Lecture 1 Flashcards
Mines and Boreholes
Mines are less than 3km usually
Boreholes are less than 10km
Both get very hot very quickly
Mean Radius of Earth
6371km
Density of Earth’s surface rocks
0.002-0.003 kgm^-3
Earth’s Average Density
0.00552 kgm^-3
Moment of Inertia of uniform sphere compared to earth
If was a perfect sphere then the moment of inertia would be 0.4Ma^2
But it is 0.33 and so mass is concentrated to the centre of the earth
South Africa Gold Mine
up to 4km deep and gets to 60-70 degrees and so pump ice slurry into mines
Every kilometre deeper you go it is roughly a 20 degree temperature increase
Earthquake properties
- Occur when faults move and release elastic energy as sound waves
- Seismology is the study of these waves
- Important in creating images on earth’s interior
Waves that propagate out from focus of earthquake
x2
- Body waves which are split in p and s waves
2. Surface waves which are restricted to the free surface or to internal interface
P waves
Arrive first and are longitudinal
also called pressure waves
S waves
Secondary or shear waves and are transverse
Example of how earthquakes can affect other parts of the world
8.2 mag in Bolivia 1994 could be felt as moving buildings in toronto 6000km away
Different mode types and definition
A normal mode is the free oscillations the earthquakes have on earth
Most common=
1. Balloon mode (entire earth expanding and contracting) period=20 minutes
2. Football mode (compression along x axis while expansion along y axis before switching) period= 54 minutes
3. Twisting mode (halves of earth rotate in different directions) period= 40 minutes
Each have different periods associated with them and the modes can get a lot more complicated
Core-mantle boundary-who discovered it
Richard Oldham in 1900 by looking at travel times from an earthquake in Assam in 1897
Who studied earth magnetic field and though the earth might be hollow
Edmund Halley
Why do p and s waves curve in earth
They change between materials and increase in velocity in the earth
At what angles do p and s waves stop and reappear
P and S waves both stop at an epicentral angle (Δ) of 104 degrees
P waves reappear at Δ=142
What is the order of wave arrival
- P waves
- S waves
- Surface waves
What is Δ?
The epicentral angle.
This is measured as the angle between the focus/epicentre, the centre of the earth, and the seismograph that recorded the wave
Wave field
A wave field is all the energy released by an earthquake.
However it is easier to visualise and calculate the field as separate rays
Epicentre angle to time before recorded
Graph 1
Core Shadow zones
The areas in which none of the waves are recorded.
What does the fact that there are no s waves recorded after 103 degrees tell us?
That the outer core is liquid as S waves cannot travel through that-Richard Oldham. We know from Inge Lenmang that the inner core is solid.
Mantle-Core boundary
All waves are refracted/reflected at this interface and p waves experience a reduction in velocity and so therefore switch directions towards the core.
The moho
the crust-mantle boundary discovered by Mohorovicic by using an earthquake in Croatia in 1909 when beyond 250km the p waves were earlier than expected
What is defined as the crust and mantle
Anything above the moho is the crust and anything below is the mantle
How was the moho found
Graph and diagram 2
- the p wave that went directly above the moho was slower than the wave that ran along the moho interface as the wave sped up from 6–>8ms^-1
Average thickness of the continental crust
35km
Average oceanic crust thickness
7km
What is the jump ion velocity at the moho caused by
- Either a change in composition or a change in mineral structure (phase change)
Graph of relative velocities of waves through the earth
graph 3
Low velocity zone at roughly 150km depth
may reflect partial melting or otherwise solid mantle. Cannot be a lot as the s waves can still travel through the majority
Increases in velocity of waves occurs at 450km and 670km why?
there are changes in the mineral structure—>a phase change due to denser materials
670km is the boundary between the upper and lower mantle
What is peridotite
A rock composed of mainly the mineral olivine which gives it the green colour. Most likely the rock thar forms the majority of the matle
What are the meteorites we use most often to estimate the composition of the earth
Carbonaceous chondrites
These meteors have a similar composition to the sun. however, that are the differences?
GRAPH 4
- N, O, C, H- mostly in gas form and so not kept in meteor as much as sun
- Li more in meteorite as cosmic ray interaction causes this to form on the surface
What is the chondritic model of earth
The idea that the earth’s composition resembles that of chondritic meteorites
Iron meteorites contain roughly 90% iron. What does this show
represent cores of small planetary bodies that were broken by collision and so most likely the earth’s core as well
Who discovered the earth’s magnetic field and when
Gauss in 1830
Describe the earth’s magnetic field
90% dipolar (like a bar magnet)
