Earth's Interior and Magnetics Flashcards
1
Q
earth’s internal structure
A
- Crust – the outer layer of rock that forms a thin skin on Earth’s surface
- Mantle – thick shell of dense rock that separates the crust above from the core below
- Core – the metallic central zone of the Earth
2
Q
how to get direct evidence of earth’s interior
A
- mining
- drilling
- physical samples (kimberlites, xenoliths, opiolites)
- remotely sensed evidence (deep earth geophysical models: seismic waves, magnetism, gravity)
3
Q
mining
A
- Deepest mine in the world: 3.9km deep - TauTona Gold mine
- High temperatures (up to 55 degrees C) and pressure on rocks
4
Q
drilling
A
- Take a core sample -> cylindrical section of rock taken with a hollow steel tube called a core drill
- We’ve drilled up to 12.5km (Kola superdeep borehole)
- Found water that originated from rock minerals
- Found microscopic fossils
5
Q
kimberlites
A
- Igneous bodies/volcanic rocks formed from mantle melts (>150km deep) that ascend quickly towards earth’s surface and crystallize
- Gives us a sample of composition of mantle rocks, and diamonds
6
Q
xenoliths
A
- Small fragments of crustal or mantle rocks that are picked up again in an ascending magma – evidence for composition, pressures, and temperatures at depths within earth
- Ex. Peridotite xenolith in basalt
7
Q
ophiolites
A
- Slivers of preserved oceanic crust that indicates its structure
- Sometimes young ocean crust or ocean crust between 2 colliding continents gets “obducted” rather than subducted
- Sections of ocean crust snipped off and preserved on a continent
8
Q
studying interior of the earth
A
- Deep interior of earth must be studied indirectly
- Direct access to crustal rocks and small upper mantle fragments brought up igneous activity
- Can only drill so deep (not to the mantle)
- Geophysics: branch of geology that studies interior of earth
9
Q
how do we know earth isn’t a uniform solid?
A
- if it were, P-wave, s-wave, and surface waves would arrive at a station at the same time, and we would compute body wave arrival time at different seismometers
- but that doesn’t happen -> we get seismic reflection and refraction
10
Q
seismic reflection
A
- occurs if there is a contrast in seismic wave velocity. The strength of the reflection depends on contrast in density
- 2-way travel time is used to calculate distance to reflection surface
- Seismic waves travel fastest through dense materials (ie. Igneous rocks)
11
Q
seismic refraction
A
- bending of seismic waves as they pass from one material to another having different seismic wave velocities
- Curved energy pathways
12
Q
evidence for existence of the core
A
- seismic shadow zones: specific areas on the opposite side of the Earth from large earthquakes do not receive seismic waves
- P‐wave shadow zone (103°‐142° from epicenter) explained by refraction of waves encountering core‐mantle boundary
- S‐wave shadow zone (≥103° from epicenter) suggests outer core is a liquid (s-waves can’t travel through liquids)
- careful observations of P‐wave refraction patterns indicate inner core is solid
13
Q
evidence for core’s composition
A
- composition inferred from its calculated density, physical and electro-magnetic properties, and composition of meteorites
- Iron metal (liquid in outer core and solid in inner core) best fits observed properties
- Inner core probably solid due to great pressure
- Probably slowly growing as core cools: about 100 degrees C per billion years
- Temperature is about 5400 degrees C -> temp. Of surface of the sun
14
Q
evidence for core-mantle boundary
A
- boundary/”D” layer is marked by great changes in seismic velocity, density, and temperature
- Hot core may melt lowermost mantle or react chemically to form iron silicates in this seismic wave ultralow-velocity zone (ULVZ)
15
Q
geothermal gradient
A
- Geothermal gradient: temperature increase with depth into earth
- tapers off sharply beneath lithosphere
- Due to steady pressure increase with depth, increased temperatures produce little melt except the outer core
