Conservative margins: processes and why they build nothing

Overview

What happens where plates slide past each other, why the earthquakes are shallow and dangerous, why there is no volcanic activity, and why no classic landform is produced.

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 conservative plate margin?

   Model answer

   Two plates slide past one another horizontally along a near-vertical fault, either in opposite directions or in the same direction at different speeds. No plate is subducted and no new crust is created, so the total surface area of the crust is conserved, which is what gives the margin its name. The movement is parallel to the boundary rather than towards or away from it.

2. 2

   If nothing is created or destroyed at a conservative margin, why is it one of the most hazardous boundaries?

   Model answer

   Because friction locks the two plates together instead of letting them glide smoothly. Stress builds up in the rock on either side of the fault, sometimes for decades or centuries, and the rock deforms elastically until the accumulated strain exceeds the friction holding the fault. The fault then slips suddenly and the stored energy is released as an earthquake. Long locked segments can store enough energy to produce very large events.

3. 3

   Why are earthquakes at conservative margins shallow, and why does that make them dangerous?

   Model answer

   The fault is a near-vertical fracture through relatively cool, brittle crust, and no cold slab is carried down into the mantle, so foci are typically within about 20 km of the surface. Shallow focus means the seismic waves reach the surface having lost little energy over the short travel path, so ground shaking at the epicentre is intense. A moderate magnitude earthquake at shallow depth can therefore do more damage than a larger one at depth.

4. 4

   Why is there essentially no volcanic activity at a conservative margin?

   Model answer

   Volcanic activity requires magma, and magma requires either the release of pressure on rising mantle, the addition of water to the mantle, or unusually hot mantle beneath. At a conservative margin the plates simply grind past one another: there is no separation to cause decompression melting, no subducting slab to release water into the mantle, and no plume. Without a melting mechanism there is no magma, so no volcanoes form.

5. 5

   What is a transform fault, and where is the movement actually taking place?

   Model answer

   A transform fault is a conservative boundary that offsets segments of an ocean ridge, running at right angles to the ridge axis. The two plates move in opposite directions only in the section between the two offset ridge segments, and that section is the active conservative boundary. Beyond the ridge segments the offset continues as a fracture zone in the sea floor, but there both sides belong to the same plate and are moving together, so there is no relative movement and no earthquakes.

6. 6

   What landforms does a conservative margin produce?

   Model answer

   It produces features of horizontal displacement rather than construction: a linear fault trace often visible as a scar across the landscape, offset features such as rivers, roads, field boundaries and ridges whose two halves no longer line up, shutter ridges blocking valleys, and small elongated basins. Where the fault bends, land is squeezed up into low ridges or pulled apart into sag ponds and small basins. None of these are large-scale constructional landforms.

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