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isostasy, gravity, magnetism, and internal heat

Earth’s gravity field

isostasyequilibrium of adjacent blocks of brittle crust

“floating” on underlying upper mantleouter layers of Earth divided into 2 based on their strength

lithosphere: rigid, solid outer layer (brittle) --strong crust and uppermost mantle

DO NOT CONFUSE WITHCRUST AND MANTLEWHICH ARE BASED ON COMPOSITION

asthenosphere: underlying denser, heat-softened, partially melted (plastic) -- weak

upper mantle

transition from lithosphere to asthenosphere reflectstemperature and rocks response to increased temperature

isostasyequilibrium of adjacent blocks of brittle crust

“floating” on underlying upper mantlei.e. mass above a certain depth must be the same

think of wood blocks in waterblock that sticks up higher

also extends farther in water

density of wood < density of water

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

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 woodwood that replaces water in the column must be thicker than water it replaces

isostasy

continental crust isless dense thanoceanic crust

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

density mantle > density oceanic crust > density continental crust

crust isless dense than

mantle

same concept as wood blocks applies to lithospheric blocks(crust and uppermost mantle)

floating on asthenosphere above the compensation depth

compensation depth

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

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

isostasya more detailed view of density differences

include sea water

&sediments

isostasyleads to “isostatic adjustment” if mass is redistributed

erosion redistributes rockfrom mountain (high)to sediment deposited

in basin (low)

less mass on mountaincauses uplift of

crust below mountain(thins and rises)

andsubsidence of basin

as mass ofsediment is added

note mountain andcrustal root below it

erosion of mountain

as mountain erodes,column becomes shorter thus,

mantle mass in column increases over time

(mass A = mass B = mass C)

A B C

A

B

C

Xmantle

crust

AX

effect on mass columns

isostasy“see” isostatic adjustment today from load of glaciers oncrust during last glaciation and unloading from melting

(possible because response of asthenosphere is slow)

process is called post-glacial rebound

isostasypost-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

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

gravitygravitational force between two objects determined

by their masses and distance between them

gravitydifferences 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 materialattracts

and extends spring

void (cave) has nomass to attract

spring

mass uniformand springis neutral

can find buried, dense things (abandoned gas station tanks)and empty spaces (caves -- don’t build)

gravitydensity differences also occur over larger areas: mountains

mass above compensation depth is uniform (isostatically balanced)--no excess or deficiency in mass; no gravity anomalies--

compensation depth

gravitymass 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

gravitymass 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

Earth’s gravity field measured from spacemass 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 toexamineEarth’sgravity

field

Earth’s magnetic fieldsurrounds 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 Sis not the sameas geographicN and S poles

(bar magnet “tilted”)

Earth’s magnetic fieldchanges through time

change in magnetic north relative to true north

1580-1970

1831-2001migration of magnetic north

consequence of rotation of outer core

N

S

Earth’s magnetic fieldreverses 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

Earth’s magnetic fieldhow do rocks and minerals acquire magnetism?

rocks and minerals at high temperatures (e.g. molten) must cool through their Curie temperatures

• above Curie temperature, atoms are random

• below Curie temperature, atoms align in domainsthat are independent of each other

• below Curie temperature, atoms align withmagnetic field if one is present (e.g. Earth)

Earth’s magnetic fieldhow do rocks and minerals acquire magnetism?

rocks and minerals that cool through Curie temperature and stay below that temperature through timerecord magnetic field AT THE TIME OF THEIR COOLING

magnetite common mineral in basalt

paleomagnetism: study of ancientmagnetic fields in rocks

--reconstruction of past fields--

thick flood basalt sequence in Brazil

Earth’s magnetic fieldexamine thick sequences of basalts to identify reversals

through time (paleomagnetism)

Earth’s magnetic fieldre-construct “normal” and “reversed” for lava sequence

Earth’s magnetic fieldcreate time-scale for magnetism from many observations

black = normal polarityblue = reverse polarity

see that lengths ofmagnetic periodsare not uniform

likely relates toturbulent flowof outer core

blue = normal polarityred = reverse polarity

reversed (orange north)

transitional (chaotic)

normal (blue north)

Earth’s magnetic fieldwhat happens during reversals?

geologic evidencesuggests that

reversals occurquickly

(a few 1000 yrs)

computer simulationsindicate that

transitions arechaotic with

many magnetic polesin odd placesi.e. not N or S

magnetic materialbelow “adds”magnetismand creates

positive anomaly

Earth’s magnetic fieldmagnetic anomalies occur in local field from magnetic rock

below surface (similar to gravity anomalies)

magnetic rocksincludeiron ore,gabbro,granite

Earth’s magnetic fieldremoval of magnetic material from near surface causes negative anomaly (example is normal faulting)

Earth’s internal heatgeothermal gradient: temperature increases with depth in the Earth--most dramatic in crust; tapers off deeper

crust: rapidincrease

in T(25°/km)

slowerincreasedeeper(1°/km)

despite increase in temperature, rocks do not meltbecause pressure also increases with depth

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

Earth’s internal heat

heat sources must be in shallow crust for crustal gradient

heat flow: gradual loss of heat from interior to surface

• magma bodies

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

Earth’s internal heatheat 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)

Earth’s internal heatobserved heat flow at Earth’s surface shows gross patterns

(red is warm; blue is cold)

red at mid-ocean ridges

blue over oldest partsof continents

Earth’s internal heatgradual 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

• downwellling (sinking of cooled material) at subduction zones• loss of heat as material moves laterally at surface