Passage IV

Earthquakes occur when rocks under stress suddenly fracture and fault, releasing seismic energy from the focus (the location of the rocks' initial fracture). In subduction zones (see figure below), where 2 plates (composed of crust and upper mantle) collide and the edge of 1 plate is forced down into the lower mantle below, some earthquake foci are located at depths of 400 km to 670 km (deep earthquakes).

Scientists have wondered what causes deep earthquakes. Below 400 km depth, rocks are under high pressure and at temperatures greater than 1, 500℃, so they should bend in response to stress rather than break, as they do above 400 km depth. Two scientists discuss possible causes of deep earthquakes.

Scientist 1

Common minerals in plate rocks, such as serpentine, contain water. As the edge of a plate descends into the lower mantle and is heated, these minerals dehydrate. This makes the rocks brittle and easier to fracture. The released water also helps lubricate existing fractures, allowing them to move. Dehydration usually occurs at depths shallower than 400 km. However, when the edge of a plate is forced down into the lower mantle, it may descend so rapidly that it remains much cooler than the surrounding mantle. In that descending plate, minerals retain water in their crystal structure to much greater depths. When they are heated to a point where they dehydrate, somewhere between 400 km and 670 km depth, the rocks become brittle and fracture, releasing seismic energy.

Scientist 2

Another common mineral in plate rocks, called olivine, changes to a denser mineral called spinel when subjected to the high temperatures in the lower mantle. This process usually occurs at mantle depths shallower than 400 km. However, when the edge of a plate is forced down into the lower mantle, it may descend so rapidly that it remains much cooler than the surrounding mantle. This allows olivine to exist well below the depths where it is normally found. At some depth below 400 km, the plate reaches a temperature that allows olivine in the rocks to suddenly change to spinel. This change causes a rapid compaction and fracturing of the rocks, releasing seismic energy.