Chapter 13 – A biography of Earth
The Earth has a history.

The estimated age of the Earth is about 4.6 Ga (billion years).
After a mountain range has eroded away, the metamorphosed rocks and plutons formed
within it can still be recognized; these relics define the position of the orogen that once
was.
Geologists use observational data to study Earth history.
 Identifying orogenies or mountain building events. Orogeny causes igneous
activity, folding and faulting deformation, and metamorphism. Ancient mountain belts that
have been eroded away can be identified by the features left behind and then dated using
radiometric techniques.
 Foreland sedimentary basins are created when materials from erosion of the
nearby mountains creates a sediment basin. The sequence of sedimentary strata in the
basin can help identify periods of uplift and erosion.
 Recognizing the growth of continents: pieces of the continental crust have different
ages depending upon when rocks were formed from magma or by metamorphosis. Rock
types making up the crust can indicate the tectonic environment in which they were
formed.
 Recognizing past environments: The environment at a particular location can change
with time and environment controls both the type of sediment deposited and the type of
organisms that lived there. For example, limestone containing coral fossils probably
developed in a shallow sea.
 Recognizing past changes in the relative sea level: changes in sea level are
reflected in changes in sedimentary deposition. For example, marine limestone
overlying an alluvial-fan conglomerate means the sea level rose at that location.
Correlating sedimentary successions at the global level helps scientists determine when
sea levels rose and fell in the past.
 Recognizing past positions of continents: using apparent polar-wander paths
can reveal the latitude of a continent in the past. Marine magnetic anomalies can tell us
how an ocean basin has gotten smaller or larger over time (< 200 million years or
Jurassic time period because of subduction of the ocean basin). Comparing fossils
found in different global locations can tell scientists whether the locations were adjacent
to one another in the past.
 Recognizing past climates: looking at fossils and rock types that formed at given
latitudes. For example, if warm-environment fossils are found near the poles, than the
environment there must have been different at that time. The ratios of isotopes for
certain elements in fossil shells can indicate historical temperature changes. Trapped air
bubbles in ancient ice cores can indicate past changes in atmospheric CO2 and other
gases.
 Recognizing life evolution: Progressive changes in the fossil line in a sequence of
strata represent changes in the assemblage of organisms inhabiting Earth through time.
An example of polar wandering using two hypothetical continents. The continents collide
at 300 Ma and move together as a supercontinent until 200 Ma and then rift apart, each
having a piece of the orogeny. The apparent polar wander paths are shown in figure 2. The
paths are separate when the continents are separate, combined when the continents
combine, then separate again after rifting.
Marine magnetic anomalies can fix the position of continents over time. Visually removing the
strips of sea floor can show us where the continents were positioned in the past. Notice in figure
2 that Europe and Africa are progressively moving away from a fixed North American plate.
The Hadean Earth.
 There is no direct record of the first 600 million years of Earth’s history because is
was too hot for rocks to form. Thus, the radiometric clock that is used to date igneous
and metamorphic rocks had not yet started “ticking”.
 No ocean because the surface of the Earth far exceeded the boiling point of water
 Scientists hypothesized that the Earth’s surface consisted of a magma ocean of
ultramafic melts that rose from the mantle and flooded its surface.
 Earth’s atmosphere = nitrogen, ammonia, methane, water, carbon monoxide, carbon
dioxide and sulfur dioxide.
 After about 4 Ga (billion year ago), the planet had cooled enough for the magma
ocean to freeze and the surface to become segmented into small, rigid plates, similar to
modern oceanic lithosphere plates.
 Subduction of these plates caused volcanic activity that yielded mafic-tointermediate composition magmas that rose, froze, and became too buoyant to be
subducted. These rigid pieces eventually collided with each other to become the first
continental crust (protocontinents). The oldest rocks on Earth came from these blocks
(4.03-2.5 Ga). This marks the start of the Archean eon.
 By 2.7 Ga, long-lived blocks of continental crust called cratons had formed. By the
end of the Archaen eon, 80% of continental crust had formed.
 Archean cratons contained gneiss (from collisional metamorphism); greenstone
(relicts of ocean crust metamorphism or basalts from rifting); granite (formed from
partial melting of continental crusts or hot spots); graywacke (marine derived
sediments of sand and clay); and chert (formed by marine silica precipitation).
The Archean eon saw the first record of life on Earth.
 3.8 Ga – traces of organic C in rocks (perhaps prokaryotic cells and
cyanobacteria)
 3.4 ga - fossils resembling cyanobacteria found in Australian rocks
 3.2 ga - Stromatolites, formed by layer upon layer of bacteria, were found in S.
Africa.
 2.5 Ga – 545 Ma – called the Proterozoic Eon
 Tectonic processes slowed and oceanic plates grew larger from
collision with other land masses
 1.8 Ga - collisions between Archean cratons, accretions of new
volcanic arcs and hot spot volcanoes created larger cratons like the
Canadian Shield, a low-lying region of exposed Precambrian rocks
forming a cratonic platform or continental platform in North America
 The Canadian shield consists of several Archean blocks sutured
together along huge collisional belts.
Most of the U.S. craton consists of crust formed when a series of volcanic island arcs and
slivers of continental crust accreted to the southern margin of the Canadian Shield between
1.8 Ga and 1.6 Ga. An orogen formed in this way is called an accretionary orogen.
Successive collisions around 1 Ga created Rodinia, a supercontinent. The last
collision forming Rodinia created a large collisional orogen called the
Grenville orogen. Metamorphic rocks from this collision crop out in eastern
Canada and in the Appalachian mountains. Between 800 and 600 Ma Rodinia
broke up and the future Antarctica, India, and Australia split away and collided
with the future South America, forming a new continent called Pannotia.
Oxygen in the atmosphere continued to increase due to the increased abundance
of photosynthetic organisms. During photosynthesis, plants take up gases like
carbon dioxide (CO2), methane (CH4), and ammonia (NH3) and give off oxygen
as a by-product. Oxygen in the atmosphere was important in the expansion and
diversification of life on earth because oxygen-dependent (aerobic) metabolism
produces far more energy than anaerobic (no oxygen) metabolism. Atmospheric
oxygen also provides the raw material to produce atmospheric ozone, an
important atmospheric gas that shields life on the Earth’s surface from
dangerous ultraviolet radiation from sunlight. Scientists deduced the
transformation from an oxygen-poor to an oxygen-rich atmosphere on Earth
from banded-iron formations (BIF) found in sedimentary strata. BIF strata are
composed of alternating layers of hematite or magnetite and chert, minerals that
precipitated out of seawater when oxygen became more abundant in the
atmosphere.
The Earth is not static; the map of the Earth is constantly changing due to plate tectonics.
The geology of the Earth is constantly changing and life on Earth is constantly changing
and adapting along with it. The progressive change of life on Earth is called evolution.
Around 1.5 Ga, eukaryotic cells appeared in the fossil record. Eukaryotic cells have a
more complex structure and are capable of building multicellular organisms. They also
photosynthesize more efficiently, so the oxygen content of the atmosphere continued to
increase. By 670 Ma, complex, shell-less organisms called the Ediacaron fauna were
inhabiting the sea. This begins the Phanerozoic eon, which encompasses the last 545 Ma
of Earth’s history. During this period, life continued to diversify and the terrestrial
continents achieved the formation we know today.
 1 Ga – Rodinia formed
 Grenville orogeny
 800-600 Ma – Pannotia formed
 When Pannotia broke up, it yielded:
 Laurentia (North America and Greenland)
 Gondwana (South America, Africa, Antarctica, India, Australia)
 Baltica (Europe)
 Siberia
Epicontinental seas flooded the continents. The only dry land in Laurentia was in the
Hudson Bay region. Depositional sediments from these seas are visible in the strata
towards the bottom of the Grand Canyon.
 Ordovician period, a collision between the eastern edge of N.A. and a volcanic arc
created the Taconic orogeny, the first stage in the development of the Appalachian
mountains.
 Over this time period, a clear record of evolution was left in the fossil record by hardshelled organisms.
 Tremendous diversification of life in the Cambrian period, perhaps due to the
continental breakup and the subsequent availability of different ecological niches.
 Proliferation of hard-shelled organisms hints that predators may have been evolving
along with the defenses of their prey.
 First vertebrate animals (jawless fish), and the ubiquitous cockroach made their
appearance.
Permian period
 gymnosperms (i.e. conifers) and cycads were widespread during the Permian
period.
 amphibians and reptiles started to appear in the fossil record. Reptiles were an
innovative introduction because they lay hard-shelled eggs, enabling these animals to
reproduce without returning to water.
 The late Paleozoic era ended with two mass extinction events when over 90% of
marine species disappeared.
A volcanic island arc collided with the eastern margin of North America to cause the
Taconic orogeny (top figure). Following erosion of the orogen created by the Taconic
orogeny, a smaller continent called Avalon collided with the coast to cause the Acadian
orogeny (figure 2).
The figure shows the Avalon
continent (including presentday Ireland and England)
heading towards collision
with the eastern coast of
North America. This is called
the Acadian orogeny.
The Late Devonian period. Most
of the U.S. is covered by an
inland sea.
Pangaea as seen from the South pole (left) The figure on the right shows different geological
features on the North American continent. Eastern N.A. collided with northwestern Africa and the
present-day Gulf Coast region squashed against the northern margin of South America. The very
forceful Alleghenian orogeny produced uplifts in the Midwest and in the present-day Rocky
Mountains. Sediments eroded from the ancestral Rockies form the red sandstone of the region.
The Mesozoic Era (245-65 Ma)
Triassic-Jurassic periods, 245-145 Ma. Rifting along the North American/African
boundary started the breakup of Pangaea. A very shallow North Atlantic Ocean was
created. Remnants of this shallow sea underlie much of the Gulf Coast region with very
thick evaporite deposits. Early in the Mesozoic, Earth was cool with low sea levels so the
interior of Pangaea was above water. Triassic strata of the SW United States consist of red
fluvial (river-lain) shales and sandstones visible in the Petrified Forest National
Monument and petrified sand dunes of the early Jurassic period appear at Zion National
Park. By the middle Jurassic, sea levels began to rise and covered the Rocky Mountain
region. On the west coast, subduction creating volcanic island arcs and caused them,
along with microcontinents and hot-spot volcanoes, to collide with North America, thus
increasing the size of the continent. These collisions caused the Sonoma orogeny and the
Nevadan orogeny.
The breakup of
Pangaea started
during the late
Triassic period
(245-208 Ma) of the
Mesozoic Era
Subduction tectonics accreted
volcanic island arcs, hot-spot
volcanoes, and slivers of continent
onto the west coast of North
America.
The late Mesozoic Era. Warmer,
greenhouse conditions caused the sea
level to rise and flood most of the
continents, producing the thick chalk
deposits in Europe and layers of
limestone and sandstone in the western
interior of North America. An interior
seaway stretched from the Gulf of
Mexico to the Arctic Ocean. Along
western North America, the Sierran
volcanic arc was active. Although the
above-ground portion of this arc has
since eroded away, the granitic
batholiths of the Sierra Nevada
mountains remain.. East of the arc the
Sevier orogeny produced a fold-thrust
belt. The Laramide orogen was, a
consequence of the convergent
boundary on the west coast and the
compressional forces it created. This
orogeny raised the present-day Rocky
Mountains.
By the Late
Cretaceous, the
Atlantic Ocean
had formed and
India moved
northward to
collide with Asia.
The breakup of Pangaea led to many mid-ocean ridges. Sea-floor spreading was faster
during the Cretaceous period and more young, ocean crust was produced which occupied
more space in the ocean, displacing larger amounts of seawater. This caused sea levels to
rise and flood the continents.
Also contributing to the displacement of sea water were large submarine plateaus
caused by hot-spot volcanoes. High levels of volcanic activity may also have influenced the
climate by increasing the amount of CO2 in the atmosphere, and therefore contributing to
climate warming and the subsequent melting of ice sheets > sea level rise.
Angiosperms (flowering plants) appeared in the late Mesozoic as well as teleost,
or modern, fish. Dinosaurs reached their peak at this time and mammal species diversified.
The K-T boundary event identified an abrupt change in fossil assemblages. It
represented a sudden extinction of most species on Earth. The dinosaurs vanished along
with 90% of plankton species and 75% of plant species. Scientists now believe that a huge
10-km wide bolide (an extraterrestrial object like a meteorite, comet or asteroid) collided
with the Earth at the site of the present-day Yucatan peninsula in Mexico. Evidence for this
collision exists in a global-wide clay layer containing iridium, an element only found in
extraterrestrial objects. Tiny glass spheres formed from the instantaneous freezing of melt,
wood ash, and shocked quartz (quartz subject to intense pressure) was also found in this
layer. The wood ash came from the forests that were set ablaze from the impact, which also
may have generated 2-km high tsunamis that inundated the continental shores. The amount
of ash and debris injected into the air probably caused darkness to reign for months and
therefore shut down photosynthesis, thus breaking the most important link in the food chain
for terrestrial animals. The area of the impact is marked by a now-buried crater called the
Chicxulub crater, 100 km wide x 16 km deep. Radiometric dating gives the date of crater
formation at 65 Ma, the time of the K-T extinction.
Different types of strata are
deposited when sea levels rise and
fall. Patterned blocks indicate
times of deposition
(transgressions). Unconformities
indicate times of erosion or no
deposition (regression). Deposition
occurs near the coasts first, then
towards the interior.
This interpretation of the
stratigraphic record was done by
Larry Sloss (1962).
History of sea level rise and fall. These seem to coincide with periods of climate
warming and cooling (ice ages).
The Cenozoic Era – to present. The final stages of the breakup of Pangaea separated
Australia from Antarctica and Greenland from North America and formed the North Sea
between Britain and Europe. The Atlantic Ocean continued to grow and the Americas
moved west, away from Europe and Africa. The two main continental orogenic systems on
the Earth today are the Alpine-Himalayan system formed when pieces of Gondwana
(Africa, India, and Australia) collided with Asia. The Cordilleran and Andean systems are
caused by subduction zones along the eastern Pacific edge.
The western margin of North America changed from a subduction zone (convergent plate
boundary) into a transform plate boundary when the Farallon Plate was subducted.
Subduction today only occurs where the Juan de Fuca Plate continues to subduct (yielding
the Cascade mountains in Washington state). The Basin and Range province developed as
the San Andreas Fault developed.
The Basin and Range province is a result of rifting in an east-west direction. This rift
has caused the region to stretch to twice its original width.. The province contains long
narrow mountain ranges separated by flat, sediment-filled basins, reflecting normal
faulting from stretching and movement on the faults creating depressions.
The Basin & Range province is a rift zone. The Colorado plateau is a craton bounded by
the Rio Grande Rift to the east and the Basin & Range to the west.
The Pleistocene ice age saw
the advance and retreat of
glaciers at least 20 times.
During a time of sea level fall,
an exposed land bridge across
the Bering Strait provided
migration routes for animals
and people from Asia into
North America and a partial
land bridge from southeast
Asia to Australia, also
allowing migration. Around
11,000 yrs ago, the climate
warmed and we are in that
period today.
A summary chart
of Earth’s history as
interpreted by
geologists. Our
ancestral humans
first appeared
during the Cenozoic
era. Homo sapiens
(our species)
diverged from
Homo
neanderthalensis
(Neanderthals)
about 500,000 years
ago. Modern
humans appeared
about 150,000 yrs
ago.
Unless the Earth collides with another a bolide, the end of the Earth will probably
occur 5 billion years from now. It’s expected that the Sun will form a red giant
(supernova) that will essentially evaporate the Earth due to our proximity to the
Sun and the immense heat generated by the supernova explosion.
End of Chapter 13