lecture 3 earths core and geomagnetism

Question 1

how was the core mantle boundary discovered

Answer 1

s h a d o w z o n e

Question 2

what is the shadow zone

Answer 2

an area of the earth where waves from an earthquakes are not detected. s waves disappear completely and p waves come back later than expected

Question 3

how do you interpret the s h a d o w z o n e

Answer 3

waves are refracted away from the surface (towards the normal) at a low velocity layer

low velocity explains why p waves arrive later than expected

s waves don’t reappear so must be liquid

Question 4

how deep is the core mantle boundary

Answer 4

2900km

Question 5

how was the inner core discovered

Answer 5

identification of PKIKP phase in shadow zone

Question 6

what does the main p wave shadow zone contain

Answer 6

some weak arrivals with long travel times

Question 7

to what did inge lehmann attribute the some weak arrivals with long travel times

Answer 7

refractions and reflections from an inner core

p wave velocities increase in inner core

s waves reappear transformed from s waves so inner core must be solid

Question 8

how deep is the inner core

Answer 8

5150km

Question 9

what is the composition of the core

Answer 9

mostly iron with lighter elements like nitrogen, sulphur and oxygen

Question 10

how do we know the cores composition

geochemical evidence

Answer 10

sun and meteorites have lots of iron

Question 11

how do we know the cores composition

geophysical evidance 1 seismology

Answer 11

constrains density and elasticity which can be tested in a lab

Seismic waves through the inner core travel about 4
percent faster when they’re moving N-S, (along spin
axis), than when they’re moving E-W which explained by computer modelling could mean the core is one giant Fe crystal!

inner core is growing slowley as it cools

Question 12

how do we know the cores composition

geophysical evidence 2 electric boogaloo geomagnetism

Answer 12

The Earth has a magnetic field, which must be
generated by convection of an electrically
conducting liquid in the outer core – liquid iron
fits the bill!

Question 13

how many degrees is the earths magnetic field tilted from earths rotation axis

Answer 13

10 degrees

Question 14

what is a dipmeter

Answer 14

• A compass whose needle can move in

a vertical plane is called a

Question 15

what is another name for angle of dip

Answer 15

magnetic inclination

Question 16

how is angle of dip measured

Answer 16

relative to the horizon

Question 17

if its horizontal dip =

Answer 17

0 degrees

Question 18

if its verticle dip =

Answer 18

90 degrees + or -

Question 19

magnetic field is strongest at

Answer 19

magnetic poles

Question 20

magnetic field is weakest at

Answer 20

magnetic equator

Question 21

how is the geomagnetic field generated

Answer 21

• The Earth’s interior is too hot (above the Curie temp) for
permanent magnetism to be possible.
• The geomagnetic field is generated by convection currents
in an electrically conducting (metallic) liquid in the outer
core. The core acts like a self-exciting dynamo. • The pattern of fluid flow is not well understood, but is
probably crucially affected by the presence of the solid inner
core, and the rotation of the Earth.
• This explains why the geomagnetic field is aligned, on
average, with the Earth’s spin axis.

Question 22

what happens every few thousand years

Answer 22

poles flip 180