The Plate versus Plume Theory

What are mantle plumes?•Stationary, long-lived areas of heat flow within the mantle•They have a long thin tail and a bulbous head that spreads 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 denser material. •32 regions where P waves slow down = magma plumes.•S wave velocities indicate that the plumes extend to the core-mantle boundary.Geochemistry•Basalts from hotspots are different from MOR basalts or Island Arcs = magma origin different source. •Oceanic crust appears to be subducted down to the core-mantle boundary = major component of the plume material.

Case StudiesIceland: MOR and hotspot – magmas are ‘evolved’ so are silicic and intermediate as well as mafic.

Mantle plumes are implicated in the break up of continents and the formation of continental flood basalts including the 65 Ma Deccan Traps in India and the 250 Ma Siberian Traps in Russia.

The Yellowstone ‘super volcano’ is widely considered to be a hotspot.

BUT.....Plumes are said to have five basic features: 1. High Temperatures 2. 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 features

If every hotspot gets a special exemption, what is actually explained? - Foulger

Volcanism arises where plate tectonics promotes melting by:•Stretching the lithosphere apart releasing pressure on mantle rock (MOR and continental rifts).•Introducing material (subduction zones).•Extension and introduction of material: cracks in plates, changing stresses across plates and bodies of old material in the mantle through plate tectonics.

•This can explain ordinary and anomalous melting—even in Hawaii, Iceland and Yellowstone. (Foulger)

The Plate Hypothesis

Some related points to consider

Mantle Plumes and Plate Hypothesis are intrinsically linked with 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 slab pull instead?

Could a combination of theories/processes exist?

THE MANTLE IS NOT MOLTEN?Rocks melt for three reasons:

increasing temperaturedecreasing pressure

addition of waterAt mid-ocean ridges, buoyant mantle rises and melts (called adiabatic melting as the temperature of the rock doesn’t change).

At subduction zones, the downgoing slab heats up and releases water, and this addition of water to the overlying mantle then causes the mantle to melt. Think of this 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 only where 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 in the magma).

Source: http://www.wired.com/wiredscience/2011/07/eriks-volcano-nightmare-why-cant-the-media-get-science-right/

Plates V Plumes

For more information use the website above.

THERE ARE MANY QUESTIONS FOR BOTH SIDES OF THE DEBATE WITH GEOPHYSISTS ARGUING WITH GEOCHEMISTS

AS NEW INFORMATION AND RESEARCH COMES TO LIGHT THEORIES OF TECTONICS CHANGE AND/OR ARE CLARIFIED