Hot spots and intraplate volcanic activity

Overview

Hot spot and flood basalts.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a hot spot?

   Model answer

   A hot spot is a localised region of unusually hot mantle, fed by a narrow plume of hot rock rising from deep within the mantle, which causes melting and volcanic activity at the surface. Crucially it is not associated with a plate margin and it stays more or less fixed in position while the plate above it moves. It is therefore an intraplate feature, capable of producing volcanoes in the middle of a plate thousands of kilometres from any boundary.

2. 2

   Why is it important to understand that a hot spot is not a plate margin?

   Model answer

   Because otherwise the distribution of volcanoes appears to contradict the theory. Hot spot volcanoes such as those of Hawaii sit in the centre of a plate, where there is no separation, no subduction and no melting mechanism of the usual kind. They are explained by a plume rising independently of plate boundaries, so the plate simply travels over the source of heat. Treating a hot spot as a margin would wrongly imply plate movement, earthquakes and boundary landforms that are not there.

3. 3

   How does a mantle plume generate magma?

   Model answer

   The plume is a column of mantle rock hotter than its surroundings, so it is less dense and rises slowly through the mantle. When it reaches the base of the lithosphere it spreads out into a mushroom-shaped head. Because the pressure at that shallow depth is much lower and the rock is unusually hot, a portion of it melts, again by decompression. The magma produced is basaltic, and it works its way up through the plate above.

4. 4

   Explain how a hot spot produces a chain of islands.

   Model answer

   The plume remains roughly stationary while the plate moves over it. A volcano built above the plume is therefore carried away from its heat source, and once it loses its supply of magma it becomes extinct. A new volcano then grows on the plate over the plume. Repeating this over millions of years produces a line of volcanoes, active at the end that currently overlies the plume and progressively older and extinct in the direction of plate movement.

5. 5

   What can be deduced from the ages of the volcanoes in a hot spot chain?

   Model answer

   Both the direction and the rate of plate movement. The direction of movement is from the active end towards the older end, and the rate can be calculated by dividing the distance between two volcanoes in the chain by the difference in their ages. A bend in the chain records a change in the direction of plate movement at the time corresponding to the age of the rocks at the bend, which is how the sharp bend in the Hawaiian and Emperor chain is interpreted.

6. 6

   What happens to a hot spot volcano after it moves off the plume?

   Model answer

   It stops erupting, and it starts to sink and to be worn away. The lithosphere beneath it cools and contracts as it moves away from the plume, so the volcano subsides, while wave attack and weathering erode it down. An island therefore becomes a low island, then a coral atoll where reefs continue to grow upwards as the base sinks, and finally a flat-topped submarine seamount below the surface. The chain therefore shows a progression in height and form as well as in age.

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