Plate boundaries II
extension
Increasing heat
causes uplift
and fault
development,
which in turn
creates steepsided valleys
bounded by
faults. A variety
of non-marine
environments
are created here.
Extrusion of basalt
along faults floods
the valleys with
magma. Many
volcanoes of the
shield variety. Nonmarine 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.
The continued spreading of the new ocean continues. Fully marine,
deep ocean sediment (clays and very fine-grained limestone) lies on
top of basalt
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 difference…remember isostacy?
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……
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 Earth’s crust is a series of blocks floating in the
upper mantle
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 = 7.9+15.5+x= 28; x=4.6km
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.
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….
A sequence of rocks that
record the rifting of a
continent.
Overlying 100’s meterst of
basalt is over 1km of
sediment recording fluvial
environments (blue and
yellow-green) overlain by
km’s of sandy-shales
representing continental
shelf environment (rusty red).
Not shown
here is the rock that overlies
the rusty-red shelf sediment;
km’s of dolostone representing
shallow ocean floor
environment.
Total thickness of sequence?
~6km.
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)
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
Rifting continues….
With continued rifting, more sediment is
laid down on the crust…it 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.
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