the oldest known rocks of North America are the 2.5-billion-year-old gran-
ites of the Canadian Shield.
The shields are associated with greenstone belts (Fig. 24). These are a
mixture of metamorphosed (recrystallized) lava flows and sediments possibly
derived from island arcs (chains of volcanic islands on the edges of deep-sea
trenches) caught between colliding continents. Although no large continents
existed during this time, the foundations upon which they formed were pre-
sent as protocontinents. These small landmasses were separated by marine
basins that accumulated lava and sediments derived mainly from volcanic
rocks that later metamorphosed into greenstone belts.
Greenstone belts occupy the ancient cores of the continents.They span an
area of several hundred square miles, surrounded by immense expanses of gneiss,Figure 24Archean
greenstone belts are the
earliest evidence of plate
tectonics.
Historical Geology
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