Magma From the Deep
The 'Big Island' of Hawaii is built of five - three of them active - shield volcanoes. Mauna Loa is the highest (4100m) and Kilauea is the most acitve volcano on Earth, producing daily eruptions of lava. The actual height of the Hawaiian volcanoes, from sea floor to their summits, is greater than the height of Everest, so the volume of lava that built them was immense.

Hawaii is located on a hot spot above a rising mantle plume. Mantle plumes are stationary long lived areas of high heat flow within the mantle. They consist of a long, thin conduit and a bulbous head that spreads out at the base of the lithosphere. As the plume head reaches the lithosphere , the reduction in pressure causes widespread partial melting of ultramafic mantle producing huge volumes of mafic magma. The mafic magma accumulates beneath the lithosphere until it is able to punch a hole through to form a shield volcano.

A geophysicist, W Jason Morgan, proposed the theory of mantle plumes in 1971. He suggested that along with mantle convection cells, mantle plumes are another way in which the Earth loses heat energy. The Earth's core is considerably hotter than the mantle. There is little evidence that any material moves across the Gutenbery boundary, so heat must be transferred across the boundary by conduction. As heat is transferred across the boundary, the material in the lowest part of the mantle becomes hotter, less dense and more buoyant. These overheated portions of the mantle will start to rise as low density diapirs that become mantle plumes. So it is possible that mantle plumes originate from as deep as the core-mantle boundary.

Moving Island Chains
All the islands of the Hawaiian-Emperor chain are made of basaltic shield volcanoes.
Apart from those on the 'Big Island', the others are extinct. Towards the northwest, they get progressively older and more eroded, continuing as a 6000km line of submarine guyots and seamounts, which extend to the Aleutian Islands.

There are two possible explanations for this pattern:

the position of the mantle plume has moved towards the southeast
the Pacific Plate has moved towards the northwest over a stationary plume

The second is more likely, obvs. A volcano builds when a vent or series of vents is created through the lithosphere. Plate movement then carries the volcano off the mantle plume so it becomes extinct and a new vent and volcano form over the stationary plume. Eventually a chain of volcanoes form, each one younger than the one before. The position and ages of these seamounts show the direction and rate of plate movement. In the case of Hawaii, the age of volcanic islands and seamounts show the hotspot is 80 million years old. A new, submarine volcano Loihi, is growing 30km south of Kiluaea, showing the mantle plume is still active. As the Hawaiian-Emperor Bend, the chain of seamounts turn north towards the Aleutian Trench, indicating that there was a change in the direction of plate movement 42 Ma.

	Created by siobhan.quirk over 11 years ago

Undergraduate Geology - Part 1 Note on Hot Spots and Mantle Plumes, created by siobhan.quirk on 15/05/2013.