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?

What is the lithosphere?

Continental crust

Oceanic crust

Upper mantle

Asthenosphere

Lithosphere

• outermost layer of Earth• crust & upper mantle• mechanically strong• brittle

Lithosphere

• underlies the lithosphere• within the mantle• mechanically weak• plastic & easily deformed

Asthenosphere

• cold (< 1300°C)

• hot (>1300°C)• lower seismic velocity zone

• partially molten (<0.5%)

• higher seismic velocity zone

Mantle

Asthenosphere

Upper Crust

Lower Crust

Upper Mantle

Mantle

6 km/s

7 km/s

8 km/s

7.8 km/s

Moho 30 km100 km

How is the lithosphere made up of distinctive layers?

How is rock strength related to temperature?

Continental crust

Oceanic crust

Upper mantle

Asthenosphere

How does heat flow vary with depth?

geotherm

Geothermal gradient = 30°C/km

Geothermal gradient

= 10°C/km

Lithosphere

Asthenosphere

(Conduction)

(Convection)

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?

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)

Focus

What evidence supports a layered internal structure of the lithosphere?

Seismic waves

2. Seismic Refraction & Reflection

surface

Layer 1

Layer 2

Refracted wave

Refracted wave along boundary

Reflected wave

Direct wave

Secondary waves

Angle of incidence is less than critical angle Angle of incidence

is equal to the critical angle

Angle of incidence is greater than the critical angle

What evidence supports a layered internal structure of the lithosphere?

Focus

1909 Mohorovicic (Yugoslavian seismologist) noted 2 sets of seismic waves arriving at seismograph stations from a single earthquake.

Station 1

P1 + S1

P2 + S2

surface

Layer 1

Layer 2

What evidence supports a layered internal structure of the lithosphere?

Focus

1909 Mohorovicic (Yugoslavian seismologist) noted 2 sets of seismic waves arriving at seismograph stations from a single earthquake.

Station 3Station 1

P1 + S1

P2 + S2

surface

Layer 1

Layer 2

What evidence supports a layered internal structure of the lithosphere?

Focus

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.

Station 3Station 1

P1 + S1

P2 + S2

surface

Layer 1

Layer 2

Station 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

Focus Seismic Station

1923 – Conrad found a third layer

Upper Crust

Lower Crust

Upper Mantle

Mohorovicic Discontinuity

Conrad Discontinuity

What evidence supports a layered internal structure of the lithosphere?

Two-way travel time in seconds

Reflective boundary (Moho)

Estimate the depth of the Moho and the thickness of the Moho from the diagram opposite, given P wave velocity is 6 km/s.

3. Seismic Reflection

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)

Geology of the Lithosphere

Describe and explain global variations in thickness of the lithosphere.

(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)

Geology of the Lithosphere

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.

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:

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)

Geology of the Lithosphere

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.

Introduction:

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)

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:

a). the internal structure and layering

b). the thickness and

c). the composition

of the lithosphere

(25)

Geology of the Lithosphere

Continental

sediment, upper crust, lower crust, upper mantle

Oceanic

sediment, basaltic, sheeted dykes, gabbro, peridotite

Seismic velocities: 2km/s – 6 – 7 – 8 (stepped & increases)

Depth to focus:

Seismic refraction & reflection:

Seismic refraction: infer differences

Drilling: testing seismic velocities in lab