This
EarthCache brings you to an outcrop of serpentinite adjacent to the
parking lot of Cuesta Canyon County Park in San Luis Obispo. The
park has a play structure, picnic benches and BBQs.
The typical water cycle we all learned as children whereas water
evaporates from the oceans, gathers in the clouds, falls back to
the earth and flows back to the ocean. While there are a variety of
additional details in this story, this is the basic idea. However,
this is only part of the story of where water circulates. There is
also what has been termed the Deep Water Cycle. The Deep Water
Cycle describes how water is transported down into the inner layers
of the earth and back up to the surface of the earth.
This story begins with mafic rocks and the largest store of
water on the earth, the oceans. Mafic rocks are low in silica and
high in magnesium and iron, and typically form in the earth’s
mantle. This deep in the earth, the pressure and temperature is
very high. The minerals that form under these conditions are stable
only under these conditions.
At divergent plate boundaries deep in the oceans, peridotite, an
igneous mafic rock from the earth’s mantle is pushed up
toward the ocean floor. In the earth’s crust, the temperature
and pressure on the peridotite is reduced. Fractures from plate
movement and cooling rock create conduits for sea water to come
into contact with the peridotite.
At low pressures and temperatures, peridotite is no longer
stable. The mafic rock begins to metamorphose. Water is
incorporated into the rocks minerals and they become serpentine
minerals turning the peridotite into serpentinite. The minerals
brucite and magnetite are also formed with the magnesium and iron
ions that are left over along with the release of hydrogen ions.
This metamorphic process is called serpentization. The general
chemical process is represented by the following chemical equation.
Magnified view of the two rocks are used instead of their chemical
formula. The peridotite picture is using polarized light producing
the false colors. H2O is the chemical formula for water and H2 is
the chemical formula for hydrogen.
The bright color of the mineral in the peridotite
is not real, it is the result of the observation under polarized
light. The two microscope view have a width of approximately 0,2
millimeters. from Ifremer
(http://www.ifremer.fr/serpentine/english/scientific-sheet-5.htm)
Serpentization incorporates approximately 300 liters or 79 US
gallons of water per cubic meter of peridotite. When applied to the
entire sea floor, this ends up being a huge amount of water.
This process appears to continue as the sea floor moves toward a
subduction zone (to the left of "I" in the diagram). As the oceanic
plate bends down underneath the continental plate (see "I" in the
figure), it fractures even more, creating conduits for more
seawater to come into contact with peridotite which then
metamorphoses into serpentinite.
The subducting plate pulls the serpentinite and
sediment down toward the mantle. As the pressure and temperature
increases, the serpentine minerals become unstable and revert back
to the original rock in a process called deserpentization,
releasing the water that was incorporated into the serpentine
minerals near area "III" in the diagram. Some of the water is
dragged further down into the mantle, but some begins to rise back
up to the surface.
One of the effects of adding water to rock is that the melting
point of the rock is reduced. This helps form large volumes of
magma that migrate up to the earth’s surface. This magma
fuels the volcanic arcs found near subduction zones. When these
volcanoes erupt, the water that was pulled down into the earth by
the serpentinite is released back into the atmosphere.
This process has been verified by analyzing the water vapor in
volcanic gases emitted by back arc volcanoes and relating the
isotopes ratios to seawater.
Logging requirements:
Send me a note with :
- The text "GC1GBY0 The Deep Water Cycle and Serpentininte" on
the first line
- The number of people in your group.
- Examine the rock and see if you can find the water.
- What features in the area did deserpentization help form.
- looking at the outcrop, did the entire rock undergo
serpentization?
The above information was compiled from the
following sources:
- Rupke, Morgan, Hort, and Connolly; Serpentine
and the subduction zone water cycle, Earth and Planetary Science
Letters 223 (2004) 17– 34
- California Geological Survey - CGS Note
41-Guidelines for Reviewing Geologic Reports; Serpentine:
California State Rock;
http://www.conservation.ca.gov/cgs/information/publications/cgs_notes/note_14/Pages/Index.aspx
- Reaction Sea-water – Mantle: production
of serpentine;
http://www.ifremer.fr/serpentine/english/scientific-sheet-5.htm
- SERPENTINE and SERPENTINITE, USGS,
http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/serpentine.html