Earth's Internal Structure

Question 1

what is inside the earth?

Answer 1

layers of different composition formed by differentiation

Question 2

how is earths volume distributed between the mantle, crust and core?

Answer 2

Crust: 1.4%
Mantle: 82.5%
Core: 16.1%

Question 3

how is earths mass distributed between the mantle, crust and core?

Answer 3

Crust: <1%
Mantle: 68%
Core: 32%

Question 4

How do we know about the continental crust?

Answer 4

We live on it
Erosion cuts through the layers
Drilling (to a few km depth)

Question 5

How do we know about oceanic crust?

Answer 5

Sample the upper layers (seafloor, drilling ships)

Ophiolites on land provide cross-section

Question 6

why does continental crust sit higher on the mantle than oceanic crust?

Answer 6

thickened continental crust -> high elevations

Question 7

what is the average thickness of oceanic crust?

Answer 7

6km

Question 8

what is the average thickness of continental crust?

Answer 8

35-40 km (max >70 km)

Question 9

what is the composition of oceanic crust?

Answer 9

Mafic (Si,Mg, Fe)

Question 10

what is the composition of continental crust?

Answer 10

Felsic (Si, Al)

Question 11

what is the relative deformity of oceanic crust?

Answer 11

relatively undeformed

Question 12

what is the relative deformity of continental crust?

Answer 12

deformed

Question 13

what is the age of oceanic crust?

Answer 13

young (<200 Ma)

Question 14

what is the age of continental crust?

Answer 14

older (max 4.4 billion years)

Question 15

what is the density of oceanic crust?

Answer 15

Higher (~3.0 g/cm3)

Question 16

what is the density of continental crust?

Answer 16

Lower (~2.7 g/cm3)

Question 17

what are the typical rocks in oceanic crust?

Answer 17

Basalt, gabbro

Question 18

what are the typical rocks in continental crust?

Answer 18

Varied, granite, gneiss

Question 19

what is the extent of the mantle?

Answer 19

Moho to ~2900km depth

Question 20

what is the composition of the mantle?

Answer 20

Ultramafic (Mg, Fe)

Question 21

what is the density of the mantle?

Answer 21

3.3 g/cm3 to 5 g/cm3

↑with depth

Question 22

what are the typical rocks in the mantle?

Answer 22

Peridotite (olivine-rich)

Question 23

what is the extent of the core?

Answer 23

Radius ~3500 km

Question 24

what is the composition of the core?

Answer 24

Outer core: Fe-Ni alloy (+ some O, Si, S)

Inner core: pure Fe

Question 25

what is the density of the core?

Answer 25

Average 11 g/cm3 (max 14)

early molten Earth: heaviest matter sank to centre

Question 26

How do Earth’s (physical/mechanical) properties change with depth?

Answer 26

Lithosphere: rigid, brittle
Asthenosphere: weak, ductile (partially molten)
Mesosphere: rigid
Outer core: liquid
Inner core: solid

Question 27

Why do Earth’s (physical/mechanical) properties change with depth?

Answer 27

Conditions change: T and P both increase with depth

Question 28

How do we know about the deep layers in the Earth?

Answer 28

Cannot sample directly
Gain information from seismic waves that travel through the Earth: body waves (P waves and S waves).
Earthquake -> seismic waves. Rays are paths at 90 degrees to the wave front
Waves travel in all directions away from earthquake focus
In uniform material -> constant wave speed in all direction

Question 29

what are P (primary) waves?

internal structure

Answer 29

Material transmits wave by compression & dilation (push-pull)
travel in solid, liquid & gas

Question 30

what are S (secondary) waves?

internal structure

Answer 30

Waves transmitted by shear motion between particles

cannot move through liquid/gas

Question 31

“Seismic waves found to arrive sooner than expected for places farther from source.” What can we infer form this?

Answer 31

Wave speed (velocity) must increase with depth
Abrupt velocity changes between “layers” -> waves reflect or refract (bend)
Downgoing waves refract to bend back toward surface

Question 32

what is the Moho (Mohorovicic) discontinuity?

Answer 32

at the crust/mantle boundary, composition change leads to abrupt change in seismic velocity

Question 33

what is seismic discontinuity?

Answer 33

abrupt change in seismic velocity -> Implies changes in composition or physical state (solid/liquid)

Question 34

How do we know the asthenosphere is partially molten?

Answer 34

P and S waves slow down through it

Question 35

How do we know that the outer core is liquid?

Answer 35

(1) S waves cannot travel through it
(2) P waves are slowed
(3) Slowed P waves must be refracted downward (not upward) -> shadow zone (where no P waves arrive)

Question 36

what are some other examples of seismic discontinuities?

Answer 36

In mantle:
410 km, and 660 km: phase changes to denser mineral structures
Deep:
2900 km – core/mantle boundary (CMB)
5150 km – inner core/outer core boundary

Question 37

How do we know that the inner core is solid iron?

Answer 37

Gravitational pull exerted by Earth requires high-density iron inner core
The whole Earth oscillates after large earthquakes so inner core must be solid