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Rocks of Malibu Creek SP – Conejo Volcanics

A cache by TerryDad2 Send Message to Owner Message this owner
Hidden : 4/21/2008
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Geocache Description:

This stop takes you to an example of the igneous rocks that are found in Malibu Creek State Park.

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 :

  1. The text "GC1BFCN Rocks of Malibu Creek SP – Conejo Volcanics" on the first line
  2. The number of people in your group.
  3. What size and color range of the breccia?
  4. 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
  • 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 U.S. Department of the Interior U.S. Geological Survey

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