Structure of the Earth

1
Plate Tectonics
Structure of the Earth
Plate Boundaries
Driving Mechanisms of Plate Tectonics
2
Structure of the Earth
The Earth can be considered as being made up of a
series of concentric spheres, each made up of
materials that differ in terms of composition and
mechanical properties.
3
Crust the outermost layer of the earth, a hard
outer shell.
Crust beneath the oceans and the continents is
different
Oceanic crust relatively thin, varying from 5 to
8 km (but thinner at Oceanic ridges).
4
Thickness ranges from 20 km to about 75 km
(beneath mountain ranges).
5
The lithosphere has the composition of the upper
mantle (Iron and Magnesium Silicates) but is
rigid like the crust.
Lower temperatures and pressures allow the
Lithosphere to be rigid.
Very thin (a couple of km) at the Oceanic Ridge
extends to 80 km depth beneath old oceanic crust
that is well away from the Ridge.
6
The crust and lithosphere float on the
underlying mantle.
Where the crust and lithosphere are thick (e.g.,
beneath continental mountains) they extend deeper
into the upper mantle.
7
The crust and lithosphere are broken up into 25
Lithospheric Plates
8
Unlike the crust, the mantle is dominated by Iron
and Magnesium Silicate minerals.
Upper Mantle near its melting point so that it
behaves like a plastic (Silly Putty is a
reasonable analogy) the upper mantle material
flows under stress.
Upper mantle material flows by convection
transfers heat from within the Earth towards the
surface.
Lower Mantle solid material, rather than plastic.
9
The Core the metallic portion of the Earth Iron
mixed with small amounts of Nickel.
Outer Core probably liquid (based on studies of
shock wave passage through the Earth).
Inner Core solid, made up of cooled liquid core
material.
10
Plate Boundaries
11
Plate Boundaries
The types of boundary between plates are
distinguished by the type of relative plate
motion along the boundary
Oceanic Ridge Divergence
Oceanic Trench Convergence
Transform Margins Horizontal slip
12
Oceanic Ridge
13
More-or-less continuous volcanic mountain chain
throughout the world's oceans.
65,000 km long.
Average width approx. 1,000 km.
Rise up to 3 km above the surrounding sea floor.
Average depth approx. 2.3 km below sea level.
14
The ridge is a Divergent Plate Margin and
divergence takes place by Sea Floor Spreading.
Older crust is pushed laterally away from the
ridge axis so that the sea floor spreads away
from the ridge axis.
15
Oceanic crust becomes older with distance from
the oceanic ridge.
16
Spreading rates (distance per year that two
points on either side of a ridge move apart) vary
N. Atlantic Ridge 3cm/yr
S. Atlantic 5cm/yr
N. Pacific 12.5cm/yr
E. Pacific 17.5 cm/yr
17
http//www.gisdevelopment.net/technology/images/im
age002.gif
18
Deep, narrow troughs the border many ocean
basins.
19
(No Transcript)
20
Thousands of kilometers long, 50 to 100 km wide
and several kilometers deep (below sea level).
Longest trench Peru-Chile trench at 5,900 km.
Deepest trench Mariana trench (western Pacific)
over 11 km deep.
21
Trenches are termed Convergent Plate Margins
because they are locations where plates converge
on, or push against, each other.
Where oceanic crust is subducted back into the
upper mantle.
22
Crust melts as it descends, beginning at 100 to
200 km depth and has melted completely by 700 km
depth.
The zone over which melting takes place is termed
the Benioff Zone.
23
Melting crust rises and penetrates overlying
crust to form volcanoes.
Material (sediment and basaltic rock) is scraped
off the subducting crust and accreted to the
over-riding crust termed the subduction complex.
24
Island Arcs parallel many oceanic trenches
arc-shaped chains of volcanic islands (e.g.,
Japan) due to the rising magma from melting
subducted crust.
25
Convergent Plate Margins
The oldest, densest crust normally descends
beneath the younger crust.
Volcanic islands develop at the surface of the
over-riding crust (forming Island Arcs).
26
Basaltic oceanic crust descends beneath lighter
continental crust.
Coastal mountain chains develop due to
compressive forces and volcanics (e.g., the Andes
of South America).
Magma material rises from descending slab and
builds volcanoes in the rising mountains.
27
(No Transcript)
28
Continental Crust-Continental Crust
Neither plate subducts (both too light).
Compressive forces driving plates fold and thrust
the continental margins forming an extensive
mountain belt (e.g., the Himalayan Mountains).
29
Plate margins along which the plates slip by each
other. Termed Transform Faults
On either side of a transform fault plate motions
are in opposite directions.
30
Transform faults displace the oceanic ridge.
31
The faster the rate of spreading the greater the
distance between transform faults.
32
The land east of the fault is on the North
American Plate the land west of the fault is on
the Pacific Plate.
The eastern side of the fault moves southeast and
the western side moves to the northwest.
Total movement along the fault has been 564 km
over the past 30 million years (1.9 cm per year).
33
Rupturing of the ground surface along the San
Andreas Fault.
34
Wallace creek crossing the San Andreas Fault
35
(No Transcript)
36
To summarize
http//www.seed.slb.com/en/scictr/watch/living_pla
net/mountains.htm
37
http//www.gisdevelopment.net/technology/images/im
age002.gif
38
But what drives plate tectonics?
Two main hypotheses
1. Convection Cells within the upper mantle
(first postulated by Arthur Holmes a year before
Wegener died).
and 2. Ridge push and slab pull.
39
Giant convection cells within the upper mantle
drag the plates along laterally.
Where convection rises sea floor spreading takes
place.
Where the convection cells descend they drag
crust down, causing subduction.
40
Heres a link to an animation showing how
convection might drive plate tectonics.
41
Ridge push and slab pull
Where new, young crust forms its weight pushes
down slope to drive the plates laterally.
Once the crust has cooled, having been pushed
away form the ridge, it sinks into the upper
mantle and helps to pull adjacent crust along.
This pushing and pulling provides the forces that
drive plate tectonics.