isostasy, gravity, magnetism, and internal heat - ut … · isostasy! equilibrium of adjacent...

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! crust and uppermost mantle!

DO NOT CONFUSE WITH!CRUST AND MANTLE!WHICH ARE BASED !ON COMPOSITION!

asthenosphere: underlying denser, heat-softened,!" partially melted (plastic) -- weak!" " " "upper mantle!

transition from lithosphere to asthenosphere reflects!temperature and rocks response to increased temperature!

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!

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 wood!wood that replaces water in the column !must be thicker than water it replaces!

isostasy!

continental crust is!less dense than!oceanic crust!

mass in column 1 = mass in column 2 = mass in column 3!density mantle > density oceanic crust > density continental crust!

crust is!less 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 thinner!if more continental crust in column -- column will be thicker!

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

isostasy!a more detailed view of density differences!

include !sea water!

&!sediments!

isostasy!leads to “isostatic adjustment” if mass is redistributed!

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!

note mountain and!crustal 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!

X!mantle!

crust!

A!X!

effect on mass columns!

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!

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

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

gravity!gravitational force between two objects determined!

by their masses and distance between them!

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!

void (cave) has no!mass to attract!

spring!

mass uniform!and spring!is neutral!

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

gravity!density 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!

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!

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!

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!

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”)!

Earth’s magnetic field!changes through time!

change in magnetic north relative !to true north!

1580-1970!

1831-2001!migration of magnetic north!

consequence of rotation of outer core!

N

S

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!

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 !

• 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 that cool through Curie temperature ! and stay below that temperature through time!record magnetic field AT THE TIME OF THEIR COOLING!

magnetite common mineral in basalt!

paleomagnetism: study of ancient!magnetic fields in rocks!

--reconstruction of past fields--!

thick flood basalt sequence in Brazil!

Earth’s magnetic field!examine thick sequences of basalts to identify reversals!

"through time (paleomagnetism)!

Earth’s magnetic field!re-construct “normal” and “reversed” for lava sequence!

Earth’s magnetic field!create time-scale for magnetism! from many observations!

black = normal polarity!blue = reverse polarity!

see that lengths of!magnetic periods!are not uniform!

likely relates to!turbulent flow!of outer core!

blue = normal polarity!red = reverse polarity!

reversed (orange north)!

transitional (chaotic)!

normal (blue north)!

Earth’s magnetic field!what happens during reversals?!

geologic evidence!suggests that!

reversals occur!quickly!

(a few 1000 yrs)!

computer simulations!indicate that!

transitions are!chaotic with!

many magnetic poles!in odd places!i.e. not N or S!

magnetic material!below “adds”!magnetism!and creates!

positive anomaly!

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

"below surface (similar to gravity anomalies)!

magnetic rocks!include!iron ore,!gabbro,!granite!

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

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

crust: rapid!increase !

in T!(25°/km)!

slower!increase!deeper!(1°/km)!

despite increase in temperature, rocks do not melt!because 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 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)!

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!

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!

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