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isostasy, gravity, magnetism, and internal heat. Earth’s gravity field. isostasy. equilibrium of adjacent blocks of brittle crust “floating” on underlying upper mantle. outer layers of Earth divided into 2 based on their strength. lithosphere : rigid, solid outer layer (brittle) --strong

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Presentation Transcript
slide2

isostasy

equilibrium of adjacent blocks of brittle crust

“floating” on underlying upper mantle

outer layers of Earth divided into 2 based on their strength

lithosphere: rigid, solid outer layer (brittle) --strong

crust and uppermost mantle

asthenosphere: underlying denser, heat-softened,

partially melted (plastic) -- weak

upper mantle

DO NOT CONFUSE WITH

CRUST AND MANTLE

WHICH ARE BASED

ON COMPOSITION

transition from lithosphere to asthenosphere reflects

temperature and rocks response to increased temperature

slide3

isostasy

equilibrium of adjacent blocks of brittle crust

“floating” on underlying upper mantle

i.e. mass above a certain depth must be the same

think of wood blocks in water

block that sticks up higher

also extends farther in water

density of wood < density of water

compensation depth

for masses to be the same above the isostatic compensation depth:

mass in column 1 = mass in column 2

masses in both columns in 2 dimensions equal

(density wood x thickness wood) + (density water x thickness water)

density water > density wood

wood that replaces water in the column

must be thicker than water it replaces

slide4

isostasy

same concept as wood blocks applies to lithospheric blocks

(crust and uppermost mantle)

floating on asthenosphere above the compensation depth

continental crust is

less dense than

oceanic crust

crust is

less dense than

mantle

compensation depth

mass in column 1 = mass in column 2 = mass in column 3

density mantle > density oceanic crust > density continental crust

if more mantle in column -- column will be thinner

if more continental crust in column -- column will be thicker

implication is that mountains have “roots” -- crust is thicker below them

slide5

isostasy

a more detailed view of density differences

include

sea water

&

sediments

slide6

crust

mantle

X

A

erosion of mountain

A

B

C

as mountain erodes,

column becomes shorter thus,

mantle mass in column

increases over time

(mass A = mass B = mass C)

isostasy

leads to “isostatic adjustment” if mass is redistributed

note mountain and

crustal root below it

A

X

erosion redistributes rock

from mountain (high)

to sediment deposited

in basin (low)

less mass on mountain

causes uplift of

crust below mountain

(thins and rises)

and

subsidence of basin

as mass of

sediment is added

B

C

effect on mass columns

slide7

isostasy

“see” isostatic adjustment today from load of glaciers on

crust during last glaciation and unloading from melting

(possible because response of asthenosphere is slow)

process is called post-glacial rebound

slide8

isostasy

post-glacial rebound still occurs in Canada & northern Europe

i.e. crust is rising -- (not isostatically balanced)

(can measure uplift rates with highly precise GPS receivers--mm’s/yr)

amount of uplift since glaciation

polar glaciers melting animation
Polar Glaciers Melting Animation

From: http://www.uni-geophys.gwdg.de/~gkaufman/work/onset/onset_ice3g.html

slide10

gravity

gravitational force between two objects determined

by their masses and distance between them

slide11

gravity

differences in density of materials (rocks) in Earth’s interior

produces small differences in local gravity field (anomalies)

can be measured with a gravimeter (attraction of spring to mass)

dense material

attracts

and extends spring

mass uniform

and spring

is neutral

void (cave) has no

mass to attract

spring

can find buried, dense things (abandoned gas station tanks)

and empty spaces (caves -- don’t build)

slide12

gravity

density differences also occur over larger areas: mountains

compensation depth

mass above compensation depth is uniform (isostatically balanced)

--no excess or deficiency in mass; no gravity anomalies--

slide13

gravity

mass above compensation depth is not uniform

-- excess mass of dense mantle below mountain (no crustal root)

compensation depth

generates increased gravity and, thus, a positive gravity anomaly

slide14

gravity

mass above compensation depth is not uniform

-- deficiency of mass below low area (too much crust)

compensation depth

generates decreased gravity and, thus, a negative gravity anomaly

slide15

Earth’s gravity field measured from space

mass in Earth “pulls” on satellites as they orbit,

causing “wobbles” in orbit paths, which are measured

--amount of wobble related to amount of mass--

GRACE

--NASA--

mission to

examine

Earth’s

gravity

field

slide16

Earth’s magnetic field

surrounds the Earth

• has north and south magnetic poles

• is detected by compasses

• is recorded in rocks and minerals as they cool

• is generated in the Earth’s liquid outer core as

it spins and produces electrical currents

Earth’s field similar

to that for

bar magnet (left)

magnetic N and S

is not the same

as geographic

N and S poles

(bar magnet “tilted”)

slide17

1580-1970

migration of magnetic north

1831-2001

Earth’s magnetic field

changes through time

change in magnetic north relative

to true north

consequence of rotation of outer core

slide18

S

N

Earth’s magnetic field

reverses over time (north and south poles flip)

--magnetic field lines reverse--

“normal” polarity: north is north and

south is south

“reversed” polarity: north is south and

south is north

after next reversal, compass needle will point south

slide19

• above Curie temperature, atoms are random

• below Curie temperature, atoms align in domains

that are independent of each other

• below Curie temperature, atoms align with

magnetic field if one is present (e.g. Earth)

Earth’s magnetic field

how do rocks and minerals acquire magnetism?

rocks and minerals at high temperatures (e.g. molten)

must cool through their Curie temperatures

slide20

magnetite common mineral in basalt

Earth’s magnetic field

how do rocks and minerals acquire magnetism?

rocks and minerals that cool through Curie temperature

and stay below that temperature through time

record magnetic field AT THE TIME OF THEIR COOLING

paleomagnetism: study of ancient

magnetic fields in rocks

--reconstruction of past fields--

thick flood basalt sequence in brazil
thick flood basalt sequence in Brazil

Earth’s magnetic field

examine thick sequences of basalts to identify reversals

through time (paleomagnetism)

slide22

Earth’s magnetic field

re-construct “normal” and “reversed” for lava sequence

slide23

black = normal polarity

blue = normal polarity

blue = reverse polarity

red = reverse polarity

Earth’s magnetic field

create time-scale for magnetism

from many observations

see that lengths of

magnetic periods

are not uniform

likely relates to

turbulent flow

of outer core

slide24

Earth’s magnetic field

what happens during reversals?

reversed (orange north)

geologic evidence

suggests that

reversals occur

quickly

(a few 1000 yrs)

normal (blue north)

computer simulations

indicate that

transitions are

chaotic with

many magnetic poles

in odd places

i.e. not N or S

transitional (chaotic)

slide25

Earth’s magnetic field

magnetic anomalies occur in local field from magnetic rock

below surface (similar to gravity anomalies)

magnetic material

below “adds”

magnetism

and creates

positive anomaly

magnetic rocks

include

iron ore,

gabbro,

granite

slide26

Earth’s magnetic field

removal of magnetic material from near surface

causes negative anomaly (example is normal faulting)

slide27

Earth’s internal heat

geothermal gradient: temperature increases with depth

in the Earth--most dramatic in crust; tapers off deeper

despite increase in temperature, rocks do not melt

because pressure also increases with depth

(big increase in T in outer core--molten)

crust: rapid

increase

in T

(25°/km)

slower

increase

deeper

(1°/km)

slide28

Earth’s internal heat

heat flow: gradual loss of heat from interior to surface

heat sources must be in shallow crust for crustal gradient

• magma bodies

• uranium-rich igneous rock (decay of U, Th, K generates heat)

slide29

Earth’s internal heat

heat flow is reasonably similar over oceans and continents

heat comes from different sources in two regions

• continental crust: radioactive decay in granites

• oceanic crust: mantle sources (no granite in oceanic crust)

slide30

Earth’s internal heat

observed heat flow at Earth’s surface shows gross patterns

(red is warm; blue is cold)

red at mid-ocean ridges

blue over oldest parts

of continents

slide31

Earth’s internal heat

gradual loss of heat from interior to surface causes

mantle convection as mechanism of heat transfer

• upwelling (rising of warm material) in mantle below mid-ocean ridges

• loss of heat as material moves laterally at surface

• downwellling (sinking of cooled material) at subduction zones