Geology Reference
In-Depth Information
sedimentary rocks in Australia with zircons (ZrSiO
4
)
4.4 billion years old indicate that source rocks that old must
have existed.
We know from studies in geology and astronomy that
Earth's rate of rotation is slowing because of tidal effects. That
is, friction caused by the Moon on Earth's oceanic waters,
as well as its landmasses, causes its rate of rotation to slow
very slightly every year. When Earth fi rst formed, it probably
rotated in as little as 10 hours. Another effect of the Earth-
Moon tidal interaction is the recession of the Moon from
Earth at a few centimeters per year. Accordingly, during the
Archean, the view of the Moon would have been spectacular.
Geologists agree that shortly after Earth formed, it was
exceedingly hot and volcanism was widespread. However,
rather than being a fi ery orb for half a billion years as was
formerly accepted, some geologists now think that Earth
cooled suffi ciently by 4.4 billion years ago for surface waters
to accumulate. They base this conclusion on oxygen-18 to
oxygen-16 ratios in tiny inclusions of oxygen trapped in zir-
con crystals that indicate reactions with surface water.
The first crust formed as upwelling mantle currents
of mafic magma disrupted the surface, and numerous
subduction zones developed to form the first island arcs
(
Canada, a large part of Greenland, the Adirondack Moun-
tains of New York, and parts of the Lake Superior region in
Minnesota, Wisconsin, and Michigan (
Figure 19.5). Over-
all, the Canadian shield is an area of subdued topography,
numerous lakes, and exposed Archean and Proterozoic rocks
thinly covered in places by Pleistocene glacial deposits. The
rocks themselves are plutonic, volcanic, and sedimentary,
and metamorphic equivalents of all of these (see Geo-inSight
on pages 508 and 509).
Actually, the Canadian shield, as well as the adjacent
platform, are made up of numerous units or smaller cratons
that amalgamated along deformation belts during the Paleo-
proterozoic. Absolute ages and structural trends differentiate
these smaller cratons from one another.
Drilling and geophysical evidence indicate that Precam-
brian rocks underlie much of North America, but beyond
the Canadian shield, they are exposed only in areas of deep
erosion and uplift. For instance, Archean and Proterozoic
rocks are present in the deeper parts of the Grand Canyon
of Arizona, as well as in the Appalachian Mountains and the
Rocky Mountains (see Geo-inSight on pages 508 and 509).
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Geologists place the beginning of the Archean Eon at
4.6 billion years ago (Figure 19.2), although the oldest
known rocks, the Acasta Gneiss in Canada, are about 4.0 bil-
lion years old. The end of the Archean Eon and the begin-
ning of the Proterozoic Eon 2.5 billion years ago is arbitrary,
but it does correspond to a time when the style of crustal
evolution changed and when rock assemblages more like
those of the present appeared in abundance.
The Archean Eon alone accounts for 45.6% of all geologic
time, yet we review both its physical and biological history in
just a few pages. One should not assume, however, that it was
an unimportant time in Earth history, though. The geologic
record is more complete for more recent intervals of geologic
time, and because Earth is so active, ancient rocks are more
likely to have been eroded or changed by metamorphism.
In short, older intervals of geologic time are represented by
smaller volumes of rock, especially sedimentary rock, and even
if preserved, they are more diffi cult to fi nd and interpret.
Figure 19.4a). Weathering of the mafic rocks of island
arcs yielded sediments richer in silica, and some of the
magma in the arcs also became more enriched in silica.
Collisions between island arcs eventually formed a few con-
tinental cores as silica-rich materials were metamorphosed
(Figure 19.4b). Larger groups of merged island arcs, or pro-
tocontinents, grew faster by accretion along their margins
than smaller ones did, and eventually the fi rst continental
nuclei or cratons formed (Figure 19.4c).
◗
CONTINENTAL FOUNDATIONS—
Continents are more than simply land areas above sea level.
Indeed, they consist of rocks with an overall composition
similar to granite, and continental crust is thicker and less
dense than oceanic crust, which is made up of basalt and
gabbro. Furthermore, a
shield
consisting of a vast area or
areas of exposed ancient rocks is found on all continents.
Continuing outward from the shields are broad
platforms
of
buried Precambrian rocks that underlie much of each con-
tinent. Collectively, a shield and platform make up a
craton,
which we can think of as a continent's ancient nucleus.
The cratons are the foundations of continents, and
along their margins more continental crust was added,
a process called
continental accretion
, as they evolved to
their present sizes and shapes. Both Archean and Proterozoic
rocks are present in cratons, many of which indicate several
episodes of deformation accompanied by metamorphism,
igneous activity, and mountain building. However, the cra-
tons have experienced remarkably little deformation since
the Precambrian.
In North America, the exposed part of the craton is
the
Canadian shield
, which occupies most of northeastern
Only 22% of Earth's exposed Precambrian crust is Archean,
with the most extensive exposures in Africa and North America
(see Geo-inSight on pages 508 and 509). Archean crust is made
up of many types of rocks, but we characterize most of them
as greenstone belts and granite-gneiss complexes, the latter
being by far the most common.
Granite-gneiss complexes
are composed of rocks as varied as peridotite and sedimen-
tary rocks; but granitic gneiss and granitic plutonic rocks are
the most common, both of which were probably derived from
plutons emplaced in volcanic island arcs (
Figure 19.6c).
Greenstone belts are subordinate, accounting for only 10 per-
cent of Archean rocks, and yet they are important in unravel-
ing some of the complexities of Archean tectonic events.
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