365 Days of Climate Awareness 23 - Crustal Processes

 

365 Days of Climate Awareness

23 - Crustal Processes

The plates which compose the surface of the earth are not still. Rather, they are in motion, either grinding alongside, sliding underneath, or smashing directly into one another. Oceanic crust is generated and destroyed continually, while continental crust, less dense than the surrounding oceanic crust, is thought to remain floating on top of the mantle and never greatly increase or diminish in size. ("Scum floating on the surface of water," is how one professor of mine described the continents.)

Tectonic plates.

Oceanic plate is generated at spreading ridges, which occur around the world. A few examples are the Mid-Atlantic Ridge and the East Pacific Rise, long, segmented chains of volcanoes produce lava which cools and forms fresh seafloor crust. The ocean spreads out in both directions from these ridges (typical rates of spread are 1 to 10 cm per year). The motion of the oceanic crust, and of the continents, is driven by the convective motion of the mantle underneath.

Plates can either slide past one another, as with the San Andreas Fault in California, or one plate can slide beneath another, as occurs on the southern coast of Alaska, along the Aleutians and through Japan, or two continents can collide and form mountains, as where the Indian Plate's collision with Eurasia is (still!) forming the Himalaya.

Oceanic crust, being denser than continental, will dive beneath the plate it collides with, and descend into the mantle, where it will ultimately re-melt. When ocean plate collides with ocean plate, one plate rides on top, and the other passes underneath. The result of an ocean plate collision is a chain of volcanoes due to the friction between the plates.

Crustal tectonic processes.

When the ocean plate is passing beneath a continent, volcanoes like the Cascades and Andes result. When one ocean plate is descending beneath another, a chain of volcanic islands like the Aleutians results. The coasts surrounding the Pacific Ocean are commonly called the "Ring of Fire" because ocean plate is spreading in both directions and passing underneath the surrounding plates, leading to a roughly circular chain of volcanoes several thousand miles long.

Isolated volcanoes, occurring over mantle "hot spots", also occur. Hawaii is one such. Iceland is simultaneously part of the Mid-Atlantic spreading ridge, and also a hotspot. Yellowstone National Park is located on a hotspot, and is known as a "supervolcano", located over two magma domes (mostly solid at the moment), the shallower of which is about 55 miles long, 25 miles wide, and about 3 to 10 miles beneath the surface. Supervolcanoes have been responsible for major climatic changes in earth's history, which we will review in future posts.

Tomorrow: Tectonics 3: Supercontinents.

Be well!