isostasy, gravity, magnetism, and internal heat

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 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

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

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

isostasy a more detailed view of density differences include sea water & sediments

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

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 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)

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--

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”)

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

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

• 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

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 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

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

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)

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

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 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)

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)

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

isostasy, gravity, magnetism, and internal heat