3.4: Geophysical Phenomena
- Page ID
- 33716
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Most earthquakes are associated with plate boundaries. Examine this map showing more than a century of epicenters to get an immediate sense of this pattern:
Figure \(\PageIndex{1}\): GigaMacro image taken from https://opengeology.org/historicalge...ate-tectonics/
Earthquakes are caused by the sudden slip of two bodies of rock past one another, after tectonic stresses have accumulated to the point that they overwhelm the resistance created by friction. Most (90-95%) of the places where rock is forced into the uncomfortable position of scraping over other rock are located at plate boundaries. The exceptions (5-10%) are at sites where the crust is being stressed for other reasons: excessive loading due to sedimentation, intrusion of magma, isostatic adjustments, or artificially induced seismicity due to elevated pore pressure.
Volcanoes
Similarly, most volcanoes on this planet are associated in some way with plate boundaries [map]. There are four major options here:
- Subduction of oceanic lithosphere under continental lithosphere (example: Andes Mountains)
- Subduction of oceanic lithosphere under oceanic lithosphere (example: the Philippines or Aleutian Islands in Alaska)
- Divergence (rifting) of continental lithosphere (example: East African Rift)
- Divergence (seafloor spreading) of oceanic lithosphere (example: Mid-Atlantic Ridge)
But volcanoes can also form in non-plate boundary settings. When we find eruptions piercing plates in locations far from their edges, many geologists invoke hot spots or mantle plumes (areas where heat is concentrated in the mantle). The idea here is that, independent of plate motions, the mantle has isolated places where rising warm material moves upward, and in so doing partially melts. The melt makes its way through the overlying lithosphere and erupts. This can occur in a couple of contexts:
- Hot spot volcanism through oceanic lithosphere (example: Hawaii)
- Hot spot volcanism through continental lithosphere (example: Yellowstone)