Download Key topics today: How do we know about the Earth`s interior structure?

Solid-earth geology:
Plate tectonics, earthquakes, and volcanoes
• Explain shape and structure of the
ocean basins
• Explain cycle of oceanic opening and
closing
• Explain many aspects of basic marine
geology
• Understand occurrence of dramatic,
sometimes destructive events like
quakes and volcanoes
Earth’s layering: By composition
Key topics today:
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Structure of the earth
Plate tectonics: Evidence and basic theory
Relate to marine geology and ocean basins
Rates of plate motions
How do we know about the
Earth’s interior structure?
• Drilling?
• Seismic waves (earthquakes)
• Other observations
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Layering of the outer part of the earth: by
physical properties
Lithosphere and asthenosphere
• Lithosphere: the rigid upper part (crust and
upper mantle)
• Asthenosphere: the squishy part just below
lithosphere
• (below these are lower mantle and core)
Moving on…
• Structure of the earth
• Plate tectonics:
– Evidence for continental drift and seafloor
spreading
– The basic theory
– Relate to marine geologic patterns and
phenomena (e.g. earthquakes, volcanoes)
– Plate motion
Continental drift
• Proposed in 1912 by Alfred Wegener
• Suggested that continents had oringially
formed a single land mass, Pangaea, and had
split apart and “drifted” to current positions
• Based on simple geological and
geographical observations (3 examples)
• Lacked a believable mechanism for
explaining how this could happen
• He died in 1936, with few believers left
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Evidence 1: fit of continents:
Evidence 2: Distribution of fossils
Evidence 3: Geological formations
From continental drift to seafloor spreading
• Mountains of similar age and composition
on either side of the Atlantic
• New technologies in the 1950s-1960’s
allow better ocean measurements:
– Depth
– Heat flow from the bottom
– Magnetic orientation of rocks
– Ages of rocks
• New picture of marine geology… Aha!
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Mid-Ocean ridges
Ocean evidence for seafloor spreading
• Noted initially by Harry Hess
• Bathymetry: Ridges look like seams
between continents
• Ridges are hot
• Ridges are young
• Ridges have magnetic reversals
preserved in their minerals
Heezen and Tharp
Magnetic
anomalies
Lava magnetization
• Ocean crust, like most
rocks, has magnetic
properties that depend
on alignment of
magnetic grains
– Set when lava cools to
align with prevailing
magnetic field
• Magnetic anomalies are
symmetric about the
ridge axis
Magnetic minerals in lava respond to prevailing magnetic field.
Orientation indicates both field direction and latitude
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Magnetic
anomalies
preserved in
seafloor
Magnetic
stratigraphy
• Reversals have
common pattern at
all ridges
• Can date lavas to
determine ages (K/Ar)
• Once known at one
site, can extrapolate
to other sites
• Basis for
magnetostratgraphy
Seafloor spreading gives new life to
the idea of continental drift
• Oceans play an active role!
• Continents carried passively as part of
rigid plates
• Ridges as “spreading centers” where
new crust is formed
• Deep trenches: regions where crust is
destroyed
Theory of plate tectonics emerges from
this synthesis (CD + SFS)
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Plate tectonic theory
Rigid lithospheric plates make up the
Earth’s surface
• Earth surface consists of rigid lithospheric plates
• They move as a result of heating and flow in the
softer asthenosphere, below
• Their interactions at plate boundaries result in
earthquakes and volcanoes
• Plate tectonics remakes the surface of the Earth!
– Continents expanded and split apart
– Ocean basins created and swallowed
• It explains diverse geological observations
• It is a theory that is constantly being tested by new
observations
Predictions of plate tectonics
Age of oldest sediments overlying basalt ocean crust
• Ocean sediments should be younger and
thinner near ridges
• Crust should cool and shrink as it moves
away from ridges, deepening ocean
• Specific ideas about where certain kinds
of EQ and volcanoes should be found
Red = young (<20 million yr); blue is oldest (>130 million yr)
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Predictions of plate tectonics
Thermal subsidence
• Ocean sediments should be younger and
thinner near ridges
• Crust should cool and shrink as it moves
away from ridges, deepening ocean
• Specific ideas about where certain kinds
of EQ and volcanoes should be found
• Ridges stand higher than surrounding basins
• (What makes a ridge a ridge!)
Predictions of plate tectonics
• Ocean sediments should be younger and
thinner near ridges
• Crust should cool and shrink as it moves
away from ridges, deepening ocean
• Specific ideas about where certain kinds
of EQ and volcanoes should be found
Where do we find earthquakes?
Shallow
Medium
Deep
What about DEEP earthquakes?
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Where are the worlds active volcanoes?
Plate boundaries - where the action is
• Convergent: 3 types
– (type of crust determines
how plates interact)
– Ocean-continent
– Ocean-ocean
– Continent-continent
• Divergent
• Transform
Convergent 1: ocean-continent
Cotopaxi Ecuador
Ocean-continent convergence and
explosive volcanism
W. coast of
S. America
• Ocean crust subducted beneath continental (why?)
• Deep earthquakes occur along the sinking slab of
ocean crust
• Trench forms along subduction zone, offshore
• Explosive volcanoes form along continent’s edge
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Convergent 2: Ocean-ocean
Convergent 3: Continent-continent
Tibetan
Plateau
Aleutian
trench
Marianas
trench
• Ocean crust is subducted beneath ocean crust
• Trench forms along subduction zone
• Island arc of volcanoes forms over subducting
slab; these are not as explosive as continental
volcanoes
Annapurna north face (Nepal Himalaya)
continent-continent convergence
• Continental crust buckles and deforms, but
subduction is difficult since continental crust is thick
and less dense
• Uplift produces high mountains (e.g. Himalaya)
• No volcanism
Divergent: mid-ocean ridge
Ridge
Lith.
Asth.
Plume
•New crust forms at center: hot, expanded, young
•Shallow earthquakes
•“Mid-ocean ridge volcanism” is not explosive
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Earthquakes at divergent boundaries
Example divergent boundary: mid-Atlantic rift
Formation of
Atlantic from
Pangaea
Transform
fault
Divergent: can also be on a continent
•Continental crust is
breaking apart and
stretching
•Shallow quakes and
volcanism that can be
explosive
•New crust will form that
has the composition of
ocean crust (basalt)
•Eventually, if motion
continues, ocean will fill
in the rift
Kilimanjaro - highest mountain in Africa
Formed by divergent plate motion and hotspots - E. African rift
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Red Sea = divergent junction
Transform
junction
• Plates sliding past
each other
• Shallow quakes but no
volcanism
• Crust is neither created
nor destroyed
• Example: San Andreas
fault
Plate mechanics
USGS map of global
bathymetry/topography
• Summary of key features
• What drives plate motion – Ridge push and slab pull
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