Phenomena, Comment and Notes
When a drop of rain carries a particle of dirt off the land and into the sea, there are repercussions from deep within Earth to the nearer reaches of space
- By Stearns A. Morse
- Smithsonian magazine, April 1996, Subscribe
When drops of rain fall, some of them flow downhill, each carrying a bit of dirt. As a result, over time the landscape changes from steep to shallow slopes. The land surface erodes away toward sea level. Like a boat in the water, Earth's continental crust displaces its own weight in the underlying layer of denser mantle rock. As its cargo of eroding soil and rock is thrown overboard, the crust loses weight and rises. To accommodate this change, some hot mantle rock must flow in beneath the continent, just as water flows in under a boat that is rising out of the water as its load is lightened. The only difference is the speed at which it happens: hot rock flows slowly, a finger's width a year.
The mantle rock that flows inward under the thinning continent must come from somewhere and, in turn, be replaced by other hot mantle rock. Where does it come from? We know that fresh mantle rock rises up at the mid-ocean ridges, where tectonic plates pull apart to release hot material coming up from below (Smithsonian, January and February 1975). Some of that mantle rock becomes oceanic crust, adding to the plates' trailing edges as they move apart. And some of it flows underneath the oceanic crust to fill the space being created by that lightening, rising continental crust. All this moving mantle rock is, in turn, replaced by the upward flow of hot rock from deeper in the planet. We face the astonishing fact that raindrops falling on land indirectly cause hot, flowing rock material to rise up from Earth's depths.
Until it nears the surface, the flowing rock we have seen in action is flowing in a plastic way, deforming like steel squeezed between rollers. But when hot rock rises from the interior under ocean ridges, it partly melts into a liquid as the pressure on it lessens. The melted rock travels through pores and cracks until it collects and rises and erupts in submarine volcanoes. The cooling lava, transferring its heat to the water, helps the Sun heat the ocean, powering the wind and producing the rain.
One drop of water; mantle flow; volcanism; rain. Loop closed. We are back at the beginning of the cycle.
Eventually, mantle rock flowing beneath the oceanic crust encounters, pushes and slides past the colder, more-rigid rock material of the continent. Something in that crust breaks, and there is an earthquake, because cold rocks break rather than flow. One drop of water. One Northridge earthquake.
The flowing mantle eventually cools and dives back down into the earth in subduction zones, producing more earthquakes and volcanoes as it does, especially in the "ring of fire" around the Pacific. No ocean floor is older than 180 million years; all of it goes down the tubes sooner or later.
If a piece of tectonic plate is subducted, its place at the surface is taken by new material somewhere else, at an ocean ridge. In North America we ride westward away from the Atlantic Ridge on the North Atlantic Plate; at the Ridge new rock is made from magmas that come up from hot mantle. Meanwhile, the western edge of the Pacific Plate slides down into the mantle. To paraphrase John Donne, no plate is an island, entire of itself. All motions are interrelated. One drop of water may set the whole thing going.
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Related topics: Geology