Mantle Plume: Definition & Explanation

Mantle Plume: Definition & Explanation
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  • 0:01 What Is a Mantle Plume?
  • 1:28 Evidence for Mantle Plumes
  • 4:02 Challenges Erupt
  • 5:36 Lesson Summary
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Lesson Transcript
Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Exactly what a mantle plume is and whether such things even exist, is not certain. Yet, they are proposed as an explanation for many volcanic features. You'll learn what geologists think they are in this lesson.

What Is a Mantle Plume?

A mantle plume is a large column of hot rock rising through the mantle. The heat from the plume causes rocks in the lower lithosphere to melt. The largest (and most persistent) mantle plumes are presumed to form where a large volume of mantle rock is heated at the core-mantle boundary, about 1,800 miles below the surface, although smaller plumes may originate elsewhere within the mantle. Once the temperature increases sufficiently to lower the rock density, a column of the hotter-than-normal rock (perhaps 2,000 kilometers in diameter) starts to rise very slowly through the surrounding mantle rocks.

Eventually, the rising column of hot rock reaches the base of the lithosphere, where it spreads out, forming a mushroom-shaped cap to the plume. The overlying lithosphere is pushed up and stretched out as the plume cap spreads. Heat transferred from the plume raises the temperature in the lower lithosphere to above melting point, and magma chambers form that feed volcanoes at the surface. This area is also known as a hot spot. Because the plume remains anchored at the core-mantle boundary, it does not shift position over time. So, as the lithospheric plate above it moves, a string of volcanoes (or other volcanic features) is created.

Sounds logical. But do they exist?

Evidence for Mantle Plumes

The evidence for mantle plumes is largely circumstantial (if not downright speculative).

Plumes and Volcanic Chains

The model was originally proposed in the late 1960s as an explanation for the Hawaiian-Emperor seamount chain, a 3,600-mile long string of volcanoes (only the southeastern-most of which are active) smack-dab in the middle of the Pacific Plate. Most active volcanoes are found at plate boundaries: either subduction zones (such as the Aleutian Islands) or divergent boundaries (like Iceland), where melting of the lithosphere is readily explained by plate tectonics. But, the Hawaiian Islands are nowhere near any plate boundary, so something else had to be creating melting conditions there. Thus, mantle plumes were proposed as the answer for how melting happened beneath the middle of a tectonic plate and persisted in one spot for tens of millions of years.

Plumes and Flood Basalts

Once the idea of mantle plumes had taken hold, they were suggested as the explanation for another puzzling type of volcanism.

More than 20 times in the geologic past, there have been eruptions of thousands of cubic kilometers of basalt lava from large rifts (cracks) in the continents. These events, and the thick layers of basalt thus formed, are known as flood basalts. Today, these places are marked by large, flat-surfaced plateaus built up from thousands of feet of basalt lava flows. Notable flood basalts include the Columbia Plateau (Washington and Oregon), the Deccan Plateau (India), the Siberian Traps (Russia), and the Karoo (South Africa).

For the most part, the eruptions are thought to be too massive to be caused by normal plate subduction, or they occurred in places where no subduction zone was active at the time. The quantity of magma necessary to yield the lava flows was extreme and required melting of large volumes of mantle rocks. So, melting within a plume's mushroom-shaped top was proposed to explain the large quantities of basalt lava. But so far, no direct connection has been made.

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