A Geologic Study of Robert Sibley Volcanic Regional Preserve

1. A GEOLOGIC
STUDY OF
GEOLOGY 103
FIELD ASSIGNMENT
BY DON GIESEN

2. 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

3. 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)

4. California Poppy
Eschscholzia californica
Native Perennial
Poppy Family
(Legard, 2006)
Ithuriel's Spear
Triteleia laxa
Native Perennial
Lily Family
(Legard, 2006)
May weed
Anthemis cotula
Introduced Annual
Sunflower Family
(Legard, 2006)
The big leaves are that of the
Cow Parsnip
Heracleum lanatum
Native 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).

5. 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.)

6. *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.).

7. 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.)

8. 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.

9. 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.

10. 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.

11. 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