Volcanoes are the surface manifestations of molten rock, or magma,
escaping from the magma chambers as lava. Volcanoes come in many
different shapes and sizes, largely dependent upon the chemical
make-up of the lava extruded. Fluidic basalt eruptions, such as the
aa eruptions of Hawaii, spread wide and add material in
relatively thin layers creating wide, massive shield
volcanoes. Medicine Lake volcano near the Lava Beds National
Monument is a California example of this kind of volcano.
Typically, the viscidity of lava in Northern California is much
higher and this creates eruptions that build volcanic domes and
with repeated eruption history, composite volcanoes or
stratovolcanoes. Mount Shasta is our California poster-child
volcano of this sort.
To earn this Earthcache, you’ll need to take a picture of you
holding your GPSr (a must) with Mount Saint John
behind you (azimuth bearing 358º, zero declination) at the
coordinates (so no pictures from Stoneyford, please). In addition,
send via e-mail, the azimuth bearing of the three volcanoes visible
from this vista: Mount Lassen, North Butte of the Sutter Buttes,
and Mt. Konocti. Set your compass to zero declination when taking
your azimuth readings.
More Technical Information
The following information comes from the USGS Cascades Volcano Observatory
website.
Lassen Peak is the largest of a group of more than 30
volcanic domes erupted over the past 300,000 years in Lassen
Volcanic National Park in northern California. These mound-shaped
accumulations of volcanic rock, called lava domes, were created by
eruptions of lava too viscous to readily flow away from its source.
Eruptions about 27,000 years ago formed Lassen Peak, probably
within only a few years. With a height of 2,000 feet and a volume
of half a cubic mile, it is one of the largest lava domes on
Earth.
Sutter Buttes is an anomalous volcanic landform rising
starkly from the flat plain of the Sacramento Valley. It is the
smallest mountain range in the world. Rising magmas uplifted
Cretaceous and Tertiary sediments, which quickly eroded. Explosive
eruptions, extending perhaps 0.5 million years, accompanied the
emplacement of viscous intrusions and extrusions at the center and
periphery of the uplift. These pelean domes, which strongly
uparched the intruded beds, are andesitic in the central core of
Sutter Buttes, and are surrounded by a halo of dacitic to rhyolitic
domes.
Mt. Konocti is a composite dacitic volcanic dome that
formed 350,000 years ago. It towers above the southern shore of
Clear Lake and is most prominent feature of the Clear Lake Volcanic
Field. The volcanic field has numerous vents and eruptive units
that can be grouped into four major episodes of activity, the first
at around 2.1 million years ago and the most recent about 10,000
years ago. A large silicic magma body about 14 km in diameter lies
some 7 km below the surface in the Clear Lake Volcanic area. This
magma bodies fuels The Geysers area which is the world’s largest
largest producing geothermal field for electricity.