Radiolarian Chert Folds - Chevrons EarthCache
Hidden : 1/6/2009
Difficulty:
2 out of 5
Terrain:
2 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:




Geology of Mount Diablo



Although Mt. Diablo is one of California's best-known landmarks, the details of its origin have long been cloaked in mystery. To try to understand the mountain's history, we must look far back in time and far beyond the peak itself, at geologic processes operating on a global scale.

Scientists have discovered that the earth's crust is divided into large fragments or "plates" that slowly move relative to one another. In fact, California appears to be the product of a long-term collision between two such plates: one including most of North America, and the other much of the Pacific Ocean floor.

Banded chert bed. Originally formed as a sedimentary cover on a distant ocean floor. In the distant geologic past, at least 200 million years ago, the ocean floor began to slide like a giant conveyor belt beneath the western edge of our continent. During this process, known as "subduction", material "riding" on the oceanic plate was scraped off on the continental edge, or stuffed under it. Ultimately some of these jumbled scrapings, called "melange", would form the rugged upper part of Mt. Diablo, but for the time being this melange material accumulated offshore.

Between the melange and the continental uplands to the east a basin formed, filled with the waters of a prehistoric sea. This sea covered much of what someday would become California's Coast Ranges and Central Valley. Into the sea washed layer upon layer of sediment, over an almost unimaginable span of time: through eighty million years when giant reptiles ruled the earth, then through another sixty million years while land masses rose and eroded back into the sea, and the climate gradually became cooler and drier. As the eons passed, countless plants and animals lived and died here, some leaving their remains in the ever-thickening beds of sand and mud.

About twenty million years ago the collision process changed in this region. The great plates continued to move past each other, but laterally now, with the San Andreas fault system developing to accommodate the sideways motions. This rearrangement of geologic forces also led to the creation of the Coast Range mountains. Sea water retreated from the area for the final time.

According to current theory, within the last four million years local fault activity brought a slab of old ocean floor into contact with the layered beds of sediment. Then, during the last million years or so, powerful compressive forces caused the land to buckle, producing a massive fold. In the center of the fold, the resistant slab of old ocean floor material became exposed in bold relief as the softer surrounding strata were worn away. And so the landform that we recognize as Mt. Diablo was born.

Even now, the mountain's evolution is not complete. Geologists suspect the peak may still be rising. Landslides re-shape its contours from time to time, and the erosion carries fresh sediment each winter to the valley flats. Every jolting earthquake and every tumbling rock are new evidence of Mt. Diablo's dynamic nature and new additions to an ancient story that continues to unfold.

The above text was taken from the video presentation "Creating A Mountain" that is part of the geology display in the Summit Museum. (ã California State Parks, 1993)



Radiolarian Chert Folds



At this location on Mt. Diablo you will see beds of chert, which itself is made of fossils, the recrystallized silica skeletons of microscopic sea creatures called radiolarians. When the radiolarians die, their skeletons slowly settle to the sea floor, where they form an ooze that eventually turns to this ribbon-like rock.

Chert forms when radiolarian skeletons, silt, and ash settle to the ocean floor together and lithify (turn to rock). The ooze must be several meters thick before this change, called diagenesis, occurs. The skeletons build up very slowly; only about 1 millimeter collects every 1,000 years.

These examples of chert are among the oldest rocks in the park. You can sometimes see the tiny radiolarian skeletons if you lick the rock to wet it and then peer through a hand lens. The skeletons look like opaque pinheads.

To log this cache, you will need to complete the following:
- Email the cache owner the (a) Color of the Chert Forms (b) The general age of the Chert Forms and (c) How were the rocks able to bend?
- Post a picture of You and your GPS along with the Chert Forms.


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