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We live on top of Earth's
crust, a thin shell representing
less than 1% of the total
volume of our planet.
Yet, its shape and
relief can tell us a lot
about the rest of our planet.
The crust is composed
of igneous, sedimentary,
and metamorphic rocks that
lie on top of a denser mantle
composed of peridotite.
The crust and mantle
are differentiated
by their chemical composition.
Since we will be talking
about mechanical properties
of these rocks, let's
look at the distinction
based on their response
to stress, lithosphere
and asthenosphere.
The lithosphere is formed by the
crust and the uppermost mantle.
It is the rigid outer
layer of the earth.
The lithosphere is
segmented by fault zones
into tectonic plates
which float on top
of the asthenosphere,
the hotter and weaker
part of the upper
mantle that mechanically
decouples the lithosphere
from the underlying convecting
mantle.
The lithosphere can deform in
many different ways, depending
on the pressure,
temperature, loading rate,
and minerals involved.
The lithosphere may
deform elastically,
it may fracture during
an earthquake, or deform
viciously, leaving intricate
patterns in the rocks
for us to analyze.
The asthenosphere, however,
deforms exclusively
by viscous flow.
How do we know all this?
It is not an easy task, since we
cannot really sample the inside
of the earth.
The deepest humans
have managed to drill
is 12.2 kilometers deep.
The Kola Borehole only got
to 0.2% of the distance
to the center of the
Earth and made it just
to a third of the
continental crust that
forms the Baltic Shield.
We have never managed
to drill all the way
to the uppermost mantle,
much less so all the way down
to the asthenosphere.
Most of our understanding of
the deep structure of the earth,
hence, relies on remote sensing
using seismic, electromagnetic,
and gravimetric observations.
From these measurements,
we can infer the presence
of discontinuities in the
rigidity, conductivity,
and density, and
make educated guesses
about their cause based
on geological observations
and laboratory measurements.
We know the
lithosphere can deform
both in the brittle
and viscous manner
because we see examples of
these deformation styles
in outcrops exhumed
from a range of depths.
We infer that the
asthenosphere deforms
by viscous flow because of the
high pressures and temperatures
occurring at the
decoupling depths.
When will the rocks fracture
and when will they flow?
Why do earthquakes occur
at certain depth intervals?
Why do we observe plate
tectonics only on earth and not
on other planets in
our solar system?
To start answering
these questions,
we need to understand
the mechanical properties
of the rocks involved, as well
as the external factors that
may affect them.
This module will focus on a
particular piece of the puzzle,
figuring out the strength
of the Earth's lithosphere.
It is not straightforward to
know the mechanical properties
of the lithosphere
and asthenosphere, as
extreme pressures,
temperatures, time,
and length scales are involved.
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