Unusual Plants & Rocks of the Presidio EarthCache
Unusual Plants & Rocks of the Presidio
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This EarthCache is located in the Presidio area of San Francisco
and it is part of the Golden Gate National Recreation Area. This
EarthCache is in a US National Park and therefore visitors should
stay on designated trails and should not collect any materials or
leave any physical markers.
Unusual Plants surviving on a Serpentinite Island:
Because serpentinite is altered rock from the Earth’s mantle, its
chemistry is unlike most other continental rocks. Serpentinite is
low in potassium and calcium, which are important plant nutrients.
It also contains high levels of magnesium, nickel and chromium that
are potentially toxic to plants. Plants living on serpentine soils
have adapted to survive these chemical conditions. Eight of the
thirteen rare plants found at the Presidio grow on serpentinite and
the relative rarity of serpentinite outcrops contributes to the
extreme rarity of these plants.
Unusual Rock:
This unique rock type is called serpentinite and is made from
oceanic crust and mantle rocks that have been altered by the
intense pressure and chemical-rich water present in areas known as
subduction zones, places where one tectonic plate moves under
another. The serpentinite visible below formed in a subduction zone
that ran along the coast here about 100 million years ago. Faulting
and uplift have subsequently moved the serpentinite to the earth’s
surface. A plate underlying the Pacific Ocean slid under the North
American Plate, rocks scraped from the base of the ocean crust
underlying the Great Valley sequence were transformed into
serpentinite.
To log this EarthCache:
1) Look at the cliff’s below at this site and E-mail us the COLOR
of the Serpentinite Rock that you see.
2) Post a photo of yourself or group taken at this site. If you do
it just right, you will have the Golden Gate Bridge in the
background of your picture.
Parking is available.
For additional information, go to the GGNRA website
http://www.nps.gov/goga
Enjoy the fantastic view of the Golden Gate Bridge!
Additional Hints
(No hints available.)