Geology Reference
In-Depth Information
The craton of a continent is created when multiple volcanic arcs (which I describe in
Chapter 9) crash together to form larger and larger land masses. Evidence for these tec-
tonic processes is found in the Archean rocks of the continental cratons.
Revving up the rock cycle
The craton of each modern continent is composed of parallel formations of granite-
gneiss complexes and greenstone belts. Together these rocks illustrate that all the pro-
cesses observed in the rock cycle today were actively taking place by 4 billion years
ago. Keep reading to find out how scientists have come to such a conclusion.
Granite- gneiss complexes
Geologists have observed on present-day earth how rocks are formed and transformed
by the processes associated with plate tectonics. (I describe these processes in Chapter
10.) The different plate boundaries and processes result in different rock types and char-
acteristics. For example, mantle upwelling at mid-ocean ridges produces dense igneous
rocks such as basalt, and continental uplift and the weathering of existing rock result in
sediment particles being transported and deposited to form sedimentary rocks.
In a few of the continental cratons, geologists have found granite-gneiss complexes that
they have dated to nearly 4 billion years ago. In Chapter 7, I note that granite is an in-
trusive igneous rock and gneiss is a metamorphic rock. For granite to form, there must
be conditions of heat and/or pressure that melt rock into magma (I describe these pro-
cesses in Chapter 10). These conditions are most commonly found at subduction zone
boundaries between two converging plates. Gneiss is formed by the intense compres-
sion and deformation of preexisting rocks, such as might occur with subduction. Thus,
the presence of granite-gneiss complexes is evidence for active tectonic processes, in-
cluding subduction zone features, as early as 4 billion years ago. (See Chapter 10 for
more on tectonic processes and subduction).
Greenstone belts
Another Archean rock formation, called greenstonebelts, provides geologists with evid-
ence of crustal rocks being weathered and forming sedimentary rocks by 2.5 billion
years ago (a process I describe in Chapter 7). Greenstone belts are mostly dense, dark
igneous rocks with minerals such as olivine in them, similar to ocean ridge basalt rocks
today. (The green color of olivine gives these rock formations their name.) Above these
igneous rocks are layers of sedimentary rock. Sedimentary rocks can form only when a
different type of rock (in this case igneous or metamorphic, because no other sediment-
ary rocks existed) is exposed to the atmosphere and is weathered into sediment
particles. The presence of sedimentary rocks on top of igneous rocks in the greenstone
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