El Modena Volcanics EarthCache
Hidden : 9/4/2012
Difficulty:
1 out of 5
Terrain:
2.5 out of 5

Size: Size: other (other)

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Geocache Description:


Did you know there are extinct volcanoes in the middle of Orange county?

On the eastern edge of Orange is a small volcanic range that emerged during the middle-to-late Miocene period, extending from 23 to 5 million years ago. Consisting of extrusive pyroclastic and flow rocks, it has three members: a basalt flow, measuring between 10 and 200 feet thick; palagonite tuff and tuff breccia, which averages 200 to 300 feet in thickness; and an andesite flow and flow breccia member, which averages 200 feet in thickness.

You are encouraged to park at the posted coords in order to view the following three features: 1) a series of dikes visible from Cannon Road, 2) a fault near the trailhead, and 3) a sample of vesicular andesite found near (or on the way to) some of the peaks. There are, of course, many other wonderful features, but these three are a good starting point for exploring the area.

I. DIKE (N33 48.109, W117 47.542)

Dikes are sheet intrusions that cut across rock structures or formations; the molten magma eventually solidifies and later appears as bands of different color or composition. Often vertical in orientation, they are typically thinner than the surrounding layers. The same type of intrusion, when found more parallel to other layers, is referred to as a "sill." Dike thickness can vary from a few millimeters to several meters, but the ones in this area can be measured in inches or feet. Locate at least one dike. What angle is it at (0 degrees is horizontal, 90 degrees is completely vertical). Is it continuous or are there visible breaks in the dike?

II. FAULT (N33 47.873, W117 47.505)

A fault is a fracture in rock, resulting in some displacement or movement of earth. At this spot, look to the west. You'll see a small fault about 15-20 feet above street level. It may be hard to see, but look for color differences and layer separation. A small fennel bush appears centered in the fault line. How far would you say the fault has moved (in inches or feet)?

III. VESICULAR ANDESITE (N33 47.908, W117 47.568)

Vesicular andesite, an igneous rock, is characterized by a pitted surface, the result of gas bubbles forming in hot magma which later cools and preserves the "path" taken by the bubbles. Follow the trail up the hill and find a sample of vesicular andesite. (You may have to expand your search area a bit.) Describe it, answering at least two of the following questions: Is it heavier or lighter than other rocks of the same size? What do you think the color of the rock means? How many cavities (bubbles) are there per square inch? How does your sample compare to other rocks of the same type found at the bottom of the hill?

Please send an e-mail with your answers to the cache owner. Photographs of the various waypoints are encouraged, but not required.

Sources:

1) In-field interview with, and documentation prepared by, members of the Department of Geological Sciences, California State University Fullerton.

2) "Volcanic Rocks of the El Modeno Area Orange County California; Description of extrusive pyroclastic and flow rocks of El Modeno volcanics of middle to late Miocene age" by Robert F. Yerkes, published in 1957 by the U.S. Geological Survey as Professional Paper 274-L.

3) "A Natural History of California" by Allan A. Schoenherr, published in 1992 by the University of California Press.

4) "Mineral Information Service," Volume 10, Number 8, published August 1, 1957 by the California State Division of Mines.

"Look deep into nature, and then you will understand everything better."--Albert Einstein

Cache Maintenance Record
File No. GC3W57A
Last Checked: NO MAINTENANCE REQUIRED!

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