Gray Earth - The Story of Earth - page 83

quake fracturing), and only a few tens of miles thick but hundreds to thousands of miles
wide.Theserigidplatessimplyslideacrosshotter,softermantlerocks.ThePacificOcean's
ring of fire defines one large plate, Antarctica and its surrounding seas another. The North
and South American Plates extend westward from the Mid-Atlantic Ridge all the way to
the Pacific coast ofthe Americas, while the Eurasian Plate extends eastward fromthe Mid-
Atlantic Ridge to the Pacific coast of East Asia. The African Plate, stretching from the
Mid-Atlantic Ridge on the west to the middle of the Indian Ocean on the east, displays
an intriguing aspect of Earth's dynamic surface: the African continent is beginning to split
apart as a new rift valley forms, marked by a string of lakes and active volcanoes, as well
as high altitudes that regularly produce the fastest distance runners in the world. Someday
Africa will become two plates, with a new expanding ocean in between.
As ocean ridges produce new plate material and subduction zones swallow up the old,
the scenario is once again complicated by Euclid: Earth is a sphere. The geometry of
plate growth and subduction on a sphere requires that some plates scrape against each oth-
er along jagged transform fault lines—hence the offset bands in Mason and Raff's fam-
ous magnetic map of the Juan de Fuca Ridge. The violent San Andreas Fault, which has
triggered many memorable California earthquakes, is another such suture. Every day more
stress builds along the fault, as the mighty North American Plate moves in a southeast-
erlydirectionrelativetothemightyPacificPlate.Everydaytheseinexorableplatemotions
bring residents of Los Angeles and San Francisco closer to the next “big one.”
So much for the simple geometry of plate tectonics. What of the epic forces that must
power plate motions? What could cause entire continents to shift, scrape, and collide over
hundreds of millions of years? The answer lies in Earth's inner heat. Earth is hot, while
space is cold. The second law of thermodynamics, a sweeping core concept of the cosmos,
states that heat always flows from hotter to cooler objects—heat must gradually disperse,
must somehow find a way to even out.
Recall the three familiar mechanisms that facilitate the transfer of thermal energy. Every
warm object transfers its heat to the surroundings in the form of infrared radiation; heat
alsomoves,albeitmuchlessefficiently,bydirectcontactor conduction ,andby convection ,
when a fluid mass flows between hotter and cooler regions. Earth must obey the second
lawofthermodynamics.Buthowcanheatmoveefficientlyfromsearingcoretocoolcrust?
Rock and magma impede infrared radiation, while sluggish conduction is not much more
efficient. So convection of softened, taffylike hot mantle rocks is the key.
Rocks at Earth's surface are hard, brittle materials, but deep inside the superheated pres-
sure cooker that is the mantle, rocks soften like butter. Over millions of years, under the
stresses of the deep interior, rocks deform, ooze, and flow. Hotter, more buoyant rocks
gradually rise toward the surface, while cooler, denser rocks sink into the depths. Great
convection cells, each thousands of miles across and hundreds of miles deep, overturn