1. The Plate versus Plume Theory

2. 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.

3. Plume Evidence
Seismic 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.

4. Case Studies
Iceland: 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.

6. BUT.....
Plumes are said to have five basic features:
1. High Temperatures
2. Rise as big blobs that raise the crust above them
3. Have narrow "tails" that reach the base of the
mantle
4. First appear with large flood basalt eruptions
5. 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

7. The Plate Hypothesis
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)

10. 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?

11. THE MANTLE IS NOT MOLTEN?
Rocks melt for three reasons:
increasing temperature
decreasing pressure
addition of water
At 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/

12. 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