Volcanic activity at constructive margins

Overview

How magma is generated where plates part, why the eruptions are fissure eruptions and mostly invisible, and why an Icelandic basaltic eruption can still throw up an enormous ash cloud.

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

   How is magma generated at a constructive margin?

   Model answer

   By decompression melting. As the plates separate, hot mantle rises to fill the space. Its temperature stays nearly constant but the pressure on it falls, and because lowering the pressure lowers the melting point, a small percentage of the rising mantle rock melts without any additional heat. The liquid produced by partially melting mantle peridotite is low in silica and therefore basaltic.

2. 2

   What are the characteristics of constructive margin eruptions?

   Model answer

   They are effusive rather than explosive. The magma is basaltic, at roughly 45-52 per cent silica, very hot at around 1100-1200 degrees Celsius, of low viscosity and with a low dissolved gas content, so gas escapes freely as it rises. Eruptions therefore consist of lava fountains and outpourings of runny lava that flow long distances, with relatively little ash, and they are frequent but usually of low violence.

3. 3

   Why does volcanic activity at a constructive margin usually take the form of a fissure eruption?

   Model answer

   Because the crust there is being pulled apart, so it is cut by long, parallel tension fractures rather than by a single central pipe. Magma exploits these cracks and erupts along a line that may be many kilometres long, producing curtains of fire and flows that spread widely on both sides. The result is a lava plateau or a series of low cones strung along the fissure, rather than one large volcano.

4. 4

   Why is most constructive margin volcanic activity invisible?

   Model answer

   Because the great majority of constructive margins lie beneath the ocean, along the mid-ocean ridge system. Eruptions there take place under two or more kilometres of water, where the pressure suppresses any explosive activity and the lava chills immediately into pillow lavas. Although this is by volume the largest volcanic process on the planet, continuously manufacturing new ocean floor, almost none of it is observed directly.

5. 5

   What kinds of volcano are built where a constructive margin does reach the surface?

   Model answer

   Low, broad shield volcanoes built of many thin basaltic flows, together with fissure systems, spatter and scoria cones along the rift, and extensive lava fields. Where fresh magma meets ice or shallow water the eruption can become explosive and produce large volumes of fine ash, but this is a consequence of contact with water rather than of the magma's own chemistry. Iceland, where the ridge is exposed on land, shows the full range.

6. 6

   Why can a constructive margin eruption in Iceland produce enormous ash clouds if the magma is basaltic and effusive?

   Model answer

   Because of the interaction between magma and ice or water, which is a separate mechanism from gas-driven explosion. When hot basaltic magma erupts beneath a glacier, the ice flashes to steam and the sudden expansion shatters the magma into very fine, glassy ash particles. The steam then drives that fine ash high into the atmosphere. The magma itself is not gas-rich or viscous, so this is a conditional case: the same magma erupting on dry ground produces quiet lava flows.

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