In the articles you just read, the authors assume you know something about faults: how they space classified, what sort of activity they experience, what sense of stress and anxiety they feel, and how to identify them top top a map. Therefore, the is time come step back a small and evaluation some straightforward material about faults and earthquakes.

You are watching: Stress at a divergent plate boundary

A fault is formed in the Earth"s crust together a brittle response to stress. Generally, the movement of the tectonic plates provides the stress, and rocks in ~ the surface break in an answer to this. Faults have actually no particular length scale. If friend whack a hand-sample-sized piece of rock through a hammer, the cracks and also breakages girlfriend make space faults. At the other finish of the spectrum, some plate-boundary faults are thousands the kilometers in length.

Fault categories

The feeling of stress and anxiety determines the kind of fault that forms, and also we generally categorize that sense of tension in three different ways:

compression,tension, andshear.

Handily, these 3 senses of stress additionally correlate through the three varieties of bowl boundaries.

Compressive anxiety happens at convergent plate boundaries where two plates move toward each other.Tensional stress and anxiety happens in ~ divergent plate boundaries where 2 plates are relocating away from each other.Shear tension is skilled at transform borders where two plates space sliding previous each other.

Source: Cross ar by José F. Vigil native This Dynamic Planet—a wall surface map developed jointly by the U.S. Geology Survey, the Smithsonian Institution, and also the U.S. Naval study Laboratory.

In terms of faulting, compressive tension produces turning back faults, tensional stress produces common faults, and shear stress produces change faults. *Terminology alert: Geoscientists refer to faults the are formed by shearing together transform faults in the ocean, and also as strike-slip faults ~ above continents. Otherwise, these two species of faults room basically the exact same thing. Check out the sketches listed below to view a cartoon of what every of these fault varieties look choose in cross-section.

Eliza"s nifty sketches

Here we have a an easy cross-section consists of three rock layers: brown, pink, and granite. You have the right to tell it"s a cross-section due to the fact that I drew a small tree (Bob Ross-style!) and also a couple of birds and also the sun.


Now we"ll apply some tensional stress and anxiety to this terrain. Tension has the result of pulling and also elongating. If this product were ductile, it would certainly stretch and get thinner, yet we are managing brittle rocks here, so rather they will certainly break. The method this typically happens is by forming a error at part angle to the bedding. Then the whole package of rocks slides follow me this fault. The kind of fault formed here is dubbed a normal fault. This terminology come from miners in Germany that noticed that many of the faults whereby they were working were of this nature, so they dubbed them "normal," an interpretation typical.


As you have the right to see, the error has had the impact of dropping the block on the appropriate with respect to the block on the left. If you saw something choose this in the field, you"d be able to tell how much balance out there was on the fault by measuring exactly how much the layers had actually moved across the fault.

If we instead use compressive stress, this has actually the effect of squeezing and also shortening the terrain. A error will form that watch an horrible lot like the typical fault in the ahead example, but the movement on this fault is in the contrary direction. This error is called a reverse fault due to the fact that it is the "reverse," meaning opposite, the normal. Reverse faults have tendency to type scarps--a scarp is the piece of rock that has actually been thrust up greater than the initial surface level.


The third typical fault type is the strike-slip fault. Strike-slip faults are unique from the previous two due to the fact that they don"t involve vertical motion. They type via shear stress. These room not as easy to acknowledge in cross-section unless there has actually been so much activity on the fault the there are totally different rock varieties on either next of the fault. Most strike-slip faults are close come vertical through respect come the bedding.

See in the computer animation below exactly how the miscellaneous fault varieties move. Computer animation is silent and also comes indigenous IRIS.

Each of these three types of faults is marked in a standard way on a geologic map. I"ve sketched those icons below.

A common fault is generally shown by a line representing the error trace with a little perpendicular heat to present the direction that the block that has actually slid down. Occasionally two parallel currently are drawn to represent plates relocating apart instead.A reverse fault is a line with teeth on it. The teeth are drawn on the next of the overriding block. At a subduction zone key boundary, the teeth room on the upper plate.A strike-slip error is drawn as a line, normally (but not always) through a half-arrow on each side to show which direction the 2 sides of the fault room moving. The example below shows a left-lateral fault.

Fault price ID check!

Can you recognize the type of faulting arising at each plate boundary in the map below? examine your prize here. (and a captioned version).

Based on a map prepared by the U.S. Geology Survey.
Source: Oregon State university

Applying what you know

Have another look at figure 1 indigenous de Boer et al., 2001 (reproduced below). What type of faulting is being illustrated on the map? can you photo in three dimensions how the lithosphere is relocating in that map? Think about it and also compare your idea to my lay out (and a captioned version).

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Source: de Boer, J. Z., Hale, J. R., & Chanton, J. (2001). New evidence for the geological origins of the old Delphic oracle (Greece). Geology, 29(8), pp. 707-710.