Download Plate tectonics II: Earth`s structure and plate boundaries

Plate tectonics II: Earth’s structure and plate
boundaries
Important: This chapter follows mainly:
Chap. 1 in Turcotte and Schubert
Chap. 2 in Fowler.
Earth structure: The main units
Compositional:
•  Crust
•  Mantle
•  Core
Rheological:
•  Lithosphere
•  Asthenosphere
•  Mesosphere
Earth structure: The main units
•  Crust versus mantle: The crust is a product of mantle melting. Typical mantle rocks
have a higher magnesium to iron ratio, and a smaller portion of silicon and aluminum
than the crust.
•  Lithosphere versus asthenosphere: While the lithosphere behaves as a rigid body over
geologic time scales, the asthenosphere deforms in ductile fashion. The lithosphere is
fragmented into tectonic plates, which move relative to one another. There are two types
of lithosphere: oceanic and continental.
•  Upper versus lower mantle: Together the lithosphere and the asthenosphere form the
upper mantle. The mesosphere, extending between the 660 boundary and the outer
core, corresponds to the lower mantle. The region between 410 and 660 km is referred
to as the transition zone.
Earth structure: Mantle phase changes
•  410 km: Above this depth the Mg, Fe, Si
and O are primarily within olivine and
pyroxene. Below this depth the olivine is
no longer stable and is replaced by a
higher density polymorph - spinel. The
material has a similar overall composition
but the minerals have a more compact
structure.
•  660 km: Below this depth the spinel gives
way to the minerals Mg-perovskite and
Mg-wustite. (In fact, Mg-perovskite is
probably the most abundant solid of the
earth since it appears to be stable through
much of the mantle.)
Earth structure: Seismic discontinuities
•  Moho: The dept at which the
P-wave velocity exceeds 8.1
Km/S is referred to as the
moho (after the seismologist
Mohorovicic). The moho is
both a seismic and a
compositional boundary,
marking the transition between
crust and mantle materials.
•  Low Velocity Zone (LVZ):
The low velocity is more
strongly visible for S-waves
than for P-waves. It marks the
boundary between the
lithosphere and the
asthenosphere.
Earth structure: Seismic discontinuities
Thickness of the Earth's
crust (by the USGS). Since
the Moho is at the base of
the crust this map also
shows depth to Moho.
Earth structure: Seismic discontinuities
The LVZ is deeper under
shield and platforms,
than it is under oceanic
basins and continental
rifts.
Earth structure: Seismic discontinuities
•  D”: There is evidence of a seismic
discontinuity about 200 km above the
core-mantle boundary (CMB). This is
known as the D" discontinuity, and while
we don't know much about it, it appears to
be ubiquitous, although its position varies
from less than 100 km to over 300 km
above the CMB.
Earth structure: Seismic discontinuities
Tomographic images of the P
and S velocity perturbation,
averaged vertically over the
deepest 1000 km of the mantle,
reveal structures with vertical
continuity over that depth range.
Negative (reddish) anomalies
indicate the presence of lower
mantle plumes.
Figure from Montelli et al., 2006
Earth structure: Seismic discontinuities
Seismic images suggesting that
some mantle plumes originate at
the D”.
Figure from Montelli et al., 2004
Earth structure: Seismic discontinuities
Earth structure: Seismic discontinuities
It has been suggested,
based on tomography (i.e.,
seismic imaging), that the D”
is a slab graveyard and/or
plume factory
Figure from: http://www.avh.de/kosmos/titel/
2002_011.htm
Earth structure: Core
The shadow zones
Earth structure: Core
Does Earth’s inner core rotate slower, faster or at the same rate
as the rest of the plant?
see animation on:
http://www.ldeo.columbia.edu/news-events/scientists-confirm-earths-inner-core-rotating-faster-than-rest-planet
Plate boundaries: MOR
•  Lithospheric plates are
created at ocean ridges.
•  The two plates on either
side of an ocean ridge move
away from each other with
near constant velocities of a
few tens of millimeters per
year.
•  As the two plates diverge,
hot mantle rock flows
upward to fill the gap.
•  The upwelling mantle rock
cools by conductive heat
loss to the surface.
•  The cooling rock accretes to
the base of the spreading
plates, becoming part of
them
Plate boundaries: MOR
•  As the plates move away
from the ocean ridge, they
continue to cool and the
lithosphere thickens.
•  As the lithosphere cools, it
becomes more dense; as a
result it sinks downward into
the underlying mantle rock.
•  The topographic elevation
of the ridge is due to the
greater buoyancy of the
thinner, hotter lithosphere
near the axis of accretion at
the ridge crest.
Plate boundaries: MOR
•  The elevation of the ocean
ridge also provides a body
force that causes the plates
to move away from the
ridge crest.
•  A component of the
gravitational body force on
the elevated lithosphere
drives the lithosphere away
from the accretional
boundary.
•  This force on the
lithosphere is known as
ridge push and is a form of
gravitational sliding.
Plate boundaries: MOR
•  The volcanism at ocean ridges
is caused by pressure-release
melting.
•  As the two adjacent plates
move apart, hot mantle rock
ascends to fill the gap.
•  The temperature of the
ascending rock is nearly
constant, but its pressure
decreases.
•  When the temperature of the
ascending mantle rock equals
the solidus temperature,
melting occurs.
Plate boundaries: MOR
•  In some localities slices of
oceanic crust and underlying
mantle have been brought to
the surface. These are known
as ophiolites; they occur in
such locations as Cyprus,
Newfoundland, Oman, and
New Guinea.
•  Field studies of ophiolites
have provided a detailed
understanding of the oceanic
crust and underlying mantle.
Plate boundaries: MOR
•  Layer 1 is composed of
sediments that are deposited
on the volcanic rocks of layers
2 and 3. The thickness of
sediments increases with
distance from the ridge crest;
a typical thickness is 1 km.
•  Layers 2 and 3 are composed
of basaltic rocks of nearly
uniform composition.
Plate boundaries: MOR
•  Layer 2 of the oceanic crust is
composed of extrusive
volcanic flows that have
interacted with the seawater
to form pillow lavas and
intrusive flows primarily in the
form of sheeted dikes.
•  A typical thickness for layer 2
is 1.5 km.
Plate boundaries: MOR
Plate boundaries: MOR
•  Layer 3 is made up of gabbros and related cumulate
rocks that crystallized directly
from the magma chamber.
Gabbros are coarse-grained
basalts; the larger grain size is
due to slower cooling rates at
greater depths.
•  The thickness of layer 3 is
typically 4.5 km.
Plate boundaries: Driving forces
Figure taken from William Lowrie textbook
Plate boundaries: Subduction
The negative buoyancy of the dense rocks of the descending lithosphere
results in a downward body force. Because the lithosphere behaves
elastically, it can transmit stresses and acts as a stress guide. The body
force acting on the descending plate is transmitted to the surface plate,
which is pulled toward the ocean trench. This is one of the important forces
driving plate tectonics and continental drift. It is known as slab pull.
Deriving forces:
•  Ridge push
•  Slab pull
Resisting Forces:
•  Viscous traction
•  Frictional resistance
Additional forces:
•  Slab suction
•  Elastic bending
Fig from Heki and Mitsui, EPSL, 2013
Plate boundaries: Subduction
•  Since the gravitational body force on
the subducted lithosphere is
downward, it would be expected that
the subduction dip angle would be 90◦.
•  In fact, the typical dip angle for a
subduction zone is near 45◦.
•  One explanation is that the subducted •  The dip of a subducting
slab is supported by the induced flow
lithosphere is a direct
above the slab. The descending
consequence of the balance
lithosphere induces a corner flow in
between the gravitational torque
the mantle wedge above it.
and the lifting pressure torque,
i.e. the slab suction.
Plate boundaries: Subduction
•  In some trench systems a
secondary accretionary
plate margin lies behind the
volcanic line.
•  This back-arc spreading is
very similar to the seafloor
spreading that is occurring
at ocean ridges.
Plate boundaries: Subduction
A number of explanations have been given for back-arc spreading:
•  Option 1, the descending lithosphere induces a secondary convection
cell (panel-a).
•  Option 2, the ocean trench migrates away from an adjacent continent
because due to the sinking of the descending lithosphere, and the backarc spreading is required to fill the gap (panel-b).
Plate boundaries: Subduction
Isotherms in a lithosphere descending at an angle of 45◦ into
the mantle
•  As the subducted lithosphere
descends into the mantle,
frictional heating occurs at its
upper boundary.
•  The effect of frictional heating
gives rise to the isotherms in
the slab.
•  The low temperatures in the descending
lithosphere cause it to have a higher density
than the surrounding mantle. The higher density
results in a body force driving the descending
lithosphere downward.
Plate boundaries: Subduction
An additional downward body force on the descending slab is provided by
the distortion of the olivine–spinel phase boundary in the slab.
The olivine–spinel phase
boundary is elevated in the
descending lithosphere as
compared with its position in
the surrounding mantle
because the pressure at which
the phase change occurs
depends on temperature.
P
spinel
olivine
T
Sketch of the Clapeyron curve, which gives the pressures
and temperatures at which two phases of the same
material, such as olivine and spinel, are in equilibrium.
Plate boundaries: Subduction
The same approach can also be
applied to the transition of spinel
to perovskite. In this case the
slope of the Clapeyron curve is
negative and the transition
occurs at a deeper depth (higher
pressure) in the slab.
P
post-spinel
spinel
T
Plate boundaries: Subduction
•  The phase change from
spinel to perovskite could act
to deter penetration of the
descending lithosphere.
•  Shallow subduction
earthquakes generally
indicate extensional stresses
where as the deeper
earthquakes indicate
compressional stresses. This
is also an indication of a
resistance to subduction.
Fig. from: Wolfgang, Meschede and Blake
Plate boundaries: Subduction
Earthquakes terminate at a
depth of about 660 km, but
termination of seismicity does
not imply cessation of
subduction.
Fig. from: Wolfgang, Meschede and Blake
Plate boundaries: Subduction
The fate of the descending plate has important implications regarding mantle
convection.
Figure from Fukao et al., 2001
Blue = fast anomaly = dense = cold
Red = slow anomaly = buoyant = hot
Plate boundaries: Subduction
Currently, it seems that the
answer to this fundamental
question is in the eye of the
beholder. (learn more at:
http://www.mantleplumes.org/
TomographyProblems.html )
Figure from Zhao et al., 2004
Plate boundaries: Subduction
The remaining of the Farallon
plate underneath N. America?
Plate boundaries: Subduction