Plate Boundaries

1
Plate Boundaries

• colllisional

2
How collision builds mtns processes are labeled
on diagram. The topography of a mtn is build from
a combination of physical (faulting and folding
pushing the surface UP) and isostatic (the crust
rises because it gets thicker and can float
higher in the mantle)
3
Generalization of the internal structure of mtns.
The region colored mdm-grey-brown (labeled
continental crust) will be metamorphosed from
the regional heat and pressure generated from the
collision. Thrust faults flank the metamorphic
belt. The diagram does not show the presence of
volcanic rocks within the mtn
4
the sequence of steps associated with accretion
of terranes, or microplates. The terrane
basically plugs up the subduction zone,
forcing it to move backward.
5
Another way in which terranes accrete to
larger plates. Again, the terrane, made of more
buoyant material, wont subduct, so the site of
subduction moves. This sequence
also illustrates an example of which
plate subducts? At first, its the ocean
crust-bearing plate of on the right, but later
it is the ocean-bearing plate on the left. The
controlling factor? The more dense plate will
subduct.
6
Basins where thick piles of sediment can
accumulate are also characteristic of mtns.
Immediate adjacent to the Canadian Rockies, for
example, is a basin that has tens of km of
sediment in it, accumulated from erosion of the
mtn as it is uplifted. How does a basin subside,
or sink, in a regime of uplift?
7
Basins are sites of sediment accumulation because
they are depressions that form from fault
movement the crust sinks as a result of the
weight of the extra pile of crust pushed on
to it. This basin then accumulates sediment,
whose weight causes further sinking, or
subsidence.
8
Geologic record of orogeny

• Instrusive (granite plutons, dikes) and extrusive
  (ash and flows) volcanic rocks
• Regional metamorphism
• Abundant faulting, especially thrust faults
• Basins with thick accumulations of sediment
  sandstones are lithic arenites and record
  deposition in non-marine environments, such as
  rivers, lakes, deltas.
• Abundant unconformities reflecting the times of
  vertical movement

9

• What does the geologic record of orogeny look
  like?
• There will be mtns, or if erosion has been very
  severe, the
• remains of the innards of mtns folded,
  metamorphic and
• volcanic rocks.
• 2. Abundant folds and thrust faults
• Stratigraphy that records the
• types of basins and sediments
• that infilled them
• 4. Lots of unconformities!

Pluton intrusion
Closure of ocean basin
The stratigraphic record of the Taconic
Orogeny in eVt wMA
New ocean seds volcanics
10
Isostacy why portions of the Earths crust are
at different elevations
A block of ice floats in water. Why does part of
it stick above Water level?
Why not?
The pressure along the dashed line Is everywhere
the same. This is called The compensation depth.
The pressure at PB must pressure at PA. The
block will rise to equilibrate pressures
11
Isostacy, continued
We can determine the pressure that is pushing the
block upin other words, how much does it have to
rise to come into equilibrium?
The block of ice will be at equilibrium when PA
PB PA is a function of the density of the
block, the gravitational acceleration downward,
and its thickness, or PA (rho ice)(g)(thick) PB
is a function of density of the water,
the gravitational accel downward, and its
thickness, or PB (rho water)(g)(thick)
12
Let ZT total thickness. Let ZU the upper part
(above water level). Let ZL the lower part
(below water level).
Then -- ZT ZU ZL if ZT 100m, Zu x
ZL100-x PA ?ice g ZT
(.917)(9.81)(100)) PB ?water g ZL (1)
(9.81)(100-x) PA PB (.917)(9.81)(100)
(1)(9.81)(100-x) x 8.3m So ZL / ZT ?ice /
?water and ZU / ZT (?water ?ice ) / ?water
If the density of ice .917 g/cm3 The density of
water 1g/cm3 grav const - 9.81 m/sec2 and the
thickness Lt100m
In other words, 92 of the ice floats below
water level
13
This is a preview of Airy isostasy Which says,
for example, that mountains are higher than
plains because they have roots. Or, continents
are higher than ocean basins because continental
crust is thicker than oceanic crust.
Sample problem if continental crust is 40km
thick, how high would the Earths surface extend
above ground (how high a mountain would be)
This is solved the same way you approach the
iceberg. What is different is the density of the
iceberg, which becomes the density of cont.
crust the density of water becomes the density
of the mantle