The
Salton Buttes are five small rhyolite domes, Red Island (actually
two domes), Rock Hill, Obsidian Butte, and Mullet Island. The
coordinates bring you to Red Island, but as you can see, it is not
an island. Rock Hill is located in the Sonny Bono Salton Sea
National Wildlife Refuge Complex. Obsidian Butte is on Imperial
Irrigation District property where there is no access from March 15
to June 15 for bird nesting. Mullet Island is really an island out
in the Salton Sea. Even though these domes are the highest points
in the area, their peaks are still below sea level.
These domes are located above the plate boundary between the
Pacific Plate to the west and the North American Plate to the east.
Deep underground the plates are moving away from each other forming
a spreading center. This area is the northern most part of the East
Pacific Rise which is a series of spreading centers that extends
southward beneath the Gulf of California.
As the plates move away from each other, magma from deep in the
earth wells up to fill in the gap that is created. This large body
of magma is tapped by the many geothermal power plants in the area
and is often called the Salton Sea Geothermal Field (Salton Sea Geothermal Field Earthcache ).
Periodically, the magma forces its way up to the service and
erupts. About 16,000 years ago, rhyolitic lava erupted through the
sediments to form these domes. (Igneous rocks are typically divided
up into four types based on the amount of silica they contain.
Silica is a common compound found in the earth’s crust.
(See
http://volcanoes.usgs.gov/Products/Pglossary/VolRocks.html for a
set of graphics on the classification of igneous rocks)
Rhyolite is a volcanic rock with the most silica content of the
igneous rocks (between 68 to 77%) and tends to be the coolest of
the igneous rocks (about 900 deg C). It is typically light red to
pink to tan and has very few if any visible crystals. Due to their
high silica content and relatively low temperature, molten rhyolite
is typically thick and chunky. This generates explosive eruptions
that tend to pile up close to the volcano or short stubby lava
flows.
Red Island (the given coordinates)
Two domes make up this island. As you might suspect, the dome gets
its name from the color of the rhyolite that make up much these
domes. The rhyolite is a red color with a variety of xenoliths
(Xenolith Earthcache and Peninsular Batholith Xenolith Earthcache )
within it. A xenolith is a piece of some other rock enclosed within
an igneous rock. The other rock is usually picked up from the walls
surrounding the igneous rock, or in this case during the explosive
eruption, and is frozen in place before it has a chance to
melt.
The rock at the given coordinates is a rhyolitic breccia because
of the number of large angular rock fragments (xenoliths) within
it. If you explore around the domes you should also find dark gray
rhyolite and obsidian.
These domes are thought to be subaqueous pyroclastic flows.
Pyroclastic flows are thick mixtures of hot, dry (not dry for long
if they were erupted under water) rock fragments and gasses that
are violently erupted from a volcano. The 1980 eruption of Mt. St.
Helens is an example of a pyroclastic flow.
Obsidian Butte (N33 10.245 W 115 38.051)
Only accessable June 16th to March
14th.
Obsidian Butte is a single dome of rhyolite. As you can see, the
character of this dome is quite different than Red Island. At the
coordinates, the dome is made up of gray to black rhyolite pumice
and obsidian. Pumice is a light air-filled volcanic glass. It can
be described as a rock sponge because of the rock is filled with
frozen gas bubbles. Pumice forms during explosive eruptions when
volcanic gasses that were dissolved in the magma suddenly come out
of solution and the magma cools quickly to trap the shape of the
bubbles. The effect is similar to the results of a shaken
carbonated drink after you open it and the foam instantly freezing.
Other types of magma will also form pumice. Elsewhere on the dome
you will find rhyolitic breccia.
Some areas of the dome have obsidian. Obsidian is a dense
volcanic glass. Obsidian forms under similar situations as pumice,
but there is not as much gas dissolved in the magma, so the glass
form a dense mass. Obsidian usually forms from rhyolitic lava. The
black color comes predominantly from tiny minerals of magnetite
trapped with in the glass. Obsidian fractures easily and forms very
sharp edges, so be careful.
These domes are not unique in the area. Deep wells drilled for
the hydrothermal plants have encountered various layers of rhyolite
and basalt that have been buried by the Colorado River sediments.
This shows that the area has been volcanically active for quite a
while and that we should expect future volcanic activity in the
area.
Logging requirements:
Send me a note with :
- The text "GCZ5WD Salton Buttes - Volcanic Domes" on the first
line
- The number of people in your group.
- Find a broken off piece of rhyolite. Before you pick it up
guess about how heavy you expect it to be. Then pick it up and see
how close you were (actual measurements aren't required just
relative observations).
- If you can get access to the second coordinates, perform the
same experiment with the rocks you find there.
- Send me a email with your results and a comparison of the
weight of the rocks found at each location. Include a reason why
they are different (or the same)
The following sources were used to generate this
cache:
- USGS, USGS/Cascades Volcano Observatory,
Vancouver, Washington , Salton Buttes Lava Domes,
http://vulcan.wr.usgs.gov/Volcanoes/California/SaltonButtes/description_salton_buttes.html,
11/09/01, Lyn Topinka
- USGS, Visual Glossary, Xenolith,
http://wrgis.wr.usgs.gov/docs/parks/rxmin/gxenolith.html, last
updated on 01/13/04
- USGS Volcano Hazards Program, Pyroclastic Flows
and Their Effects,
http://volcanoes.usgs.gov/Hazards/What/PF/pcflows.html, Last
modification: 20 December 1999 (SRB)
- USGS Volcano Hazards Program, Photo glossary of
volcano terms: Pumice,
http://volcanoes.usgs.gov/Products/Pglossary/pumice.html, Last
modification: Monday, 04-Sep-2000 22:44:39 EDT (SRB)
- USGS Volcano Hazards Program, Photo glossary of
volcano terms: Obsidian,
http://volcanoes.usgs.gov/Products/Pglossary/obsidian.html, Last
modification: Monday, 04-Sep-2000 22:44:29 EDT (SRB)
- PAUL T. ROBINSON and WILFRED A. ELDERS,
Quaternary volcanism in the Salton Sea geothermal field, Imperial
Valley, California, Geological Society of America Bulletin: Vol.
87, No. 3, pp. 347–360
- Photos by TerryDad2