Download Unit 5 - Structure and Composition of the Earth

Structure and
Composition of the Earth
• The solid earth can be divided into:
_______________
• (water, organic substances and skeletal
matter) - solid and liquid - and includes
all forms of life (e.g. plants and animals)
and their products (e.g. skeletons) both
on land and in the sea
_______________
• (fresh and salt water, snow and
ice) - mainly liquid, some solid includes all forms of water
Internal structure of the
Earth
• Which includes:
– _______________ (normal silicate rocks
such as granite and basalt) - solid
– _______________ (ferromagnesium-rich
silicate rocks) - solid
– _______________ (iron-nickel alloy) - liquid
upper part and solid lower part
Crust
• The crust makes up only 0.5 % of the
Earth's total mass and can be
subdivided into two main parts:
– _______________
– _______________
• Both differ in thickness, density and
composition.
• Although oceanic crust covers
approximately 61 % of the Earth's
surface, it only comprises some 30 %
of the crustal mass, as the
continental crust is much thicker.
Continental Crust:
• From the surface of the Earth down
to 30 km - 50 km.
• _______________appears to be
stratified (layered) and becomes
denser with depth.
Oceanic Crust:
• From the surface of the Earth down to
10 km - 12 km
The Moho
• The _______________ between the
crust and mantle is known as the
Mohorovicic Discontinuity (Moho).
Mantle
• The _______________ is thought to be
primarily composed of rocks rich in
magnesium and iron.
• We can subdivide it into an
– upper mantle
– transition zone
– lower mantle
Upper Mantle
• Measured from the base of the crust
down to 400 km
• 10 % of the Earth's total mass
Transition Zone
• 400 km - 1000 km below the Earth's
surface
• 17 % of the Earth's total mass
Lower Mantle
• 1000 km - 2900 km below the Earth's
surface
• 41 % of the Earth's total mass
The Core
• The _______________ is marked as that
point within the Earth where S-waves
cannot penetrate.
• It is believed to be composed primarily of a
nickel-iron alloy, consisting of a
_______________ outer zone, and a
_______________inner zone.
• It is also marked by an abrupt increase in
_______________.
• Outer core
– 2900 km - 5000 km below the Earth's
surface
– 30 % of the Earth's total mass
• Inner core
– 5000 km - 6370 km below the Earth's
surface
– 2 % of the Earth's total mass
Cross Section of
the Earth
• Not to scale
History of Plate Tectonics
• Plate tectonic theory had its beginnings
in 1915 when Alfred Wegener proposed
his theory of "_______________."
• He was one of the first to realize that the
Earth's surface has changed through
time, and that continents that are
separated now may have been joined
together at one point in the past.
• Paleontologists had also found that
there were fossils of similar species
found on continents that are now
separated by great geographic distance.
• Wegener's ideas were very
controversial because he didn't have an
explanation for why the continents
moved, just that there was
observational evidence that they had.
• At the time, many geologists believed
that the features of the Earth were the
result of the Earth going through
cycles of heating and cooling, which
causes expansion and contraction of
the land masses.
• Although Wegener's "continental drift"
theory was later disproved, it was one of
the first times that the idea of crustal
movement had been introduced to the
scientific community; and it laid the
groundwork for the development of
modern plate tectonics.
• _______________deals with the study of
the motion and deformation of the Earth's
crust.
Plates
• Plate tectonic theory tries to account for the
movement of the crust throughout geologic
time.
• _______________ are rigid bodies of rock that
essentially float atop a region of partial melt called
the _______________.
• The plates comprise the _______________,
composed of the crust (which is the rigid,
outermost layer of the Earth) and the solid portion
of the upper mantle.
Plate Boundaries
• _______________are found at the edge
of the _______________ plates and are
of three types:
– Convergent
– Divergent
– Conservative
• The three boundaries are characterized
by their distinct _______________.
Divergent Boundary
• The first sort of plate
boundary is called a
divergent boundary, or
spreading center.
• At these boundaries, two
plates move away from one
another.
• As the two move apart, midocean ridges are created as
magma from the mantle
upwells through a crack in
the oceanic crust and cools.
• This, in turn, causes the growth of
oceanic crust on either side of the
vents.
• As the plates continue to move, and
more crust is formed, the ocean basin
expands and a ridge system is
created.
• Divergent boundaries are responsible
in part for driving the motion of the
plates.
• As you can imagine, the formation of the
new crust on either side of the vents
would act to push plates apart, as we
see at the Mid-Atlantic Ridge, which
helps to move North America and
Europe further and further apart.
Convergent Boundaries
• These are plate
margins where one
plate is overriding
another, thereby
forcing the other into
the mantle beneath it.
• These boundaries
are in the form of
trench and island arc
systems (volcano).
• Convergent boundaries also explain why
crust older than the Cretaceous cannot be
found in any ocean basin-- it has already
been destroyed by the process of
subduction.
• Subduction zones are the location of very
strong earthquakes, which occur because
the action of the down going slab interacts
with the overriding slab.
• The "Ring of Fire" around the margins
of the Pacific Ocean is due precisely to
the subduction zones found around the
edges of the Pacific plate.
• Sometimes, when there is a convergent
boundary between two continental plates,
subduction cannot occur.
• Since continental crust is more buoyant, or less
dense, than oceanic crust, one plate does not
easily override the other.
• Instead, the plates crumple as they plow
into one another, and a very high
mountain range is created.
• The Himalayas in India are the result of
two continental plates (the IndoAustralian and Eurasian plates) colliding
head on.
Conservative Boundary
• It is called conservative because
plate material is neither created
nor destroyed at these
boundaries, but rather plates slide
past each other.
• The classic example of a transform
plate boundary is the San Andreas fault
in California.
• The North American and Pacific Plates
are moving past each other at this
boundary, which is the location of many
earthquakes.
Faults
• _______________ are surfaces along
which rocks have fractured and been
displaced.
• The tectonic stresses caused by plate
motions build up over time and eventually
cause breaks in the crust of the Earth
along which the rocks sporadically grind
past one another.
• When this happens, earthquakes occur.
Earthquakes
• Earthquakes are the Earth's natural
means of releasing _______________.
• When the Earth's plates move against
each other, stress is put on the
_______________.
• When this stress is great enough, the
lithosphere breaks or shifts.
• There are many different types of
earthquakes:
– Tectonic
– Volcanic
– Explosion
– Collapse
• The type of earthquake depends on
the region where it occurs and the
geological make-up of that region.
• _______________ - occur when rocks in
the earth's crust break due to geological
forces created by movement of tectonic
plates
• _______________ - occur in conjunction
with volcanic activity
• _______________ - small earthquakes in
underground caverns and mines
• _______________ - result from the
explosion of nuclear and chemical
devices
Seismology
• _______________ is the study of the
origin and propagation of elastic waves
through planetary bodies.
• It was originally regarded as the study
of earthquakes.
• There are three types of waves
that are created when stress is
released as energy in
earthquakes:
–P
–S
– Surface waves
• P-waves:
Primary or Compressional
P Waves
waves are the _______________ type of
wave and have velocities of 5 km/s at the
top of the crust, 8 km/s at the top of the
mantle, and 14 km/s at the bottom of the
mantle.
• S-waves:
Shear or Transverse waves
S Waves
that travel at slower speeds than Pwaves.
Surface Waves
• Slowest
• These waves move
close to or on the
outside surface of
the ground.
Richter Scale
• Using a
seismogram, the
time difference
between the
recording of the P
wave and the S
wave is
determined and
matched to a
corresponding
distance value.
• Close-up of
Government Hill
Elementary
School
Anchorage,
Alaska. The
school was
destroyed by the
1964 Prince
William Sound
earthquake,
magnitude 9.2.
Note the person
in the upper right
portion of the
image(in the red
box) for scale.
• Fence offset 8 ½
feet by the 1906
San Francisco,
California
earthquake.
Magnitude 7.3