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Volcanism
Earth’s internal heat
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Drives most geological processes including volcanism and plate tectonics
Leftover heat from its formation 4
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Common component is silica (SiO2) at 45-75% by weight
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2-3% by weight
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The viscosity of magma depends on its silica
and gas content ad temperature
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Formation of Magma
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Decompression Melting – temperature stays the same, but the pressure exerted decreases
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Flux Melting – gaseous substances are added into the hot solid rocks where it react with rocks
and weaken them, causing them to change from a solid state to a liquid
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Heat Transfer Melting – melting of surrounding rocks caused by hot magma bringing additional
heat
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The melting temperature of the rock, called eutectic temperature, is lower than the melting
temperature of its constituent minerals
o When a rock begins to melt, only certain minerals are melted
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Fractionation - If the magma is separated from rock that has not undergone complete melting,
the magma will consist of components that melted at lower temperature and the remaining
solid rock will contain components with higher melting temperature
o The magma that is formed first tends to be richer in silica

Volcano
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Vent that serves as the conduit of lava or molten rock that reaches the surface
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The event when the lava is spewing out of a volcano is called an eruption
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The composition of the lava controls the type of eruption
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The explosive eruption is associated
with lavas having high silica content and is more viscous
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There are different kinds of volcanoes according to their shape:
o Shield volcano – forms a very broad dome with a gentle slope that covers a wide area
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They are generally composed of intermediate to felsic rocks and
they tend to build the large and high volcanic edifice
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It is composed of
cinder with mafic composition
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o Mid-ocean ridges – chain of volcanoes on the ocean floor formed on divergent
boundaries
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Lavas with smooth surfaces are called
Pahoehoe, while those with very rough surfaces are called aa
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Pumice is also
made of volcanic glass but with a frothy texture characterized by lots of open spaces
caused by gas bubbles
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Lapilli is pea- to marble-size fragments of lava
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o The lithified volcanic ash is called tuff
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o Larger non-streamlined chunks of lava or preexisting rocks are called blocks
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A pyroclastic deposit that is dominantly composed of pumice is
called ignimbrite
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There are 1,500 active volcanoes in the world
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”
The explosivity of volcanoes are scaled using the Volcanic Explosivity Index (VEI)
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The scale increases logarithmically
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Tambora
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Ash from eruptions provide nutrients for the soil and lava forms new landforms
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In most cases, but not all, this involves the rock being deeply buried beneath other rocks, where
it is subjected to higher temperatures and pressures than those under which it formed
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Greek: meta = after, morph = form, so metamorphism means the after form
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Protolith is transformed into new metamorphic rocks
Transformation occurs due to processes:
o Recrystallization – no chemical composition change, only physical
o Neo-crystallization – new arrangement of mineral, chemical
o Pressure Solution – subduction, pressure and water, dissolved minerals precipitate
o Plastic Deformation – too much heat and pressure, crystals are elongated

How Metamorphism Occurs
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Heat and Pressure
o Metamorphism occurs at a pressure of about 12 kilobars (depth of 40km)
o Metamorphism occurs at temperature roughly between 200-850 degrees Celsius
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In geology,
however, we restrict diagenetic processes to those which occur at temperatures below 200oC
and pressures below about 300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3
kilobars of pressure (1kb = 100 MPa)
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Rocks can be subjected to these higher temperatures and pressures as they are buried
deeper in the Earth
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The upper limit of metamorphism occurs at the pressure and temperature where melting of the
rock in question begins
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Most metamorphism results from the burial of igneous, sedimentary, or pre-existing
metamorphic to the point where they experience different pressures and temperatures than
those at which they formed
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Although most metamorphism involves
temperatures above 150°C, some metamorphism takes place at temperatures lower than those
at which the parent rock formed
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Metamorphic grade is a
general term for describing the relative temperature and pressure conditions under which
metamorphic rocks form
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Generally characterized by an abundance of hydrous
minerals
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As grade of metamorphism increases, hydrous minerals
become less hydrous, by losing H2O, and non-hydrous minerals become more common
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Occurs in subduction zones
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Occurs in divergent plate boundaries
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Occurs in collision zones and
transform fault boundaries
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o Diagenesis – Sedimentary rock to new rock (but below 12 kilobars of pressure)

Index Minerals
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Geologists also tend to look at specific index minerals within the rocks that are indicative of
different grades of metamorphism
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2
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The upper and lower limits of the ranges are intentionally vague
because these limits depend on a number of different factors, such as the pressure, the amount
of water present, and the overall composition of the rock
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o San Andreas Fault and Anatolian Fault are notable examples of this
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o Fault - A break in rock along which a vertical or horizontal rock movement has occurred
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These lines are often lines of
weakness which allow molten rock to rise up the surface when there is volcanic activity
nearby
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San Andreas Fault, Anatolian Fault
o Normal faults – creates space, two blocks of crust pull apart, stretching the crust into a
valley
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Commonly
found in collision zones
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The different styles of faulting can also combine in a single event, with one
fault moving in both a vertical and strike-slip motion during an earthquake
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In geology, stress is the force per unit area that is placed on a
rock
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o Confining stress – a deeply buried rock is pushed down by the weight of all the material
above it
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Compression is the most common stress at convergent plate boundaries
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Rocks under tension lengthen or break apart
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o

Shear stress – when forces are parallel but moving in opposite directions, the stress is
called shear
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Strain and Deformation
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When stress causes a material to change shape, it has undergone strain or deformation
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A rock’s response to stress depends on the rock type, the surrounding temperature, and
pressure conditions the rock is under, the length of time the rock is under stress, and the type of
stress
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Sudden stress, such as a hit with a hammer, is more likely to make a rock break
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Folding
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Two forces push towards each other from opposite sides, the rock layers will bend into folds
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Folds
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Rocks deforming plastically under compressive stresses crumple into folds
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If the rocks experience more stress, they may undergo more folding or
even fracture
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The rocks dip away from the center of the fold
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▪ When rocks arch upward to form a circular structure, that structure is called a
dome
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The youngest rocks are at the center and the
oldest are at the outside
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o Theory solidified in the 1960s
In plate tectonics, Earth’s outermost layer, or lithosphere—made up of the crust and upper
mantle—is broken into large rocky plates
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o Due to the convection of the asthenosphere and lithosphere, the plates move relative to
each other at different rates, from two to 15 centimeters (one to six inches) per year
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The idea that continents moved over time had been proposed before the 20th century
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Wegener
published two articles about a concept called continental drift in 1912
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The continents we see today
are fragments of that supercontinent
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In addition, South America and Africa looked like they could fit
together like puzzle pieces
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o An American geologist named Harry Hess proposed that these ridges were the result of
molten rock rising from the asthenosphere
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Millions of years later, the crust would
disappear into ocean trenches at places called subduction zones and cycle back into
Earth
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In 1963, a Canadian geologist, John Tuzo Wilson, proposed that volcanic island chains, like the
Hawaiian Islands, are created by fixed “hot spots” in the mantle
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As the plate moves over the hot spot, one volcanic island after another is formed
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Today, the theory is almost
universally accepted
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Plates move around on top
of the mantle like rafts
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Theory of Plate Tectonics – theory that pieces of Earth’s lithosphere are in constant motion,
driven by convective currents in the mantle
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Convection currents in the mantle move the plates as the core heats the slowly flowing
asthenosphere (the elastic/plastic-like part of mantle)

Plate Boundaries and Interactions
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Faults – fractures on Earth’s crust resulting from the movements of the tectonic plates
o Divergent Plate Boundary
▪ Plates move away from each other
▪ Examples are Africa’s Rift Valley and Mid-Ocean Ridges (Eurasia and North
America)
o Convergent Plate Boundary
o Two plates slide toward each other and form either a:
▪ subduction zone – one plate is denser than the other plate
▪ orogenic belt – continental plate crumples and is uplifted
o Oceanic – continental – Andes Mountains, Cascade Range
o Oceanic – oceanic – Marianas Trench
o Continental – continental – orogenic belt (orogenesis) Himalayas and Alps
o Transform Plate Boundary
o Plates slide against each other in opposite direction
o Earthquakes are common
o San Andreas Fault

Relative and Absolute Dating
Earth Dating Methods
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Geologists may use rock stratigraphy to precisely date fossils or rocks based on their placement
strata, or the series of events as manifested in the rock strata

Relative Dating
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3
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5
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7
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Stratigraphy is important in
relative dating because the order of events is recorded or can be found in the order rock
formations
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Law of Original Horizontality
o Layers of sediments are originally deposited horizontally
o Factors affecting this law:
▪ -Erosion
▪ -Earthquakes
▪ -Faults
▪ -Deposition
Unconformities
o Boundary between two layers of non-continuous ages
▪ usually marked by an erosional surface and is often irregular
o Angular Conformity
o Younger sediments rest upon the eroded surface of tilted or folded older rocks,
o Disconformity
o Contact between younger and older beds is marked by a visible, irregular or
uneven erosional surface
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o Nonconformity
o Develops between sedimentary and older igneous or metamorphic rock that has
been exposed to erosion
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Radioactive decay
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Spontaneous breakdown of the nucleus to release matter and energy
Isotopes
➢ two or more atoms that have the same number of protons but different numbers of
neutrons
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• Radiometric dating – uses the half-life of atoms to figure out the age of the rock layers
the atoms are in
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1 x 10^8 years or 2
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The period is the basic unit of geological time in which a single type of rock system is formed
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Two or more eras form eon, the largest division
of geologic time
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Concept:
o Eon
▪ Era
• Period
o Epoch

Precambrian
1
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6 to 3
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Archeozoic Eon (Archean)
▪ 3
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5 billion years ago
▪ “Ancient Life”
▪ First lifeforms evolve
3
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5 billion years ago to 540 MYA
▪ Multi-celled, animals appear, including sponges
▪ Single supercontinent called Rodinia
▪ First multicellular life; colonial algae and soft-bodied invertebrates appear
Phanerozoic Eon
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Paleozoic Era
o Bracketed by two of the most important events in the history of animal life:
▪ Multicellular animals went dramatic diversity explosion
▪ Largest mass extinction of mostly 90% of marine organisms
1
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540 to 500 mya
b
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Ordovician Period
a
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Silurian Period
a
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Devonian Period
a
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“The Age of Fishes”

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5
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360 to 325 mya
6
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325 to 280 mya
7
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280 to 248 mya
b
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o Depleted state was followed by an explosion of new life forms such as dinosaurs,
mammals, birds, and flowering plants
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Triassic Period
a
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Supercontinent Pangea
c
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Small, fast dinosaurs appeared for the first time
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Ichthyosaurs (marine reptiles) swam in the seas
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2
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208 to 146 mya
b
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Hot and Dry, with strong seasonality at first, changing to warm and moist with no
polar ice and vast flooded areas
d
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3
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146 to 65 mya
b
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Continents almost had their modern-day look
c
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Later, sea levels dropped, seasonality increased,
and there were greater extremes in temperature between the poles and the
equator
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Dinosaurs flourish
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The
oldest known ants, snakes, and butterflies arose towards the end of the Mesozoic
era
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o Also called as the age of mammals
o 65
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Tertiary Period (65 – 1
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Paleogene (65 – 24 mya)
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Eocene (58-38 mya)
▪ Mammals abound, rodents appear
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Oligocene (38 – 24 mya)
▪ Starts with a minor extinction (36 mya)
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Grasses common
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Neogene (24 – 1
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Miocene (24 – 5 mya)
▪ More mammals, including horses, dogs and bears
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South American monkeys, apes in southern Europe, Ramapithecus
ii
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8 mya)
▪ First hominids (Australopithecus)
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Megalodon swam
the seas
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Quaternary Period
➢ “The Age of Man”
➢ 1
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Pleistocene (1
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11 mya)
▪ The last Ice Age
▪ The first humans (Homo sapiens) evolve
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A mass extinction of large mammals and many birds
happened about 10,000 years ago, probably caused by the end of the
last Ice Age
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Holocene (11,000 ya to today)
▪ Human civilization



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