Transcript This Friday`s Geology Seminar speaker

This Friday's Geology Seminar speaker:
"High latitude marine
climate proxies: Oxygen
isotopes, biogenic
silica, and diagenesis"
Friday, 1:00 p.m.
Mudd 218
Process & Tectonic
Geomorphology,
part II:
Energy Sources
Landforms we see are the result of the current balance between
DRIVING FORCES
and the
RESISTING FRAMEWORK
(mainly the lithology and structure)
Energy Sources - "Driving Forces" - are
both internal and external to the Earth:
1. Rotational inertia and lunar/solar gravity
2. Terrestrial gravity
3. Radioactive decay
4. Solar radiation
The Driving Forces for Geomorphic Change
1. Rotational inertia and Extraterrestrial Gravity
a) bulge of geoid (Earth is an oblate spheroid or ellipsoid)
(The mouth of the Mississippi River is 200 m farther from
the center of Earth than the headwaters!)
 potential energy is slowly released as the Earth's
rotation slows.
b) TIDES - produced by gravitational pull of
Sun (~ 30%) and Moon (~70%) on the Earth
These are most important directly as they influence
the levels over which coastal processes are effective.
However:
1. Tidal scouring in oceans, large lakes, and on their
margins can be significant, especially in macrotidal
environments, where tide range is great (> 3 m +/-)
(more when we look at coasts).
2. Earth tides - to 30 cm amplitude
- may foster jointing in weaker rocks (debated)
- internal friction may be a minor heat source
- slows rotation of Earth ca. 1 day/yr/107 years
Driving Forces (cont.)
2. Terrestrial Gravity - What goes up, must
come down!
- average elevation continents is 850 m a.s.l.
- converts potential energy to kinetic energy,
through precipitation and stream flow,
glaciers, mass wasting, etc.
- isostatic rebound (from deglaciation &
response to erosion) and tectonic
uplift increase gradients and therefore
potential energy
Driving Forces (cont.)
3. Radioactive Decay ---> HEAT!
a) Geothermal heat flow - dissipates 80-90% by conduction
(the average near-surface geothermal gradient is ca.
20°C/km depth)
b) Mantle plumes, convection cells and plate tectonics are the
product of the remaining 10-20% of the heat.
(Note: the vertical scale of the crust is highly exaggerated here.)
Is it Orogeny or Epeirogeny?
(Only my wife knows for sure...)
1) Epeirogenic effects - regional to subcontinental
in scale
- affect cratonic blocks (continental interiors) NOT the plate margins!
- broad uplifts and downwarps
E.g., the Colorado Plateau
E.g., the
Paleozoic
Michigan
Basin
- some probably caused by broad mantle plumes
(and associated downdraws)
- Western North America is believed to be uplifted
by shallow subduction of the largely gone
Farallon Plate.
- Colorado Plateau rising ca. 0.2 mm/year for the
past 8 m.y
- northern Utah & western Wyoming – are rising as
much as 3-5 mm/yr
2) Orogenic effects - mountain building at plate margins
a) Volcanism - 1-10% of all energy release in this system
- concentrated in relatively small area (including the
mid-oceanic rise and ridge system)
“Subduction leads to
orogeny…”
Igneous activity has ALSO created many of the world's
oceanic plateaus (green)
b) Seismic activity - another 1-10% of the energy
produced by internal radioactive decay is released
in this manner - produces jointing and faulting
- produces fault scarps & fault-line scarps
Below, the fault-line scarp along
the San Andreas Fault, Carrizo
Plain, California.
Above, the fault scarp
produced by the Landers,
California, earthquake of
1992.
(Volcanism and tectonics produce constructional landforms)
Actual fault surface
Fault-line scarp at the base of the Sierra Nevada, western Nevada.
The same fault-line scarp, farther south, just east of Yosemite N.P.
Fault scarps in Armenia
(1988; above) and Borah
Peak, Idaho (1983;
right).
c) collisional zones and folded mountain belts
e.g., Himalaya: Everest is 8700 m at summit but net uplift is only 0.2 mm/year
Mountains rise slowly!
and only as long as uplift exceeds erosion!
Driving Forces (cont.)
4. Solar Radiation (99+% of total energy input to
system!)
a. ca. 30% of total incoming radiation is reflected back
into space from clouds, ice, trees, water...
… this is called the ALBEDO of the Earth
 BUT the albedo can be highly variable on a local
basis!
Lake Superior & Environs in
early spring (white is snow)
Africa
(b) Albedo is also
very latitudeand seasondependent.
We get net energy
gain (as heat) only
between ~38oN and
38oS.
(c) This differential energy transfer is what drives the winds ….
and in turn, the marine
currents ….
…. which then in turn set up the hydrologic cycle.
Without this, we have very little weathering or erosion!
97% of all global water is in the oceans
2% is in glacial ice
1% elsewhere (atmosphere, biosphere, groundwater, magma,
streams and lakes)
Ocean currents transfer
warmth from the
Equator towards the
poles (e.g., Gulf Stream
and northern Europe)
(d) Biosphere and fossil fuels - represent stored solar energy from
the past
- their consumption
(burning) produces
intense local heat;
the CO2 given off is
a "greenhouse gas"
because it is opaque
to IR (InfraRed
radiation), so heat is
trapped in the
atmosphere
e) Solar variation
this
Just a 1% variation in
solar radiation could
change Maine from
what you know to ….
or to this ….
HOWEVER, no variation approaching that has ever been
measured, ALTHOUGH…..
.. variations of a small fraction of 1%, reflected by the "Maunder
Minimum," are believed by some to have been responsible for
and "
"
The impact was significant on glaciers world-wide.
Snowline in SE Alaska. In 1800, the
entire fjord in the foreground was
filled with glacial ice.
1890
ice
margin
The Emmons Glacier, on the NE
flank of Mt. Rainier in Washington
State.
Midre Lovénbreen, northern Svalbard. The Little Ice
Age maximum extent of the glacier is marked by a
prominent moraine in the glacier forefield. Most
retreat has occurred since 1900.
Carroll Glacier, in SE Alaska, reflects these changes in photos taken
in 1906 (left) and 2004 (right); Muir Glacier (below) is shown in
2004 photos by Bruce
1950 and 2004.
Molnia, USGS
Cotopaxi, a volcano of Ecuador, atop the folded and
faulted sedimentary strata of the Andes