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Plates, Continents, Oceans through time.

The Precambrian
Most of the earth's history took place in the Precambrian. In fact, 88% of geologic time is assigned to this
vast subdivision of the geologic time scale. It began with the origin of the planet about 4.5 billion years ago (That
is 4,500 million years ago or 4,500,000,000 years ago). It ended when creatures developed hard parts (such as
shells) about .545 billion years ago.

Despite the vast amount of time involved, geologists know less about the Precambrian than they do about
the paltry 545 million years that followed. There are several reasons for this. (1)Early-formed rocks have been
destroyed or obliterated by tectonic processes as well as by the surficial processes of weathering and erosion.
(2)Precambrian rocks are largely buried beneath younger rocks and therefore inaccessible to geologists' direct
observation. (3)The Precambrian fossil record is very skimpy.

Nevertheless, geologists have groped through the fog of time and pieced together a plausible history of the
Precambrian. Precambrian time is subdivided into two vast eons, the Archean and the Proterozoic. The boundary
between the two is placed at 2.5 b.y. (billion years ago).

Archean Rocks

The oldest rock yet found is a gneiss, metamorphosed from granite, from northern Canada,
radiometrically dated at 4.1 billion years (b.y.). The next oldest rock is from Greenland and is about 3.8 b.y..
Significantly, it was a sedimentary rock before metamorphism. To produce the sedimentary rock there had to have
been weathering and erosion as well as deposition in a marine environment. Thus the atmosphere and ocean must
have been developed on earth by 3.8 b.y.

Precambrian (Archean and Proterozoic) rocks are found in cratons, stable interiors of continents. The
large central area of the United States and Canada between the Rocky Mountains and the Appalachian mountains
is a craton. Most of the craton in the United States is covered by layers of Paleozoic and younger rock, but in much
of Canada, the Precambrian rocks are exposed in a shield. A shield is a large region where the exposed bedrock is
Precambrian igneous and metamorphic rocks. (See P,M,C chapter 20.)

Archean rocks differ somewhat from later rocks. This probably reflects a hotter outer part of the earth.
Plates were thin, plate motion faster. There were probably more plates and more rapid destruction than at present.
Two types of terranes developed, gneiss terranes and greenstone belts. The gneisses are strongly metamorphosed
granites and sedimentary rocks. We interpret them as proto-continents. Greenstone belts are linear bodies of
green rock. They are mildly metamorphosed volcanic and sedimentary rocks. They include pillow basalts and
sediment that are indicative of a deep water environment. They appear to have been rapidly developed oceanic
crust deformed by being squeezed between "islands" of gneisses.

PROTEROZOIC

The Proterozoic began 2.5 b.y. ago. Gneiss terranes merged and continents began to grow. Mountain
building episodes (orogenies) and plate tectonics took place much as they do at present. "Normal" sedimentary
and igneous rocks formed. With the larger, stable continents, rocks would weather and the resulting sediment was
deposited in a variety of environments, including the shallow water of submerged portions of the continents. Thus,
limestone, sandstone and shale became common.
Archean Rocks

PROTEROZOIC

Read textbook (M,P, & C) "Dance of the Continents" in chapter 5. A supercontinent formed from 2.0-1.8
b.y. and broke up by the end of the precambrian,

PALEOZOIC

NORTH AMERICA IN THE PALEOZOIC

By the end of the Precambrian, the supercontinent had split apart. North America was smaller than at
present and isolated from other continents. The break up did not affect what is now Australia, Antarctica, South
America, Africa and India which remained together in the early Paleozoic as a supercontinent called
Gondwanaland.

During the Paleozoic (570-245 m.y.) seas rose and fell several times. When sea level was high, most of
the North American continent was covered by shallow seas. These shallow, warm seas hosted a prolific and
abundant marine fauna. The water was warm because North America was at the equator for much of the
Paleozoic. Sediment deposited by these seas resulted in a relatively thin cover of sedimentary rocks (sandstone,
limestone and shale) over the Precambrian rocks in the continental interior (the craton). What would become
North America's Appalachian mountains began the Paleozoic as sediment collecting in deeper water east of the
craton.

The Continental Interior

Sediment was deposited during the times when sea level was high and the craton covered with shallow
water,. When sea level was low, erosion of the sedimentary rocks would take place. In some parts of the craton,
the sedimentary strata were warped into broad basins or arches. elsewhere the layers remained horizontal. Where
circulation of sea water was restricted, usually due to reefs, evaporites formed.

One sequence of evaporites is found in the Michigan Basin. During the Silurian and Devonian, A barrier
reef surrounded the basin. The shallow water in the basin evaporated, resulting in deposition of rock salt, gypsum
and other evaporites.

Late Paleozoic sedimentation created economically important coal in the east (notably in Pennsylvania,
West Virginia, Tennessee, Kentucky). The coal beds resulted from a cycle of marine and non-marine
sedimentation during the Pennsylvanian period. The coal formed from swamp vegetation that partially decayed and
was buried under additional sediment (sand and clay). There were ten cycles of rising and falling of sea level
associated with the coal deposits. Glaciation in Gondwanaland probably caused the changes in sea level.

During the Permian, there was a major recession of the seas. Most of the sedimentary rocks formed
during this time are terrestrial (most are red beds). An exception was an ocean basin in southwest Texas and New
Mexico in which sedimentation took place. A reef (built by coral and other fauna) developed around the basin (the
El Capitan Reef) and the basin filled with sediment, including evaporites. These were overlain by red beds
(terrestial deposits). The area, known as the Permian Basin, is important for oil production.

The Appalachians

The Appalachian mountain system is the product of three Paleozoic orogenies. (See PM&C, p. 508-509)
An orogeny, an episode of mountain building, is associated with plate convergence. The first orogeny, the
Taconic Orogeny, took place during the Ordovician and Silurian periods. During the Cambrian and early
Ordovician, the eastern margin of North America was a passive continental margin, which is to say that no
subduction or plate activity was affecting it. In the Ordovician, the Proto-Atlantic ocean began to close.
Subduction took place off the coast of the North American craton. Oceanic-oceanic convergence resulted in an
island arc; originally a series of volcanic islands that latert developed into a linear land mass. The compressive
forces of the continental-oceanic convergence warped the crust into a trough between the craton and the arc.
Vigorous erosion of the volcanic arc resulted in thick sediments filling the trough. The resulting sedimentary rocks
became folded and faulted with continuing compression during the orogeny.

The Acadian orogeny was the second mountain building episode to affect the Appalachians. This
occurred during Devonian and Mississippian times. It involved continent-continent plate convergence, much like
the collision between India and Asia that created the present Himalaya. As the Proto-Atlantic ocean closed,
Europe and North America collided. This resulted in a high central land mass with intensely deformed and
metamorphosed rocks and intrusion of granite plutons. On the east and western flanks of the highlands, sediment
filled troughs, first as marine deposits, then these were overlain by younger terrestrial deposits. Sandstone found in
New York state, deposited west of the highlands, is almost identical to the Old Red Sandstone found in Britain.
The highlands were probably mountains as high as today's Rockies. The remains of that mountain belt are the
eastern portion of the present Appalachians and, across the Atlantic, the Caledonian mountains of Britain and
Scandinavia.

The final orogeny, the Alleghenian orogeny, was also due to a continental-continental collision. It took
place during Pennsylvanian and Permian times. The African portion of Gondwanaland collided with North
America in the Southern part of the Appalachians. This resulted in extensive folding and thrust faulting of
previously deposited sedimentary rocks. The Ouachita Mountains of Texas, Arkansas and Oklahoma were also
produced by this collision. After the Appalachian orogeny, the Appalachians were slowly eroded and quietly
uplifted, ultimately carved into the present low mountains.

The convergence of Africa, Europe and North America was only part of the gathering of all of the
continents to form the supecontinent Pangea during the Permian.

The Western Margin

During Cambrian time, the western margin of North America was passive. A thick sequence of sediment,
derived from the craton, built a continental shelf and continental slope. The margin became active in Ordovician
time. An island arc developed as a result of subduction. Volcanically-derived sediment was added to the
sedimentary sequence. Plate convergence resulted in orogenic activity beginning in the Devonian and lasting into
the Permian period. Rocks derived from the island arc were thrust over craton-derived rocks to the east.
Further east (Colorado, Wyoming, etc.), the Ancestral Rockies formed during Mississippian and
Pennsylvanian times. They are unusual because the deformation took place in the craton. The activity probably
was not related to plate motion but involved uplift along steep reverse faults.

MESOZOIC

The Mesozoic, often referred to as the "Age of Dinosaurs", began about 245-250 million years ago and
ended around 65 million years ago. The three periods and their ages are:

MESOZOIC PLATES

Pangea, the supercontinent that formed toward the end of the Paleozoic, began breaking up in the
Triassic. (See PM&C, fig. 19.2, p. 461.) The break-up was initially into two supercontinents, more or less like the
two early Paleozoic supercontinents, Gondwanaland and Laurasia. Subsequently, South America split from
Africa. North America and Europe separated as the modern Atlantic Ocean basin developed through divergence.
By Cretaceous time, the fragmentation of the continents was at its greatest extent ever. The existence of smaller
continents resulted in milder weather worldwide. Cretaceous climate was warmer than at present.

The rifting of Europe and North America began around 200 million years ago. The Triassic rift valleys
bounded by normal faults along the eastern margin of North America are products of the beginning of the breakup.
During the Jurassic, the North Atlantic Ocean was merely a narrow sea, characterized by the deposition of
evaporites.

India deserves mention as the greatest traveler from the break-up. India split from its position in
Gondwanaland and began its journey northward. Eventually (in the Cenozoic) it would crash into Asia.

In contrast to the passive margin along eastern North America, the western margin became active.
Western North America had complicated subduction. Orogeny (mountain building), including extensive igneous
activity resulted in much of the rocks and mountains of the North American Cordillera.

In California, the Sierra Nevada batholith formed as a product of oceanic-continental collision. The very
thick sequence of sedimentary and volcanic rocks were intruded by a huge volume of granitic magma. Blobs of
magma rose from depth to coalesce into what is now exposed as the Sierra Nevada Batholith. This took place
during approximately 100 million years during the Jurassic and Cretaceous period. The western coastline for
North America during this time was approximately where Sacramento is today. The trench, marking the top of the
subduction of the oceanic crust, was in the Coast Ranges. The complex of rocks that were deposited by erosion of
the Sierra Nevada and pulled down the subduction zone along with the Pacific oceanic crust. These rocks would
eventually (in the Cenozoic) be uplifted from their great depth to become the present-day Coast Ranges.

In the Cretaceous, a vast inland sea extended from the Gulf of Mexico through much of the Midwest and
part of the Rocky Mountains, up to Alaska. The sea became expelled as mountain building migrated eastward to
include uplift of the Rocky Mountains.

CENOZOIC

The Cenozoic Era began 65 m.y. and is divided into two periods, the Tertiary and the Quaternary. These,
in turn, are divided into epochs. We do not need to be concerned with the epochs of the Tertiary. However, as we
live in the Quaternary, you should know that its two epochs are the Pleistocene (the epoch that included the glacial
ages) and the Recent (more formally known as the Holocene).

ROCKS AND PLATES

In Europe, the Alps were created as the ocean basin closed when southern Europe, North Africa and
northern Europe collided. Thick layers of marine-deposited limestone and other sedimentary rocks were tightly
folded and reverse-faulted. The intense compression squeezed the original, horizontal layers covering a 300 mile-
wide basin into the 100 mile-wide Alpine mountain belt. (See PM&C p. 506, fig. 20.11.)

The Himalaya began forming when India collided with Asia. They are still growing as the continent-
continent convergence continues.

In western North America, plate activity got complicated. The mid-oceanic ridge (East Pacific Rise)
collided with the trench offshore from western-moving North America. The San Andreas Fault (a transform fault)
and the Gulf of California (with a diverging boundary) were created in the process. Reorganization of plate motion
produced (1) upwarping of the Rocky Mountains, (2) stretching of the Basin and Range (creating the normal-fault
bounded mountain ranges), and (3) creation of the Cascade Range- a new chain of volcanoes. (The andesitic,
composite volcanoes were a product of subduction in the still ongoing oceanic-continental convergence. Mt. St.
Helens' eruption is but the latest tiny bit of activity.)

Central America swung into place late in the Tertiary, joining North and South America. This had
interesting effects on life. Numerous species of animals became extinct (particularly in South America) because of
exposure to new predators.