The trail to this location is an easy walk along a fire
road in Wildwood Park up until last few hundred feet. This
last stretch is a little steep in sections with the final
climb onto the rocks.
The Conejo Volcanics are a variety of lava flows composed of
basalt, andesitie, diacite, and combinations of these volcanic
rocks. These rocks and similar rocks can be found throughout the
Santa Monica Mountains and the Channel Islands. Conejo Mountain
(about 4 miles to the southwest across the 101 freeway) is one of
two major volcanic plugs that were source for these volcanic
rocks.
When the Pacific Plate and the North American Plate began to
slide past each other forming the San Andreas Fault. As these
plates began to slide past each other, a large block broke off and
began spinning around in between the two plates like a gear. This
block includes the Transverse Ranges and made up of the roughly
east-west mountain ranges in this area of California. Most other
mountains in California are roughly north-south trending.
As this block spun, gaps in the crust formed and were filled by
these various volcanics. The eruptions began in the Miocene about
17 million years ago and likely lasted for 1,000,000 years.
Initially the eruptions were under the ocean but eventually formed
an island with a pair of volcanoes that reached at least 5,000 feet
high. Since then the volcanic rocks have been tilted down to the
north and all the southern slope of the volcanoes have been eroded
away.
The rocks at this location are a volcanic breccia. There are a
variety of rock pieces (clasts) that are large and angular held in
a fine-grained volcanic matrix. The clasts are pieces of rock from
the surrounding area and prior eruptions that got broken up and
mixed in with lava as it erupted. Based on where these rocks are in
relation to the plug (Conejo Mountain to the southwest) this area
erupted on the side of the volcano into the air after the volcano
had built up out of the ocean (as compared to an underwater
eruption)
As you look out to the northeast, you can see another set of
cliffs made up of similar material. From this distance you can see
that there are distinct layers suggesting multiple eruptions that
built up on one another. You can also see the general tilt of these
layers.
Logging requirements:
Send me a note with :
- The text "GCZKJ4 Conejo Volcanics – andesitic breccia" on the
first line
- The number of people in your group.
- Based on your observations of the general tilt of the layers of
volcanic rocks in hills to the north and northeast, do you agree
with the direction of the tilt given above and do you think they
have all been tilted the same amount?
- How many different kinds of rock make up the clasts in the
breccia (you can base this just on color)?
The above information was compiled from the
following sources:
- VOLCANIC GEOLOGY OF THE SANTA MONICA MOUNTAINS,
Presenters: Eugene Fritsche, Roberta Harma, and Karen Savage. Coast
Geological 5-18-04 Newsletter regarding their program for the
evening. http://www.vgms.org/bt/vgms0406.htm
- Miocene: Rifting and rotation, volcanism,
crustal upwelling and deposition in marine basins, by Tanya
Atwater,
http://emvc.geol.ucsb.edu/download/powayeoc.php