OC103 Lesson #9: Hot Spots and Mantle Plumes: Tails You Win, Heads You Die We have used Plate Tectonic Theory to explain most of the major features we see along plate boundaries in the ocean basins, such as mid-ocean ridges and deep-sea trenches, but you can see on the map below that there are some long chains of underwater volcanic mountains that do not correspond to any plate boundary. Examples include the long chain of seamounts that extends to the northwest from the Hawaiian Islands, and the line of seamounts that extends from eastern India and Bangladesh southward across the Indian Ocean. This lesson provides an explanation for these long chains of volcanic seamounts that is consistent with plate tectonics, but also may have some serious implications for the past survival of life on Earth.Age Progression of the Hawaiian Islands There are other examples, but Hawaii is the classic case of a volcanic island chain in the middle of a tectonic plate. It is far from any plate boundary, so cannot be related to seafloor spreading nor subduction. One obvious clue that these volcanic seamount chains have fundamentally different origins than seafloor spreading centers is that the seamounts are not all the same age, but instead have a systematic progression of ages. In the Hawaiian Islands, the youngest island is the Big Island of Hawaii, where active volcanism continues today, and islands to the northwest of the Big Island get progressively older. The oldest major Hawaiian island, Kauai, is the northwesternmost of the major islands, and formed from volcanic eruptions about 4 million years ago. The small islands to the northwest of Kauai are even older. Farther to the northwest, the islands have been eroded down to below sea level, but a long chain of seamounts continues to the northwest (just barely visible as a thin yellow line on the map on the previous slide) all the way up to the oldest seamount (about 80 million years old) in the northwest corner of the Pacific Ocean near Siberia (see figure below labeled with ages of the islands and seamounts in millions of years).Hotspots and Moving Plates Such a progression in ages for the Hawaiian islands and seamounts is exactly what would be expected if the Pacific Plate were moving across a fixed point of repeated melting and volcanism. Geologists call this the “hotspot” hypothesis because it involves the tectonic plate moving across a fixed point of persistent volcanism (that is, a hot spot). The hotspot sends lava up through the plate and onto the seafloor, but as the plate moves along it carries away the island or seamount formed by this lava, and a new volcano begins to form in its place. The generic figure below illustrates how this works (with the hotspot labelled as a "mantle plume"), with a plate moving across the top of the hotspot and a new island forming, but then being carried away by the moving plate and eroded away by weather and waves. In Hawaii, the young, actively forming volcanic island currently above the hotspot is the Big Island, while the older islands of Maui, Oahu, Kauai, etc., are being eroded as they are carried away from the hotspot by the moving plate. The link below shows an animation of how volcanoes repeatedly form above a hotspot, only to be carried away by the moving plate (https://www.youtube.com/watch?v=AhSaE0omw9o).Hotspots Caused by Mantle Plumes Because hotspots remain relatively fixed compared to the moving plates, they are thought to be rooted deep within the mantle, and rise to the surface as mantle "plumes", much as a blob in a lava lamp rises (see figure below left). These mantle plumes are solid, but can slowly deform and flow as they very slowly (<1 m/yr) rise to the surface. When the blob of warm material rising from deep in the mantle reaches the surface it melts and erupts as lava (see figure below center of a Hawaiian lava flow). Over time, the plate moving over the hotspot creates a long chain of islands and seamounts that are young on one end of the chain, and progressively older toward the other end of the chain. Hotspots are very useful in plate tectonics because they provide a way for us to measure how far and in what direction a plate has traveled relative to the mantle, simply by backtracking along the trail of seamounts leading away from a hotspot. When scientists conducted laboratory experiments trying to recreate how plumes work in the mantle, they discovered that when a new plume rises through the mantle for the first time, it does not rise as a narrow column, but develops a large "head" and a narrow "tail" as shown in the figure below of a time series of four snapshots of a rising plume (these photos are of a laboratory experiment in a tank containing very thick fluids with physical properties similar to the mantle). Credit: Peter Van Keken, University of MichiganSome Unpleasant Consequences of Mantle Plumes The narrow tail of the plume is what creates the hotspot and long chain of volcanic seamounts that are so useful in tracking plate motions back in time. This useful feature of hotspots is the "Tails You Win" part of the mantle plume story, but the plume head may not be quite as friendly a feature. As scientists started to think about what happens if a new plume with a large head reaches Earth's surface, they realized the consequences of all of that hot magma contained in the plume head suddenly erupting onto the surface. A massive outpouring of lava over very short time period is a likely result. Some hotspot seamount trails can actually be tracked back to large piles of volcanic rocks that erupted over very short time periods, covering huge areas with volcanic rocks up to several km thick. These masses of lava appear to represent plume heads erupting onto the surface. On land, these outpourings are called "flood volcanism" or “flood basalts” because they inundated large areas and left behind enormous piles of volcanic rocks (specifically, a common volcanic rock called basalt). The map below shows in red the locations of these large areas of volcanic rocks, including the Columbia River Flood Basalts in Idaho, Oregon, and Washington. These examples of flood volcanism might be no more than geological curiosities if not for the fact that volcanic eruptions are known to affect Earth's climate. It is therefore reasonable to conclude that massive volcanic eruptions could have a drastic affect on Earth's climate. Credit: The Geological SocietyErupting volcanoes