Constructive margins: processes, ocean ridges and rift valleys

Overview

Form of an ocean ridge, axial rift valley and pillow lavas.

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 happens at a constructive plate margin?

   Model answer

   Two plates move apart. The reduction in pressure on the underlying mantle causes it to melt partially, and the basaltic magma produced rises to fill the gap, cools and solidifies as new oceanic lithosphere welded to the trailing edges of both plates. Crust is therefore created, which is why the margin is called constructive. Typical separation rates are around 1-10 cm a year.

2. 2

   Why does the mantle melt beneath a constructive margin when no extra heat is added?

   Model answer

   Because of decompression melting. As the plates separate, hot mantle rock rises to fill the space. Its temperature barely changes as it rises, but the pressure on it falls sharply, and lowering the pressure lowers the melting point of the rock. The mantle therefore crosses its melting curve without getting any hotter, and a small percentage of it melts. The liquid produced from partially melting mantle peridotite is basaltic.

3. 3

   Describe the form of an ocean ridge.

   Model answer

   An ocean ridge is a submarine mountain chain running along a constructive margin, typically rising 2-3 km above the surrounding abyssal plain and up to 1000 km or more wide, forming a continuous system tens of thousands of kilometres long around the globe. It is highest and youngest at its axis and slopes gently away on both sides. It is offset repeatedly along its length by transform faults, giving it a stepped rather than a straight course.

4. 4

   Why does an ocean ridge stand higher than the ocean floor either side of it?

   Model answer

   It is a thermal effect rather than a pile of erupted material. Newly formed lithosphere at the axis is hot, and hot rock is less dense and more expanded, so it floats higher. As the lithosphere moves away from the axis it cools, contracts, becomes denser and thickens, so it sinks. Ocean depth therefore increases predictably with the age of the crust, which is why the ridge crest is elevated and the flanks slope away.

5. 5

   What is an axial rift valley, and why is it not found on every ridge?

   Model answer

   An axial rift valley is a steep-sided trough, often 1-3 km deep and tens of kilometres wide, running along the crest of an ocean ridge, formed where the crust is stretched and blocks drop down between parallel faults. It is characteristic of slow-spreading ridges, where limited magma supply means faulting and stretching dominate. Fast-spreading ridges receive magma quickly enough to keep the axis topped up, so they have a smooth or slightly domed crest instead.

6. 6

   What are pillow lavas, and why do they form at ocean ridges?

   Model answer

   Pillow lavas are rounded, sack-shaped masses of basalt, typically under a metre across, stacked on top of one another. They form when lava erupts under water: the outside chills instantly against the sea water into a glassy skin, the skin inflates as more lava is forced in, then it splits and a new lobe buds out. Because almost all constructive margin volcanic activity is submarine, pillow lavas are the normal form of new ocean floor.

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