1. isostasy, gravity, magnetism, and internal heat
Earth’s gravity field

2. 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

3. 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

4. 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

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

6. isostasy leads to “isostatic adjustment” if mass is redistributed A X
mantle
crust A X
note mountain and
crustal root below it A X
erosion redistributes rock
from mountain (high)
to sediment deposited
in basin (low)
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
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

7. 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

8. 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

9. Polar Glaciers Melting Animation
From:

10. gravity gravitational force between two objects determined
by their masses and distance between them

11. 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)

12. 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--

13. generates increased gravity and, thus, a positive gravity anomaly
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

14. generates decreased gravity and, thus, a negative gravity anomaly
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

15. 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

16. 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”)

17. Earth’s magnetic field
changes through time
migration of magnetic north
change in magnetic north relative
to true north
consequence of rotation of outer core

18. Earth’s magnetic field
reverses over time (north and south poles flip)
--magnetic field lines reverse-- N S
“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

19. 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)

20. 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--
magnetite common mineral in basalt

21. thick flood basalt sequence in Brazil
Earth’s magnetic field
examine thick sequences of basalts to identify reversals
through time (paleomagnetism)
thick flood basalt sequence in Brazil

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

23. blue = 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
blue = normal polarity
red = reverse polarity
black = normal polarity
blue = reverse polarity

24. Earth’s magnetic field
what happens during reversals?
geologic evidence
suggests that
reversals occur
quickly
(a few 1000 yrs)
reversed (orange north)
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)

25. 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

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

27. 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)

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

29. 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)

30. red at mid-ocean ridges
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

31. 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