Hollister and Pinnacles
Field Trip Notes
The goal of this field trip is to see examples of geomorphology related to the Calaveras
Fault and the volcanic rocks associated with the Pinnacles National Monument. The
Calaveras Fault is a transform fault. In Hollister, we will see examples of offset curbs and
other affects of the fault in an urban setting.
Stop #1 The Calaveras Fault at Hollister
This section of the Calaveras fault is creeping (see all the EQs in the diagram
below). We will look for evidence for the creep as we look at cultural features (houses,
curbs, and sidewalks) as we walk around in downtown Hollister. What evidence do you
see of the Calaveras Fault? What is the sense of offset along the Calaveras Fault?
The diagram below shows a cross section of seismicity along the SAF. Note all of the EQ
activity south of San Juan Bautista and how little there is to the north. The line labeled
“1989” represents the 1989 Loma Prieta EQ and its associated aftershocks. Much of this
region would be filled in with seismicity if this diagram had been made within the last 10
years.
Stop #5 The Pinnacles
We’ll spend the afternoon at the Pinnacles. The Pinnacles Volcanic Formation (PVF) in
San Benito County is an early Miocene (22Ma to 24Ma) sequence of andesite, dacite, and
rhyolite flows with interbedded pyroclastic and volcaniclastic rocks. Most of what we
will see is the pyroclastic and volcaniclastic rocks.The volcanic field is located within the
Salinian block, east of the San Andreas Fault. The volcanic rocks form a west-dipping
sequence, truncated on the northeast by the Chalone Creek Fault, which is an early trace
of the San Andreas Fault (the San Andreas Transform has migrated through time). The
Neenach Volcanic Formation (NVF) in located near Gorman, California in the
westernmost Mojave Desert east of the San Andreas Fault. The Neenach volcanics are
also composed of the same calc-alkaline andesite, dacite, and rhyolite flows with
interbedded pyroclastic and volcaniclastic rocks as the PVF (see Fig. 14-9). Structurally,
the NVFdips northwest and is broken by many faults and truncated on the southwest by
the San Andreas Fault.
Work done by former UCSC graduate student Vincent Mathews suggests that these two
formations were contiguous across the San Andreas Fault Zone at the time of their
formation 23.5 million years ago and have since been separated and transported via the
San Andreas Fault to their present locations. The Pinnacles-Neenach correlation provides
some of the most conclusive evidence of large-scale, right-lateral displacement on the
San Andreas Fault Zone. The criteria are as follows: (1) A precise distance of separation
can be determined because both areas are directly adjacent to traces of the San Andreas
Transform; (2) The age of the formations can be can be determined accurately because
the rocks are volcanic and suitable for isotopic age dating; (3) The correlation is based on
10 different rock types, each with distinct petrologic features.
At what kind of plate boundary does rhyolite typically erupt?
Is this the same kind of plate boundary that exits today in this region?
What are the sedimentary and igneous features of these rocks you see at the Pinnacles?
What about their eruption caused them to contain aspects of both igneous and
sedimentary rocks?
What is special about the rocks at the Pinnacles to allow them to be used to measure slip
rates along the San Andreas Fault?
Directions
We will make two field trip stops Hollister and the Pinnacles.
1. Leave MPC and drive north on highway 1.
2. Take the highway 156 exit and drive east on 156 to highway 101.
3. Drive north on highway 101.
4. Take the 156 East exit towards San Juan Batista
5. Drive east on 156.
6. At approximately 3 or 4 miles past San Juan Batista, turn right onto San Juan
Road towards downtown Hollister. (follow the signs to downtown Hollister)
7. San Juan Road will turn into 4th Street. (I believe this is a 156 business route as
well)
8. As you get into a residential neighborhood near downtown Hollister, turn right on
Powell street. (turn right on West street if you miss Powell)
9. Turn left on 6th street and park near Dunne Park. It’s a nice little park with swings
and grassy fields.
10. Meet for a tour of the Calaveras Fault.
11. Back to the cars to continue East to San Benito Street.
12. Turn right on San Benito Stree, which is also State route 25. Follow signs to keep
driving south on state route 25 towards and through the small town of Tres Pinos
and Paicines. You will find no stores or gas stations south of Hollister.
13. Drive for a long ways south on highway 25.
14. Turn right (west) on state route 146. We will probably regroup at the intersection
of 25 and 146. We will drive as a group to park entrance station and then to the
parking area a little farther up the road. You will get into the park for free.
15. Enjoy the Pinnacles
16. Drive home the way you came. One small shortcut is to bypass downtown
Hollister by turning left on Union Road just before you get back to downtown
Hollister.
17. I hope to return by 4 pm.