# Large scale topography

```Whole Earth morphology
Large scale topography
What are the largest topographic
features of the Earth?
Oblateness?
pole
– By 21 km (~12 mi)
• Why?
Results in a slow flow of
mass towards the equator
Whole earth morphology
versus
Size and rheology matter
• Size drives the internal pressure and the
time it takes to cool down
• Rheology drives how “well” the inner
material flows and moves
Low pressure
High pressure
P1 = P2 = P3 =&gt; no pressure gradients
Is the surface of the earth smooth
or rough?
Why is there a
bimodal distribution
of elevation?
3.8 km
23 km
5 km
13 km
The continents ride higher in the mantle
due to lower density and greater thickness – isostatic balance
Isostasy: Floating materials in a denser fluid
• How thick a column of rock is needed to exert a pressure
of 1 bar (or 105 Pa) at the Earth’s surface. How thick a
column of water is needed? How thick a column of air,
given its density is that of air at the Earth’s surface, 1.22
kg/m3?
gH  10 5 Pa
H  10 5 / (9.8  )
H rock  10 5 / (9.8 * 2700)  3.78m
H water  10 5 / (9.8 *1000)  10.2m
H air  10 5 / (9.8 *1.22)  8364m  8.4km
Response of the mantle and crust
• http://geology.utah.gov/utahgeo/gsl/flash/lb_flash.htm
Calculate the expected deflection of the lithosphere beneath a thick
icesheet.
Say the icesheet is 4000 m thick, and ice has a density of 917 kg/m3. (The
Antarctican icesheet is roughly this thick at its maximum thickness.)
The thickness of the crust (of density 2700 kg/m3) beneath the center of
the icesheet and the region outside of it is the same.
The density of the upper mantle that gooshes out of the way to allow this
deflection of the surface is 3300 kg/m3.
What I want to know is how far down the rock is depressed beneath the
i gH  c ghc  c ghc  m gz
i
917
z 
H
4000  1111m
m
3300
Rock Cycle
Weathering, transport, deposition
• Repeated creation and destruction of crustal material (rocks
•
and minerals)
Volcanoes, folding faulting, uplift
– bring rock, water, gas to the Earth surface
• Rocks disintegrates
– weathers by exposure to water and air
• Transport by gravity, water, wind
– weathering products go back to the ocean
• Deposition and burial
– formation of sedimentary rocks
• Deep burial
– metamorphic rocks
• Uplift, intrusion, or extrusion
– rocks exposed, process begins again
Landform creation by:
Weathering, transport, deposition
• Weathering agents move into soil and rock along
a weathering front
– brings fresh rock up
• Weathered material (regolith; soil) on surface +
•
material brought in by wind, water, ice, animals
= weathered mantle
Mantle remains in place or moves downslope by
gravity
– water can also carry it downslope
– wind can remove it
Brook manual, p. 2-4
• Q 2-3
– Magnetic declination: the difference between
geographic north and magnetic north
– Magnetic declination of Philipp quad=
• 7.5 deg E
– Verbal scale in in/mi
• Philipp: 1” = 62,500” = .98mi
• Kingston: 1” = 24,500” = .38mi
– Verbal scale in cm/km
• Philipp: 1 cm = .625km
• Kingston: 1cm = .24 km
Brook manual, p. 2-10
• Q 2.6
– Horiz scale is
• 1:25,000 or 1” on map = 25,000” on ground
– Vert scale is
• 1” = 100’ or 1”=1200”
– VE=Horiz scale/Vert scale =
• 25,000/1200=20.8
• Q 2.7
– Average gradient (or slope) = rise/run, in words
– 500 ft/(520 yds x 3 ft/yd)=500 ft/1560 ft = 1 ft/3.12
– for every vert. ft of elev gain, the dist moved in 3.12ft
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