Title: Feedback in geological processes:
1Feedback in geological processes What
instability can do for you! Dr. Peter
KelemenArthur Storke Professor of Earth
andEnvironmental Sciences
- Originally presented
- 10 March 2007
- Earth2Class Workshops for Teachers
2Selected Regents ES Core Concepts Applicable to
Todays Discussion
- 2.1a. Earth systems have internal and external
sources of energy, both of which create heat. - 2.1b. The transfer of heat energy within Earth's
interior results in the formation of regions of
different densities. These density differences
result in motion. - 2.1j. Properties of Earth's internal structure
(crust, mantle, outer core, inner core) can be
inferred from the analysis of the behavior of
seismic waves (including velocity and refraction.)
3- 2.1k. The outward transfer of Earth's internal
heat drives convective circulation in the mantle
that moves the lithospheric plates comprising
Earth's surface. - 2.1l. The lithosphere consists of separate
plates that ride on the more fluid asthenosphere
and move slowly in relationship to one another,
creating convergent, divergent, and transform
plate boundaries. These motions indicate Earth
is a dynamic geologic system. - gt These plate boundaries are the sites of most
earthquakes, volcanoes, and young mountain
ranges. - gt Compared to continental crust, ocean crust is
thinner and denser. New ocean crust continues to
form at mid-ocean ridges.
4- 2.1m. Many processes of the rock cycle are
consequences of plate dynamics. These include
production of magma (and subsequent igneous rock
formation and contact metamorphism) at both
subduction and rifting regions regional
metamorphism within subduction zones and the
creation of major depositional basins through
down-warping of the crust. - 2.1n. Many of Earth's surface features are the
consequence of forces associated with plate
motion and interaction. These include mid-ocean
ridges/rifts subduction zones trenches/island
arcs mountain ranges (folded, faulted, and
volcanic) hot spots and the magnetic and age
patterns in surface bedrock.
5- 1.2g. Earth has continuously been recycling
water since the outgassing of water early in its
history. This constant recirculation of water at
and near Earth's surface is described by the
hydrological (water) cycle. - gt Water is returned from the atmosphere to
Earth's surface by precipitation. Water returns
to the atmosphere by evaporation or transpiration
from plants. A portion of the precipitation
becomes runoff over the land or infiltrates into
the ground to become stored in the soil or ground
water below the water table. - gt The amount of precipitation that seeps into the
ground or runs off is influenced by climate,
slope of the land, soil, rock type, vegetation,
land use, and degree of saturation. - gt Porosity, permeability and water retention
affect runoff and infiltration. Soil capillarity
influences this process.
6- 3.1b. Minerals are formed inorganically by the
process of crystallization as a result of
specific environmental conditions. These
include cooling and solidification of magma
precipitation from water caused by such processes
as evaporation, chemical reactions, and
temperature changes rearrangement of atoms in
existing minerals subjected to conditions of high
temperature and pressure.
7Basic Types of Feedback Mechanisms
-
8Basic Types of Flow
- Flow refers to the movement of a fluid liquid
or gas - Some familiar examples of flow include
- Gravity flow precipitation, water running
downhill, groundwater movement
9http//ga.water.usgs.gov/edu/earthgwdecline.html
10Another familiar example of flow
- Density-driven Convection
http//www.hanksville.org/daniel/geology/convectio
n.html
11A third example Forced Flow
- Pressure, such as that exerted by a pump, can
force a fluid to move against gravity or other
conditions by expending energy - Conditions inside Earths mantle could be
considered to create natural pumps under
appropriate circumstances, perhaps with
radioactivity supplying the energy
12Porosity and Permeability
- In order for a fluid to move through any
substance, there must be - POROSITY empty space
- PERMEABILITY connections between pore spaces
- These conditions may exist naturally or be
created by the process involved, such as
dissolving the matrix (surrounding material).
13Reactive Porous Flow"
- When a liquid solvent (like water) passes
through a partially soluble porous matrix (like
salt), fingers - dissolution channels may
form along the direction of flow. This could
happen because dissolution of salt makes more
pore space, which in turn allows fresh water to
flow faster in some places compared to others.
This feedback between dissolution and water flow
causes an "instability", leading to exponentially
growing channels with predictable shapes.
14Reactive Porous Flow"
- Such a process is important in transporting lava
from the Earth's interior toward the surface.
Magma literally dissolves channels that permit
faster flow upward against gravity.
http//www.windows.ucar.edu/tour/link/earth/inter
ior/lava.html
15- Similar processes may form erosional channels,
for example channels that form on beaches at low
tide, when the groundwater comes out onto the
beach surface. These erosional processes form
coalescing networks where many small channels,
all active at once, feed a few smaller ones.
This leads to a "fractal" structure
16http//earthobservatory.nasa.gov/Newsroom/NewImage
s/images.php3?img_id17437
17- Depositional processes, during reactive porous
flow and sediment transport, produce downward
divergence of channels, for example forming wide
deltas where rivers begin to deposit sediment.
However, this is not completely symmetrical
compared to erosion. - In depositional networks, often only one or two
channels are active at a time, with flow jumping
suddenly from old to new channels. This can give
rise to very periodic behavior, for example
periodic volcanic eruptions from a single
volcano, and periodic spacing of volcanic vents
along a rift zone.
http//www.geographyhigh.connectfree.co.uk/s3river
sgeoghighlandforms.html
18Peter Kelemen writes
- Earthquakes may arise from another type of
periodic feedback process, in which gradual
elastic loading leads to sudden brittle or
viscous failure. I've been studying a potential,
new earthquake mechanism involving "frictional"
heating the strength of many materials decreases
exponentially with increasing temperature. In a
narrow shear zone under stress, slow sliding
leads to heating, which dramatically weakens the
material, increasing the sliding rate, and in
turn increasing the heating rate.
19Such investigations require study of Earths
interior, which is really more complex than the
usually onion or rigid layer model
http//www.windows.ucar.edu/tour/link/earth/image
s/earthint_image.html
20Peter Kelemen also notes
- Building on intuition gained from studies of melt
transport, erosion and earthquakes, my colleagues
and I have been thinking about feedback processes
in forming solid carbonate minerals from CO2 in
the air and ocean. High atmospheric CO2 is
contributing to global surface temperature
increase, rising sea level, and melting ice
caps. Most specialists agree on this. As a
result, it would be good to develop a method for
taking atmospheric and oceanic CO2 and
"sequestering" it in solid calcium and magnesium
carbonate minerals that are stable over very long
periods of time.
21- Many participants will have seen travertine
terraces in Yellowstone National Park, and some
will have seen the huge carbonate chimneys on the
seafloor in the recent IMAX movie, "Aliens of the
Deep." We think that the process involved in
carbonate deposition in these springs can involve
positive feedback - temperature and volume
changes driven by the chemical reactions that
form carbonate and associated minerals may heat
the rocks and form fractures, in turn increasing
reaction rates and the flux of water through
source rocks rich in calcium and magnesium.
22Conclusion
- With this background, well take a break and
let Peter Kelemen set up. - Later today, well try to create some
simple classroom activities that will allow
students to investigate concepts about flow and
feedback.