounts, which are so old they no longer reach above sea level. The oldest of the Emperor seamounts is about to subduct into the Aleutian trench off of Alaska; no one knows how many older volcanoes have already subducted. It’s obvious from looking at the Emperor seamounts that the Pacific plate took a large turn. Radiometric dating has shown that turn to have taken place about 43 million years ago.
The Hawaii hotspot may also have been moving southward during this time. Still, geologists can use some hotspot chains to tell not only the direction but the speed a plate is moving. Hot spots are also found under the continental crust, although it is more difficult for the magma to make it through the thick crust and there are few eruptions. One exception is Yellowstone, which creates the activity at the Yellowstone hotspot. In the past, the hotspot produced enormous volcanic eruptions, but now its activity is best seen in the region’s famous geysers.
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