Plate boundaries II

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Plate boundaries II

• extension

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Increasing heat causes uplift and
fault development, which in turn creates
steep- sided valleys bounded by faults. A
variety of non-marine environments are created
here.
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Extrusion of basalt along faults floods the
valleys with magma. Many volcanoes of the shield
variety. Non- marine sediments continue to
accumulate. With continued basalt eruption
the density of the crust in this
region increases, and subsidence, or sinking,
occurs. Flooding by ocean water creates a small
ocean. The type of sediment becomes marine
in the basin, flanked by shelves.
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The continued spreading of the new ocean
continues. Fully marine, deep ocean sediment
(clays and very fine-grained limestone) lies on
top of basalt
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Why do the oceans sit so topographically low when
they are so hot and buoyant?

• The topographic difference between the continents
  and oceans is due to two factors the thickness
  difference and the density differenceremember
  isostacy?

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Not only is the thickness of the continents
different from the ocean basins, so is the
composition, and that controls density. For
example
We could ask the question, how much lower will
the gabbro sit in the mantle than the granite?
What do we need to know in order to evaluate this
question? Thickness of granite continent
28km Thickness of gabbro ocean crust
7.9km Density granite 2.67g/cm3 Density gabbro
2.99g/cm3 Gravit.const 9.81m/sec2
AND
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Note that the granite and gabbro are floating in
the mantle.so we need to consider the thickness
and density of mantle holding up the thinner
crust of gabbro above the compensation level.
Is better thought of as

The Earths crust is a series of blocks floating
in the upper mantle
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Where Zc thickness cont crust (28 km) Zo
thickness of ocean crust (7.8km) Zm thickness
of upper mantle (x) ?H difference in elevation
From total P of cont total P of ocean, or
(density ocean)(thick ocean) (density mantle)
(thick mantle) (density cont) (thick
cont) Then (2.99) (7.9) (3.3)(Zm) 2.67(28) Zm
15.5km So ?H 7.915.5x 28 x4.6km In
other words, the oceans sit 4.6km below the
continents, just because of density differences.
This is an example of Pratt isostacy elevation
differences are the result of density differences.
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Meanwhile, back in rifted margins.

• What is the geologic record of a rifted
  continental margin?
• Ex, the geology of western vermont 600 million
  years ago.

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A sequence of rocks that record the rifting of a
continent. Overlying 100s meterst of basalt is
over 1km of sediment recording
fluvial environments (blue and yellow-green)
overlain by kms of sandy-shales representing
continental shelf environment (rusty red). Not
shown here is the rock that overlies the
rusty-red shelf sediment kms of dolostone
representing shallow ocean floor
environment. Total thickness of sequence? 6km.
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The geologic record of rifting

• Extrusive volcanics
• Normal faults
• Non-marine sediments, over time covered by marine
  sediments
• Very thick sequences of sediments and volcanics
  because of the subsidence, or sinking that
  happens (due to isostacy)

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How do you make what was continental crust more
dense ocean crust?
Here is a map showing a rift forming in
continental crust
Here is a cross section through the crust,
showing the formation of normal faults and the
intrusion of magma from the mantle along these
faults
The fault-bounded basin floods with water,
sediment is deposited, along with
continued extrusion of mafic volcanic rocks
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Rifting continues.
With continued rifting, more sediment is laid
down on the crustit causes the crust to sink
because of its weight. The ongoing extrusion of
mafic magma continues to cause the lower
density granite to be replaced by higher density
basalt and gabbro. This causes further
subsidence. Crust that was once only low density
granite is now a mix of granite and gabbro. The
higher density causes the crust to sink lower
into the mantle.
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To summarize

• Continental crust converts to oceanic crust
  during rifting, as more and more mafic gabbro and
  basalt is extruded and intruded into the granite.
  Over time it volumetrically overwhelms the
  amount of granite in the rift, and you have
  primarily just mafic igneous crust.
• This crust subsides (Pratt isostacy) as a
  function of its density, which means that it
  floats lower in the mantle, topographically lower
  than continental crust.
• By the time of step 2, the basin has also flooded
  with ocean water, and is called an ocean basin