[ home port | lectures | labs | surf | email prof. ]

The Plate Tectonics model nicely explains about 95% of the volcanism on Earth -- that found along spreading ridges, where plates are spreading apart, and that occurring at subduction zones, where one plate is sinking back into the mantle beneath the other. In looking at your map of the ocean floor, however, you’ve probably noticed prominent chains of volcanic islands and undersea mountains (called "seamounts") that occur throughout the ocean basins. One familiar example is formed partly by the Hawaiian Islands in the central Pacific. Notice, however, that these islands are just the southeast end of a much longer linear array of volcanoes that stretch to the west (as the Hawaiian Ridge) and then continue to the north (as the Emperor Seamounts). How were these formed so far from any plate boundary?

The answer is a phenomenon unrelated to plate tectonics. We now view these lines of volcanoes as the result of stationary hotspots, which are focused, hotter-than-usual regions that lie in the top part of the mantle, just under the plates. From time to time, magmas from the hotspot rise up through the plate (because such melts are less dense than the plate and are buoyant) and form a volcano on the plate surface. If enough magma is erupted the volcano grows up and above sealevel to form a volcanic island, as in our 50th state! Because the plate is moving from east to west over the hotspot, the volcano is carried away to the west and another is born over the hotspot. With time, then, a chain of volcanoes results simply from moving the plate continuously over the hotspot. The volcanoes become inactive once they drift away from the source of mantle melting. When they reach this stage, erosion wears them down and their weight causes the plate to sag slowly -- thus islands sink and become coral atolls, then seamounts. The change in orientation of the Hawaiian hotspot track, between the Hawaiian Ridge and the Emperor Seamounts apparently reflects a change in the direction of Pacific plate motion about 45 million years, which is the age measured for rocks dredged from the volcanoes at the "bend".

Hotspots underlie many familiar volcanic areas, like Hawaii, Yellowstone Park, and Iceland. This intense volcanic activity has left trails on each of the overlying plates and gives you a record of the direction and speed of plate motion in the past. But what causes hotspots?? Because hotspots have long lifetimes and remain stationary while plates move over them, we speculate that they are tied into a stable pattern of convection of the mantle, in which warm material from very deep rises as narrow columns, like a water fountain, called Plumes. Heat rising with these plumes ultimately flows into the upper mantle where it drives plate motions.

How do hotspots begin? The answer is, Catastrophically! If you follow any of the longer hotspot tracks from current activity back to its beginning, you end up at enormous accumulations of lava flows, usually on the margins of continents, called Large Igneous Provinces (shortened to "LIPs"). Remember, "igneous" means formed from magma. The volume of lava flows in a typical LIP is several millions of cubic km -- to give you idea of the size, such a volume would cover the states of Oregon and Washington with a thickness of 5 km (3 miles) of lava flows! Now, the really astounding part of this is that these LIPs each formed in a very short time (geologically speaking) -- often less than one million years. For this to happen, eruption rates had to be enormous -- several hundred times typical volcanic areas today. That’s why LIPs are often called catastrophic events. The other important aspect is that LIP events appear to coincide with many environmental crises in the geologic record, such as mass extinctions. Remember the poor dinosaurs? Their extinction coincides (at 65 m.y. ago) with a huge LIP in western India, called the Deccan flood basalts. This event also coincided with a large meteorite that hit the Earth on the northern edge of the Yucatan peninsula (Mexico). Scientists are still trying to understand the consequences of meteorite impacts and catastrophic volcanic episodes on the biosphere -- clearly both can have devastating effects.

So, in summary, our planet has two ways that it operates, each driven primarily by heat (convection). The first is Plate Tectonics, driven by convective flow of the upper mantle. This is a continuous, steady-state process that opens and closes ocean basins, through continent separation and collision. The second is Plumes-Hotspots, driven by convective flow from the deep mantle. This is also continuous and produces hotspot tracks, but the initiation of new plumes produces LIPs, which are intermittent events. Both plate tectonics and LIPs have important effects on the surface environment, both the ocean and atmosphere, but the first is gradual and long-term, while the second is catastrophic and intermittent.

http://dusk.geo.orst.edu/oceans/lec08.html