Geology Reference
In-Depth Information
upwelling basaltic magma to form new oceanic crust. Notice
in Figure 23.3 that the Arabian plate is moving north, so it,
too, causes some of the deformation taking place from the
Mediterranean through India.
In the meantime, the North and South American plates
continued their westerly movement as the Atlantic Ocean
basin widened. Subduction zones bounded both continents
on their western margins, but the situation changed in North
America as it moved over the northerly extension of the East
Pacific Rise, and it now has a transform plate boundary, a
topic we discuss more fully in a later section.
and
Figure 23.1a-c) record the presence of a shallow sea
in the continental interior, terrestrial deposition, lava fl ows,
and volcanism on a huge scale in the Pacific Northwest.
Exposure of Cenozoic rocks in the east are limited, except
for Ice Age deposits, but there are a few exceptions, includ-
ing Maryland, where Miocene- and Pliocene-age rocks are
present (Figure 23.1d).
Although we emphasize the evolution of mammals in
this chapter, equally important events took place among
plants. The angiosperms continued to dominate land-plant
communities, as they had during the Cretaceous, now consti-
tuting more than 90% of all land plants. Birds evolved during
the Jurassic, but the families now common appeared during
the Paleogene and Neogene, reached their maximum diver-
sity during the Pleistocene, and have declined slightly since
then. Following the extinctions at the end of the Mesozoic,
marine invertebrates diversifi ed, giving rise to the present-
day familiar fauna of the seas.
In this chapter, we also examine the primates—in par-
ticular, the origin and evolution of humans. We should point
out that new discoveries and new techniques for scientific
analyses are leading to new hypotheses about human ances-
try. In previous editions of this topic, we stated that the ear-
liest fossil evidence of hominids (humans and their extinct
ancestors) is from 4.4-million-year-old rocks in Africa. Since
then, discoveries have pushed the age back to almost 7 million
years. And by the time you read this chapter, new discoveries
may have changed some conclusions.
◗
Cenozoic orogenic activity took place in two major zones
or belts: the
Alpine
-
Himalayan orogenic belt
and the
circum
-
Pacifi c orogenic belt
(Figure 23.3). Both belts are made up of
smaller segments known as
orogens
, each of which shows
the features of an orogeny—deformation, metamorphism,
emplacement of plutons, and thickening of Earth's crust.
The
Alpine-Himalayan orogenic belt
extends eastward
from Spain through the Mediterranean region, the Middle
East and India, and on into Southeast Asia (Figure 23.3).
Ongoing deformation, volcanism, and seismicity remind us
that it remains active. Remember that during the Mesozoic,
the Tethys Sea separated much of Gondwana from Eurasia,
but this sea closed during the Cenozoic when the African
plate collided with the huge landmasses to the north.
During the
Alpine orogeny
, deformation occurred in a
linear zone extending from Spain eastward through Greece
and Turkey. Mountain building in this region yielded the
Pyrenees between Spain and France, the Apennines of Italy,
as well as the Alps of mainland Europe. Plate convergence
also produced an almost totally isolated sea in the Mediter-
ranean basin where Late Miocene deposition in an arid envi-
ronment accounts for evaporite deposits up to 2 km thick.
The collision of the African plate with Eurasia also
accounts for the origin of the Atlas Mountains of north-
west Africa, and further to the east, Africa continues to force
oceanic lithosphere northward beneath Greece and Turkey.
Erupting volcanoes in Italy and Greece and seismic activity
throughout the entire region indicate that southern Europe
and the Middle East remain geologically active.
The
Himalayan orogen
also resulted from plate conver-
gence, but in this case, two continental plates collided, giv-
ing rise to the loftiest mountains on Earth. During the Early
Cretaceous, India broke away from Gondwana and moved
north, and as it did so, oceanic lithosphere was consumed at
a subduction zone along the southern margin of Asia. The
descending plate partially melted, yielding magma that rose
to form granitic plutons and a chain of volcanoes in what
is now Tibet. The Indian plate eventually collided with and
became sutured to Asia, resulting in deformation and uplift
of the Himalayan orogen (see Figure 10.22).
AN OVERVIEW
The progressive fragmentation of Pangaea accounts for the
present distribution of Earth's landmasses. Moving plates
also directly affect the biosphere because the geographic lo-
cations of continents profoundly infl uence the atmosphere
and hydrosphere.
Notice from
◗
Figure 23.2 that as the Americas sepa-
rated from Europe and Africa, the Atlantic Ocean basin
opened, fi rst in the south and later in the north. Spreading
ridges such as the Mid-Atlantic Ridge and East Pacifi c Rise
were established, along which new oceanic crust formed
and continues to form. However, the age of the oceanic
crust in the Pacifi c is very asymmetric, because much of the
crust in the eastern Pacifi c Ocean basin has been subducted
beneath the westerly moving North and South American
Plates (see Figure 2.13).
Rifting and the separation of landmasses is not restricted
to the Triassic. In fact, Neogene rifting began in East Africa,
the Red Sea, and the Gulf of Aden (
Figure 23.3). Rifting
in East Africa is in its early stages, because the continen-
tal crust has not yet stretched and thinned enough for new
oceanic crust to form from below. In the Red Sea, rifting
and the Late Pliocene origin of oceanic crust followed vast
eruptions of basalt, and in the Gulf of Aden, Earth's crust
had stretched and thinned enough by Late Miocene time for
◗
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