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Alum Rock
Park, East San Jose
Refer to general
field trip information.
Alum
Rock Park, San Jose
Self-guided Field Trip
Alum Rock Park is
located in the western foothills of the Diablo Range of north-central
Santa Clara County. Located about 5 miles east of San Jose, the park represents
an opportunity to study a remarkable variety of geologic features. The
park was established in 1872 and is the oldest city park in California.
The entrance from
Alum Rock Avenue is closed due to a landslide. From San Jose State University
one must drive south on 280 to 680. Take the McKee Exit. Turn right or
toward the hills on McKee and left on White Road. Follow White Road to
Penitencia Creek Road. Turn right on Penitencia Creek Road. Enter the
park at this entrance. The park Website is:
http://www.ci.san-jose.ca.us/cae/parks/arp/index.html.
Purpose: to observe
first-hand, some of the geologic features we have talked about in class.
These include landslides, weathering processes, stream formation, and
several different types of rocks.
Field trip stops are
keyed to the map found on the attached map (handout) or on the map displayed
at the park Website. You do not need to complete the handout in order.
This will depend on where you park.
Stop I. Alum Rock
Rest Area (near the trolly trestle)
Look for the white
powdery alum on the surface of the rock.
1. Describe the rock: color, texture grain size.
2. This rock is a rhyolite that has been extensively altered.
a. What does this suggest about its origin?
b. Why do you think this rock stands up above the surrounding rocks?
c. How does this information relate to Bowen's reaction series?
Stop II. Mineral Springs
Loop Trail
Walk back to the parking lot by the Youth Science Institute. Take a left
at the gate leading to the Minerals Springs Loop Trail.
1. The rocks you are examining are named the Monterey Formation. This
is a siliceous-rich shale that was deposited in California during the
Miocene (about 15 million years ago). The shale is considered a biochemical
sedimentary rock because of the high organic content. Diatoms are single-celled
plants that live in the photic zone (upper 200 meters in the ocean). They
are abundant in this rock. This is the source rock for most of the oil
in California. Extensive sediment were deposited throughout southern California
due to a special oceanographic setting during the Miocene.
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a. Describe
the rock outcrop to the left of the creek in terms of color, grain
size, and sorting.
b. Draw a diagram of the outcrop or portion of the rock exposed
along the trail.
c. Describe the processes that produced what you see. (Remember:
sedimentary rocks are originally deposited in horizontal layers)
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2. Walk roughly northward
(straight, along the path) beside the creek. Examine the rocks in the
stream. You may want to walk to the bridge, you can walk closer to the
stream. You might get a better view of these rocks.
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a. What are
the rock's size and shape?
b. Describe the color, grain size, and any other characteristics.
These rocks are sandstone.
c. Describe and make sketches of the fossils in the rocks.
d. Do you think these rocks formed in ocean water or lakes? Explain
your reasoning.
e. Where do you think the original source for these rocks is located?
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3. Examine the water
in one or more of the grottos along the stream.
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a. Look for
stalactites (from above) and stalagmites (from below). Explain how
each of these features form.
b. Look for the tufa deposits. What is tufa and how do these deposits
form?
c. The water temperature in the grottos is cool, warm, or hot? Why?
d. Do you detect any odor? If so, describe.
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Stop III. The San
Jose Highlands Slide.
To get to the last stop, you can either walk or drive to the Penetencia
Road entrance. From the entrance parking lot, walk toward the park. After
crossing the stone bridge, you will come to a blocked road on the east
side of the road. Walk up this road to answer the following questions.
This area was developed in the late-1960s. All houses were removed by
the mid-1970s. Notice the topography of the area. The steep scarps now
covered by grass are the heads of landslides. The flat areas are where
the material moves. This is called hummocky topography.
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a. Draw a diagram
illustrating these features or the overall appearance of this area.
b. How would you rate the stability of the remaining houses?
c. Define the term mass movement.
d. What are the triggers to mass movement?
e. Why do you think this area is prone to mass movement?
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