Hotspots and Mantle Plumes
A hotspot is a place in the upper mantle of the Earth at which extremely hot magma from the lower mantle upwells to melt through the crust usually in the interior of a tectonic plate to form a volcanic feature.
• These are hotspots beneath the lithosphere caused by rising plumes of hot mantle material.
• Can form volcanoes on surface (examples include Hawaii and Yellowstone hotspots (Figures 4-26 and 4-27).
-Volcanoes are mostly mafic (basaltic) as these lavas are very hot and very fluid from deep sources.
-Less common are felsic (rhyolitic) magmas as they are thicker and less prone to flow.
• Hotspots can occur beneath any crustal type (OC or CC).
• Where they form a trace consisting of a chain of volcanoes (like in Hawaii's Emperor Seamount Chain, Figure 4.26)
• Hotspots can exist in about the same place for 10's of millions of years
• The Hawaiian Hotspot has existed for about 60 million years; the youngest part of the Emperor Seamount Chain.
• The oldest part of the Emperor Seamount Chain has already been subducted (destroyed).
• There are hundred of hotspots located around the world. Some are larger and more active than others.
• Most hotspots are located under the interior sections of lithospheric plates, but some occur near divergent plate boundaries.
• Paleomagnetism in rocks on the ocean floor associated with hotspots provides a method for determining speed and direction of plate motions.
• We are not sure of the exact mechanism that forms hotspots, there are some ideas (see below).
|Figure 4.26. Map of the Hawaiian Hotspot and the Emperor Seamount Chain in the Pacific Ocean basin.
|Figure 4.27. The Yellowstone Hotspot is beneath the North America continent and is slowly migrating eastward as the continent is moving westward.
Yellowstone National Park Hotspot:
• The Yellowstone hotspot currently under continental crust (Figure 7.50).
• Yellowstone is a very large and complex supervolcano.
The Yellowstone supervolcano is at the eastern end of a long chain of progressively older supervolcanoes that formed along the trace of the Snake River Plain (Idaho, Washington, and Oregon)
• The hotspot had formed volcanoes of both basaltic and rhyolitic composition.
• Has had large rhyolitic eruptions 3 times in the last 2 million years
• This hotspot appears to have been present for less time than the Hawaiian hotspot (about 17 million years).
|Hot topic: Are hotspots related to "astroblemes" (large asteroid impacts)?
Earth has been hit by many asteroids throughout the geologic past. If Earth didn't have an atmosphere and active plate tectonics it would appear heavily cratered like the moon!
Current research suggests that massive asteroid impacts can deeply penetrate and fracture the lithosphere, allowing craters to flood with lava—the magma generated by both the impact and material flooding upward to the surface from deeper down. The question is, do astroblemes turn into hotspots? Also, it has been suggested that shock waves from a massive impact can travel through the Earth and will concentrate energy at the "antipoles" - resulting in deep fracturing of the lithosphere, resulting in massive volcanic eruptions. Examples of two possible "antipole" eruptions include the formation of the Deccan Traps in India (opposite the K/T boundary impact ~66 million years ago in the Yucatan region of Mexico. Another massive flood eruption occurred about 250 million years ago, forming the massive Siberian Traps (massive flood basalts that formed about the time of the great end-of-Permian extinction).