Geography Reference
In-Depth Information
TheTheoryofPlateTectonics explainswhathasbeenhappeningovertheyears.Hereareafewpoints
from that theory:
Tectonic force has broken up the lithosphere into 13 large pieces, or plates
.
Tectonic force causes the plates either to move apart, collide, or slide by one another.
Mountains, volcanoes, and earthquakes result when plates collide.
Earthquakes may also result when plates slide by each other.
Taken together, these statements constitute much of the Theory of Plate Tectonics.
How fast are the continents drifting?
Not very fast at all. Indeed, the proverbial “snail's pace” is much, much faster. Just as a
hoot, I measured the distances between a couple of pairs of points on opposite sides of
the Atlantic Ocean where the Old and New Worlds once were joined. The average dis-
tance is about 4,000 miles. Geologists say “the big breakup” occurred about 200,000,000
years ago. That means the Atlantic Ocean has widened from zero to 4,000 miles over a
period of 200,000,000 years. I'll spare you my math, but it shows that the Atlantic Ocean
has widened at a rate of about 1.25 inches per year. That, of course, involves two plates
that are moving in opposite directions, so the speed of a single plate is about half the
speed of 1.25 inches per year. (In case you are wondering, if Columbus repeated today
his voyage of discovery, he would have to sail about 210 feet farther than he did in 1492.)
In other words, things move slowly, but consistently. We have good evidence that the
speed of plate movement varies somewhat, but at any rate, the slowest snail would leave
the fastest tectonic plate far behind.
Tectonic force has broken the lithosphere into 13 plates that vary in size. The Pacific Plate, for ex-
ample, covers millions of square miles. In contrast, the Juan de Fuca Plate, which borders the Pacific
Northwestern States of the U.S., is barely visible on the map (as seen in Figure 6-2). Every plate is
either on the move or is being affected by the movement of a neighboring plate. Arrows on the map
indicate the direction in which different plates are moving. As geographers have seen, the Mid-At-
lantic Ridge marks a boundary at which neighboring plates are moving apart. Other spreading sea
floors also are evident on the map. But, if plates are moving apart along some of their boundaries, that
