Ophiolites are the best evidence for ancient plate motions. They are sec-
tions of oceanic crust that peeled off during plate collisions and plastered onto
the continents. Blueschists (Fig.29),which are metamorphosed rocks of sub-
ducted ocean crust, also shoved up onto the continents.This resulted in a linear
formation of greenish volcanic rocks along with light-colored masses of granite
andgneiss, which are common igneous and metamorphic rocks that comprise
the bulk of the continents. In addition, many ophiolites also contain ore-bear-
ing rocks, which are important mineral resources throughout the world.
The cratons were numerous. They ranged in size from about one-fifth
the area of present-day North America to smaller than the state of Texas.The
cratons were also highly mobile and moved about freely on the molten rocks
ofthe upper mantle called the asthenosphere. They were independent mini-
continents that periodically collided with and rebounded off each other. The
collisions crumpled the leading edges of the cratons, forming small parallel
mountain ranges perhaps only a few hundred feet high.
All cratons eventually coalesced into a single large landmass several thou-
sands of miles wide called a supercontinent, which existed as early as 3 billion
years ago. At the points where the cratons collided, mountain ranges pushed
upward. The sutures joining the landmasses are still visible today as cores of
ancient mountains called orogens, from the Greek oros,meaning “mountain.”
The original cratons formed within the first 1.5 billion years of Earth’s exis-
tence and totaled only about a tenth of the present landmass.
Figure 28The cratons
that comprise the
continents.ARCHEAN ALGAE