AboutDr Thomas Bell Expertise I can answer questions regarding surface earth processes and the chemical transformations that sediments and rocks undergo with burial. I can also answer questions regarding deep time, the evolution of the elements, and the last 4.5 billion years of earth history. I specialize in metallic ore forming processes, the major geologic time periods when they were produced and what they tell us about the evolution of our planet. Learn more about my professional interests at Stratamodel.com.
Experience I am a professional consulting geologist with a background in the petroleum, mining, environmental, and geotechnical industries with over 25 years of experience.
Education/Credentials Ph.D., Geology, University of California at Berkeley, 1984
M.A., Geology, University of California at Berkeley, 1980
B.S., Geology, San Jose State University, 1978
Expert: Dr Thomas Bell Date: 5/23/2008 Subject: round chert-like rocks
Question I have found many hand-sized rocks in a field near where I live (border of Los Angeles-Ventura counties, California). They are white, translucent white to bluish white, often with creases. Some have flow marks and under hand lens have bubble marks .Some are smooth and rounded on one side and flat and dimpled on the other (as if cooled on something like a sponge). Also I have found many tiny moonstone-type pebbles of translucent white to blue. Have also found these same types in a soft green lava-like boulder.
My question is...are these agates (some have bands, cavities with nearly formed crystals) or cherts or....things from outer space:}? just kidding. This area has been under the sea often in the past couple hundred million years, plus some volcanic action, etc. I was wondering if my specimens were formed by sedimentation/layering of chemicals or by being spewed out of a volcano. The variety of crystalline shapes/cavities in the rocks is wonderful.
I am afraid that the field i have been exploring is going to be turned into yet another office or condo complex, so am collecting all my children and I can. Thank you. Kathy Riser
Answer Kathy,
Agate is microcrystalline quartz and is not really a geologic term. This is a term lapidarist's use for colored or banded chert. You can test your samples by trying to scratch the surface with a screw driver or knife blade. If it is chert, you will not be able to scratch the surface and you may even see that some of the steel from you tool has come off on the surface.
Sounds like you are looking at the Miocene Conejo Volcanics. There are lots of interesting volcanic structures in these rocks and they have produced some very interesting landscape. Mobile silica is common to nearly all volcanic rocks as they are erupted, when they weather after eruption, and when they are buried by younger rocks. Nearly all volcanic rocks have some amount of glass associated with them. Volcanic glass is produced when magma is cooled before mineral crystals can form. All glass is fairly reactive and readily dissolves in water releasing the silica to form low temperature minerals like microcrystalline quartz (chert). Voids left in the volcanic rocks during eruption like vesicles (small gas bubbles preserved in lava), open fractures, and even the molds of burned vegetation can be filled with chert as hydrothermal or even cool groundwater dumps this excess silica to fill the voids.
If you want to learn a little more about your local geology, try this website: http://www.csun.edu/~hcgeo007/day1.html#10 or use a search engine to find out more about the Conejo Volcanics.
