A GEOLOGIC STUDY OF

GEOLOGY 103FIELD ASSIGNMENTBY DON GIESEN

Once an active volcano over 10 million yeas ago Round Top is one of four volcanos in the region. One of the highest peaks in the area, Round Top is 1,763 feet above sea level ("Sibley Volcanic Regional Preserve," n.d.). At the time of its eruption this volcano was “born southeast of modern-day San Jose (Slack, 2005). Over the 10 million years the land has shifted and upturned along the Wildcat and Calaveras Faults, and through excavation of materials from rock quarries in the 1940’s, layers have been exposed showing sedimentation underneath lava flows (Slack, 2005).

*Sibley Volcanic Regional Preserve

Illustration by Carl Buell (Slack, 2005)

This is what the eruption may have look like 10.2 million years ago, looked upon by the long-necked camels, three-toed horses, and other creatures form the middle Miocene Epoch. Today, modern horses are permitted to be ridden through parts of the park, however they are descendants of horses brought from the east when settlers traveled to North America.

(Zell, 2010)

California PoppyEschscholzia californica

Native PerennialPoppy Family

(Legard, 2006)

Ithuriel's Spear Triteleia laxaNative Perennial Lily Family(Legard, 2006)

May weedAnthemis cotula

Introduced AnnualSunflower Family(Legard, 2006)

The big leaves are that of the Cow Parsnip

Heracleum lanatumNative Perennial

Carrot Family(Legard, 2006)

This are is home of many native and introduced wild flowers. There are over “2,000 species of native and naturalized plants [which] grow wild in the San Francisco Bay Area” (Legard, 2006).

Left is a basalt dike which fed into the crater of the volcano. As the dike cut through the surrounding pebbly mudstone is created

Autoclastic basaltic breccia, seen on the left.("Sibley Volcanic Regional Preserve," n.d.)

*Quarry and Labyrinth

Seen here is a large pit dug for quarying basalt, which began in the

1940’s (Slack, 2005). What was exposed is the interior of Round Top

volcano you can see, on the steep cliff, lave which capped off the crater after it was filled ("Sibley Volcanic Regional

Preserve," n.d.).

Here you can see a large conglomerate of river gravels, sand, and mudstone, known as Orinda Formation ("Sibley Volcanic Regional Preserve,"

n.d.). These formations sometimes contain fossils of plants and animals. ("Sibley Volcanic

Regional Preserve," n.d.)

In another quarry pit there can be seen a set of thick lava flows tilted, nearly vertical, from uplift. ("Sibley Volcanic Regional Preserve," n.d.). The uplift caused and an angular unconformity, seen in the picture on the left.

Here is a sequence of lava (right) atop basaltic ash, the resulting brown on the right is do to the high temperatures of the lava baking the layer underneath it. These bake zones cause the steam which makes the iron in the bands to oxidize make the red color ("Sibley Volcanic Regional Preserve," n.d.)

("Sibley Volcanic Regional Preserve," n.d.)

At the top of this picture you can see the possible beginnings of an angular

unconformity. As weathering and erosion have eaten away at this tilted

sedimentation, dirt and soil have been deposited on top.

Here is yet another example of severe shifting and uplift as a result of tectonic shifting along fault lines, which shows two separate layers of lava flows have been lifted. It can be assumed that the lava flow on the left is younger than that on the right, through the law of superposition, because you can see the red and brown bake zone to the left of the lava flow.

*An uplifted crosscut of two separate lava flows with a period of sedimentation between them.

References

Legard, W. (2006). Sibley volcanic wildflowers. Retrieved June 12, 2016, from http://www.ebparks.org/Asset2341.aspx

Monroe, J. S., & Wicander, R. (2015). The changing earth: Exploring geology and evolution (7th ed.). Stamford, CT: Cengage.

Sibley Volcanic Regional Preserve. (n.d.). Retrieved from http://www.ebparks.org/parks/sibley.htm

Slack, G. (2005, April 1). Voice of the Volcano - Bay Nature. Retrieved from http://baynature.org/article/voice-of-the-volcano/

Zell, H. (2010, September 29). Equine evolution [Skeletal Evolution]. Retrieved from

https://commons.wikimedia.org/wiki/File:Equine_evolution.jpg