Download Earth History 870:035

Earth History 870:035
• Course goal:
– To describe the history of Earth and its inhabitants
• Most of Earth’s history predates humanity, so
it has not been observed
– Therefore, we will emphasize how scientists
interpret evidence for reconstructing Earth history
Earth History, Ch. 1
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Major Events in Earth’s History
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Origin of the Earth
Formation of continents
Origin of life (simple cells)
Origin of multicellular life
Invasion of land by plants and animals
Formation of major mountain chains
Assembly and break-up of supercontinent Pangaea
Five great mass extinctions
Age of dinosaurs
Origin of mankind
Earth History, Ch. 1
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Earth is a system of highly
integrated, interdependant processes
geosphere
atmosphere
biosphere
Earth History, Ch. 1
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Earth’s physical history and biologic history
must be studied in tandem because they are so
intricately linked: e.g.,
• The global flora influences atmospheric O2 and
CO2 levels, and thus climate
• Plants and animals play a role in both mechanical
and chemical weathering of rocks, and in the
deposition of some sedimentary rocks
• Physical factors influence biogeography,
evolution, and extinction
Earth History, Ch. 1
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How do we study Earth’s history?
• Uniformitarianism: Conceived by James
Hutton, popularized by Charles Lyell: The
principle that laws of nature are constant
– “Actualism”— using knowledge of
modern processes to interpret ancient
products by applying uniformitarianism
– “the present is the key to the past”
Earth History, Ch. 1
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Uniformitarianism/Actualism illustrated
Modern process:
current ripples
Ancient product:
inferred current ripples
Earth History, Ch. 1
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Uniformitarianism/Actualism
(some limitations)
• Although physical laws may be constant through
time, rates sometimes vary!
– e.g., Earth’s rotation is slowing down
• Some geologic processes are not observable
– e.g., those that occur deep within the Earth
• Some conditions change through time
– e.g., increase in atmospheric O2
• Some geologic features are created by
catastrophic events that seldom occur
– e.g., huge meteorite impacts separated by millions of
years
Earth History, Ch. 1
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Rock Review
“The pages of Earth History”
• Mineral = a naturally occurring inorganic
solid with a particular chemical composition
and characteristic internal structure
• Rock = an aggregate of one or more
minerals
Earth History, Ch. 1
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Rock types
• Igneous—those rocks that have crystallized from
molten magma
• Sedimentary—those rocks that consist of
cemented or compacted particles
• Metamorphic—those rocks that form as a result
of the alteration of pre-existing rocks by heat
and/or pressure
Earth History, Ch. 1
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Igneous
• Intrusive igneous rocks solidified at depth
within the Earth and usually have coarse
crystals (e.g., gabbro)
• Extrusive igneous rocks solidified at the
Earth’s surface and have microscopic
crystals (e.g., basalt)
Earth History, Ch. 1
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Intrusive igneous rock (granite)
Earth History, Ch. 1
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Sedimentary
• Sandstone and Shale are formed by cementation
and/or compaction of grains from pre-existing
rocks
• Limestone is formed by cementation of
biologically secreted shell material
• Evaporites and other chemical sedimentary rocks
are formed by inorganic precipitation of minerals
from water
• Sedimentary rocks are usually layered or bedded.
An individual layer is called a stratum (plural =
strata)
Earth History, Ch. 1
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Sedimentary rocks (strata)
Earth History, Ch. 1
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Metamorphic
• Pre-existing rocks can be metamorphosed
when subjected to heat and/or pressure.
This alteration takes place without the rock
becoming molten.
• Any rock can be metamorphosed
– Igneousmetamorphic
– Metamorphicmetamorphic
– Sedimentarymetamorphic
Earth History, Ch. 1
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Metamorphic rocks (foliated)
Earth History, Ch. 1
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Classification of rock bodies
• A formation is a distinctive, mappable
body of rock
• A formation may be subdivided into
members
• Two or more formations may be united into
a group
Earth History, Ch. 1
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The rock cycle
• Any rock can
be transformed
into another
rock of the
same kind or
either of the
other two kinds
Earth History, Ch. 1
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Steno’s laws for sedimentary rocks
• Principle of superposition = in an undisturbed
succession of rock, the oldest strata are at the
bottom and progressively higher strata are
progressively younger
• Principle of original horizontality = strata are
horizontal or nearly horizontal at the time of
deposition
• Principle of original lateral continuity = strata
are continuous or nearly continuous within the
confines of the basin of deposition
Earth History, Ch. 1
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Steno’s Laws
Superposition
Original horizontality
Original lateral continuity
Earth History, Ch. 1
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Ordering of geologic events
• Principle of intrusion states that an intrusive
igneous rock body is younger than the rock it has
intruded
• Principle of components states that when
fragments of one rock body are found within
another, the second rock body is younger than the
first
• Principle of cross-cutting relationships state that
when one geologic feature cuts another, the
feature being cut is older than the feature cutting
across
Earth History, Ch. 1
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Unconformities
• An unconformity is a surface of erosion or nondeposition that separates rock bodies
– Angular: rocks beneath the unconformity surface are
tilted relative to those above it
– Disconformity: rocks beneath and above the
unconformity surface are parallel, but the surface itself
is irregular
– Nonconformity: the unconformity surface separates
crystalline rocks below from sedimentary rocks above
Earth History, Ch. 1
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Unconformities
Earth History, Ch. 1
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Angular
unconformity
Earth History, Ch. 1
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Dating the geologic record
• Relative ages of rocks can be determined by
fossil succession
– Each interval of geologic history was
characterized by an assemblage of plants and
animals that was distinct from the assemblages
of older and younger intervals
• Absolute ages can be determined by the
orderly decay of natural radioactive
materials
Earth History, Ch. 1
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The geologic time scale
• Earth’s 4.6 billion year history is divided
into three great eons
Phanerozoic Eon
Abundant, multicellular organisms
0.54 billion
Proterozoic Eon
Mostly unicellular organisms
2.5 billion
Organization of the Earth’s internal
layering
Archean Eon
4.6 billion
Earth History, Ch. 1
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Structure of the Earth
Layers are distinguished
on the basis of seismic velocity
contrasts, which reflect
differences in density
Seismic boundary between
crust and mantle is
known as the “Moho”
Earth History, Ch. 1
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Lithosphere and Asthenosphere
Lithosphere = the outer rigid layer consisting
of the crust and uppermost mantle
crust
Moho
Asthenosphere = the underlying plastic layer
Consisting of partially molten mantle
mantle
Lithospheric plates move relative to
one another, driven by thermal convection
within the asthenosphere. The movement
of plates is called “plate tectonics.”
Earth History, Ch. 1
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Crust/mantle vs.
lithosphere/asthenosphere
Earth History, Ch. 1
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Plate tectonics
• Study of the interaction among lithospheric
plates (processes and products)
• Earth’s surface is made up of about a dozen
large plates in constant motion relative to
one another
• Convection is the driving force behind
plate tectonics
Earth History, Ch. 1
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Lithospheric plates
Earth History, Ch. 1
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Convection
heat from
interior
Earth History, Ch. 1
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Interactions between the water cycle
and the rock cycle
• The water cycle and the rock cycle are
intricately linked
– Water plays a key role in weathering and
erosion of rocks
– Water plays a key role in deposition of
sediments
– Water is subducted along with lithospheric
slabs along subduction zones, then recycled
into magma
Earth History, Ch. 1
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heat from sun
Water cycle
Earth History, Ch. 1
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Directional changes in Earth’s history
• Evolution—the irreversible progression
over time of life forms from simplest cell to
complex organism
• Decrease through time in Earth’s internal
heat
• Increase through time of atmospheric O2
Earth History, Ch. 1
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