Geology of the Lithosphere
2. Evidence for the Structure of the Crust & Upper Mantle
•
What is the lithosphere and what is the structure of the lithosphere?
•
What evidence supports a layered internal structure of the lithosphere?
•
What is the thickness of the lithosphere and why does it differ between
continents and oceans?
Lithosphere
What is the lithosphere?
• outermost layer of Earth
• crust & upper mantle
• mechanically strong
• brittle
Lithosphere
Continental crust
• cold (< 1300°C)
Oceanic crust
• higher seismic velocity zone
Upper mantle
Asthenosphere
• underlies the lithosphere
Asthenosphere
Mantle
• within the mantle
• mechanically weak
• plastic & easily deformed
• hot (>1300°C)
• lower seismic velocity zone
• partially molten (<0.5%)
How is the lithosphere made up of distinctive layers?
6 km/s
Upper Crust
100 km
Lower Crust
Moho
7 km/s
Upper Mantle
8 km/s
Asthenosphere
7.8 km/s
Mantle
30 km
How is rock strength related to temperature?
Continental crust
Oceanic crust
Upper mantle
Asthenosphere
How does heat flow vary with depth?
Geothermal gradient = 30°C/km
Lithosphere
(Conduction)
Asthenosphere
geotherm
Geothermal gradient
= 10°C/km
(Convection)
What evidence supports a layered internal structure of the lithosphere?
Vp = P-wave velocity
Vs = S-wave velocity
κ = bulk modulus (a measure of incompressibility or resistance to change in
volume without change in shape)
μ = shear modulus ( a measure of rigidity or resistance to change in shape
without a change in volume)
ρ = density
What evidence supports a layered internal structure of the lithosphere?
What evidence supports a layered internal structure of the lithosphere?
1. Seismic Wave Velocities
P & S wave velocities
increase when:
• rigidity increases
• compressibility decreases
S waves cannot travel
through liquids
(compressible & no rigidity)
What evidence supports a layered internal structure of the lithosphere?
2. Seismic Refraction & Reflection
Focus
Angle of incidence is greater
than the critical angle
surface
Layer 1
Seismic waves
Secondary waves
Angle of incidence
is less than critical
angle
Layer 2
Angle of incidence
is equal to the
critical angle
Refracted wave
Refracted wave along boundary
Reflected wave
Direct wave
What evidence supports a layered internal structure of the lithosphere?
Focus
Station 1
surface
P1 + S1
Layer 1
P2 + S2
Layer 2
1909 Mohorovicic (Yugoslavian seismologist) noted 2 sets of seismic
waves arriving at seismograph stations from a single earthquake.
What evidence supports a layered internal structure of the lithosphere?
Focus
Station 1
Station 3
surface
P1 + S1
Layer 1
P2 + S2
Layer 2
1909 Mohorovicic (Yugoslavian seismologist) noted 2 sets of seismic
waves arriving at seismograph stations from a single earthquake.
What evidence supports a layered internal structure of the lithosphere?
Focus
Station 1
Station 2
Station 3
surface
P1 + S1
Layer 1
P2 + S2
Layer 2
The point where both sets of P + S waves arrive at the same time is
known as the Point of Transformation and is used in mathematical
equations to work out the depth to Layer 2.
Working out the depth to a boundary using seismic waves
= thickness of the layer
= distance at which both direct and
indirect waves arrive at same time (point
of transformation)
= velocity of Layer 1 (7 km/s)
= velocity of Layer 2 (8 km/s)
Work out the depth to the
boundary if the point of
transformation was:
• 50 km
• 200 km
What evidence supports a layered internal structure of the lithosphere?
Focus
Seismic Station
Upper
Crust
Conrad
Discontinuity
Lower
Crust
Mohorovicic
Discontinuity
Upper
Mantle
1923 – Conrad found a third layer
3. Seismic Reflection
Estimate the depth of
the Moho and the
thickness of the Moho
from the diagram
opposite, given P wave
velocity is 6 km/s.
Two-way travel
time in seconds
Reflective boundary
(Moho)
Geology of the Lithosphere
a). Explain how the velocities of P and S waves are used to
interpret the
i). mechanical properties and
ii). thickness of the lithosphere
b). Explain why the thickness of the lithosphere differs
between oceans basins and continents.
(25)
Geology of the Lithosphere
Explain how an investigation of P and S wave velocity-depth
curves provides evidence for the outer layering of the
Earth. You should restrict your answers to the layers
above the base of the asthenosphere.
(25)
Geology of the Lithosphere
Describe and explain global variations in thickness of the
lithosphere.
(25)
Describe and explain global variations in thickness of the
lithosphere.
(25)
Geology of the Lithosphere
Describe and explain how a study of earthquake body
waves, provide evidence for the variation in thickness and
mechanical properties of the lithosphere and
asthenosphere.
(25)
Describe and explain how a study of earthquake body waves, provide
evidence for the variation in thickness and mechanical properties of
the lithosphere and asthenosphere.
(25 marks)
Introduction:
The lithosphere is the outer most part of the Earth and includes the crust
and upper mantle. It constitutes a single rigid unit which is relatively cold
(<1300ºC) and brittle. The thickness of the lithosphere varies over the
Earth’s surface. Beneath the continents the lithosphere is between 100 to
400km thick, but beneath the oceans it varies with age, with young oceanic
lithosphere being as little as 10km thick and old oceanic lithosphere about
120km thick. Below the lithosphere the asthenosphere is hot, relatively
weak and ductile. This essay will show how the study of P and S wave
velocities, refraction and reflection provide evidence for this structure.
Geology of the Lithosphere
Discuss the use of seismology in the distinction between
the lithosphere and asthenosphere. Comment on the
possible significance of temperature in the formation of
the asthenosphere.
(25)
Discuss the use of seismology in the distinction between the
lithosphere and asthenosphere. Comment on the possible significance
of temperature in the formation of the asthenosphere.
(25)
Introduction:
The lithosphere is the outer most part of the Earth and includes the crust
and upper mantle. It constitutes a single rigid unit which is relatively cold
(<1300ºC) and brittle. Below the lithosphere, the asthenosphere is hot,
relatively weak and ductile. This essay will show how the study of
earthquake body wave (P and S waves) velocities can be used to provide
evidence for this distinction.
Geology of the Lithosphere
Explain how earthquake waves can be used to provide
information about:
a). the internal structure and layering
b). the thickness and
c). the composition
of the lithosphere
(25)
Geology of the Lithosphere
Explain how earthquake waves can be used to provide
information about:
Continental
Oceanic
a). the internal structure and layering
Seismic velocities: 2km/s – 6 – 7 – 8 (stepped & increases)
b). the thickness and
sediment,
upper crust,
lower crust,
upper mantle
sediment,
basaltic,
sheeted
dykes,
gabbro,
peridotite
Depth to focus:
Seismic refraction & reflection:
c). the composition
Seismic refraction: infer differences
Drilling: testing seismic velocities in lab
of the lithosphere
(25)