What are mantle plumes?•Stationary, long-lived areas of heat flow within themantle•They have a long thin tail and a bulbous head thatspreads out at the base of the lithosphere.•A HOTSPOT exists above a magma plume.•The magma is mafic and produces a shield volcano.
Plume EvidenceSeismic Topography•P and S waves slow down through hotter densermaterial.•32 regions where P waves slow down = magmaplumes.•S wave velocities indicate that the plumesextend to the core-mantle boundary.Geochemistry•Basalts from hotspots are different from MORbasalts or Island Arcs = magma origin differentsource.•Oceanic crust appears to be subducted down tothe core-mantle boundary = major component ofthe plume material.
Case StudiesIceland: MOR and hotspot – magmas are ‘evolved’ so aresilicic and intermediate as well as mafic.Mantle plumes are implicated in the break up of continentsand the formation of continental flood basalts including the65 Ma Deccan Traps in India and the 250 Ma Siberian Trapsin Russia.The Yellowstone ‘super volcano’ is widely considered to bea hotspot.
BUT.....Plumes are said to have five basic features:1. High Temperatures2. Rise as big blobs that raise the crust above them3. Have narrow "tails" that reach the base of the mantle4. First appear with large flood basalt eruptions5. Produce a trail of progressively younger volcanoes erupting from the plume tail.But not one hotspot shows all five featuresIf every hotspot gets a specialexemption, what is actually explained? -Foulger
The Plate HypothesisVolcanism arises where plate tectonics promotes meltingby:•Stretching the lithosphere apart releasing pressure onmantle rock (MOR and continental rifts).•Introducing material (subduction zones).•Extension and introduction of material: cracks inplates, changing stresses across plates and bodies of oldmaterial in the mantle through plate tectonics.•This can explain ordinary and anomalous melting—evenin Hawaii, Iceland and Yellowstone. (Foulger)
Some related points to considerMantle Plumes and Plate Hypothesis are intrinsically linkedwith the structure of the mantle itself so:Do convection currents exist in the mantle?Is core heat conducted into the mantle?Is the movement of the surface plates driven by cold slabpull instead?Could a combination of theories/processes exist?
THE MANTLE IS NOT MOLTEN?Rocks melt for three reasons: increasing temperature decreasing pressure addition of waterAt mid-ocean ridges, buoyant mantle rises and melts (called adiabatic melting asthe temperature of the rock doesn’t change).At subduction zones, the downgoing slab heats up and releases water, and thisaddition of water to the overlying mantle then causes the mantle to melt. Think ofthis like adding salt to ice to get it to melt at a lower temperature.Only in rare situations does rock melt merely from increasing heat, usually onlywhere magma is in contact with wall rocks.Magma makes it to the surface, usually due to the buoyancy of the magma(thanks to its density contrast with the surrounding rocks and/or the bubbles inthe magma).Source: http://www.wired.com/wiredscience/2011/07/eriks-volcano-nightmare-why-cant-the-media-get-science-right/
Plates V PlumesFor more information use the website above.THERE ARE MANY QUESTIONS FOR BOTH SIDES OF THE DEBATE WITHGEOPHYSISTS ARGUING WITH GEOCHEMISTSAS NEW INFORMATION AND RESEARCH COMES TO LIGHT THEORIES OFTECTONICS CHANGE AND/OR ARE CLARIFIED