which are the metamorphic equivalents of granites and the predominant
Archean rock types (Fig. 25).Their color derives from chlorite, a greenish, mica-
like mineral. The existence of greenstone belts is evidence that plate tectonics
(the interaction of crustal plates) might have operated as early as the Archean,
some 2.7 billion years ago, with small tectonic plates clashing with each other
as much as 4 billion years ago.The best-known greenstone belt is the Swaziland
sequence in the Barberton Mountain Land of southeastern Africa. It is more
than 3 billion years old and attains a thickness of nearly 12 miles.
Caught in the Archean greenstone belts are ophiolites, from the Greek
word ophismeaning “serpent.” They are slices of ocean floor shoved up onto
the continents by drifting plates and are as much as 3.6 billion years old. Pil-
low lavas (Fig. 26) are tubular bodies of basalt extruded undersea. They also
appear in the greenstone belts, signifying that the volcanic eruptions took
place on the ocean floor.Thus, these deposits are among the best evidence that
Figure 25A gr anite
pegmatite dike in
Precambrian gneiss in
Bear Creek Canyon,
Jefferson County,
Colorado.
(Photo by J. R. Stacy,
courtesy USGS)ARCHEAN ALGAE