Feedback in geological processes:

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Feedback 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

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Selected 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.)

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• 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.

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• 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.

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• 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.

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• 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.

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Basic Types of Feedback Mechanisms

• Positive
• Negative

-
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Basic 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

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http//ga.water.usgs.gov/edu/earthgwdecline.html
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Another familiar example of flow

• Density-driven Convection

http//www.hanksville.org/daniel/geology/convectio
n.html
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A 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

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Porosity 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).

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Reactive 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. 

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Reactive 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
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• 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

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http//earthobservatory.nasa.gov/Newsroom/NewImage
s/images.php3?img_id17437
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• 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
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Peter 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. 

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Such 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
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Peter 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. 

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• 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. 

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Conclusion

• 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.