Parking for this EarthCache is at either the north- or
southbound rest stop along I-5; however, the northbound rest
stop has a marginally better view. The secondary coordinates
are for a road cut along I-5, but there is no place to stop,
so it is a drive by view. If you are traveling north to south,
watch the hills to the east of the freeway as you approach the
coordinates so you can answer the logging questions. If
traveling north, pay attention to the secondary coordinates
(N41 25.763 W122 25.019)as you pass through them.
To the southeast of the coordinates is the modern Mt. Shasta
Volcano
(
Visit Earthcache). It is thought that this volcano is composed
of up of at least 4 individual volcanic cones that are made up of
pyroxene andesite
geology.com
link and
Wikipedia link)
The oldest of these is thought to be a bit more that 100,000 years
old. However, there are even older volcanic rocks (593,000 years
old) in the area indicating that older volcanic cone(s) existed in
the area.
It is from one of these older volcanoes that a massive debris
avalanche covered the area to the northwest with about 26 km3 of
debris. It is thought that this avalanche occurred between 300,000
and 380,000 years ago. The avalancehe has been described as one of
the largest known
Quaternary
landslides on Earth.
The avalanche is made up of two
facies, the block
facies and the matrix facies. These deposits are recognized over an
area covering 420 km2 and extends as far as 43 km to the northwest
near the Shasta River Gorge.
The block facies is made up of huge blocks of pyroxene andesite
that are up to hundreds of meters in diameter. These blocks form
the core of the many rolling hills and ridges that are seen from
the base of the modern Mt. Shasta Volcano to the northwest.
The matrix facies covers the tops of the block facies and the
flat areas between them. These deposits are made up of material
ranging in size from boulders to silty sand. These sediments are
also composed of pyroxene andecite.
The cause of the avalanche is not known, but based on historical
rock avalanches on modern volcanos it could have been started by a
steam explosion or earthquake. It is unlikely that an eruption
started the avalanche since there is no evidence of a magma
blast.
Stopping at the secondary coordinates is illegal since it is at
a road cut on I-5. This will be a hard one to view since it will
have to be done at freeway speeds.
Logging requirements:
Send me a note with :
- The text "GC1X3AK Ancient Mt. Shasta Debris Avalanche" on the
first line
- The number of people in your group.
- At the coordinates, what is the range of height of the hills to
the east. Are these made up of the block or matrix facies?
- As you travel north, does the height of the hills increase or
decrease? Why?
- In the road cut, what type of avalanche facies does the freeway
go through?
- What has been done to the road cut to protect the
road?
The following sources were used to generate this
cache:
- Crandell, et al, 2009, Catastrophic debris
avalanche from ancestral Mount Shasta volcano, California, Geology,
1984; 12; 143-146
- Crandell, Gigantic Debris Avalanche of
Pleistocene Age from Ancestral Mount Shasta Volcano, California,
and Debris-Avalanche Hazard Zonation, U.S. Geological Survey
Bulletin 1861