Parking in Malibu Creek State Park requires a fee, but
will get you quite a bit closer to the earthcache. Free
parking is available on surrounding streets, but it gets very
crowded on weekends. A walk along a wide fire road followed by
a narrower boulder strewn path brings you to Rock Pond
Igneous, metamorphic, and sedimentary rocks can be found in
Malibu Creek State Rock. This spot lets you examine the igneous
rocks found in the park. Examples of the park’s metamorphic and
sedimentary rocks can be located by finding their respective
earthcaches within the park.
Igneous
rocks form from the direct cooling of liquid rock. Liquid rock
is called magma while it is still inside the earth and lava when it
reaches the surface. Magma typically cools slowly due to the
insulation effect of the surrounding rock so they often grow large
crystals. This type of igneous rock is called plutonic. Lava, with
out the benefit of insulation, cools too quickly to form crystals.
These igneous rocks are called volcanic.
The igneous rock here is called basaltic breccia and is part of
the Conejo Volcanics. The Conejo Volcanics are an entire series of
igneous rocks that came from a volcano (possibly 3) located in the
western Santa Monica Mountains. One of the eruptions blew apart the
overlying rock into large angular chunks (the breccia) as it spewed
lava, in this case basalt and ash. As this mix of material came
down and/or flowed across the landscape it cooled with the pieces
of other rock inside it combining to become the basaltic breccia.
You can also see the gas that was still trapped in the basalt as it
cooled and formed small bubbles in the rock.
Logging requirements:
Send me a note with :
- The text "GC1BFCN Rocks of Malibu Creek SP – Conejo Volcanics"
on the first line
- The number of people in your group.
- What size and color range of the breccia?
- How big are the frozen gas bubbles in the breccia?
Additional Info:
Igneous rocks are classified based on their composition. See
this USGS website for a set of graphics on how they are
classified.
As you look southeast you there is a ridge of the basaltic
breccia going up the ridge. When these rocks formed they were
laying relatively flat across the landscape. Over time they have
been pushed up along with the surrounding region forming the Santa
Monica Mountains.
The above information was compiled from the
following sources:
- Volcanic Rocks
http://en.wikipedia.org/wiki/Igneous_rock
-
http://volcanoes.usgs.gov/Products/Pglossary/VolRocks.html
- Preliminary Geologic Map of the Los Angeles 30´
x 60´ Quadrangle, Southern California Version 1.0 Compiled by
Robert F. Yerkes and Russell H. Campbell Open-File Report 2005-1019
Detailed Description of Map Units, version 1.0
http://pubs.usgs.gov/of/2005/1019 U.S. Department of the Interior
U.S. Geological Survey
