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Ancient Mt. Shasta Debris Avalanche

A cache by TerryDad2 Send Message to Owner Message this owner
Hidden : 08/10/2009
3 out of 5
1 out of 5

Size: Size:   not chosen (not chosen)

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Geocache Description:

Between 300,000 and 360,000 years ago a huge debris avalance spread out from ancient Mt. Shasta Volcano covering the landscape as far as 43 km to the north with pieces of pyroxene andesite.

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 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 :

  1. The text "GC1X3AK Ancient Mt. Shasta Debris Avalanche" on the first line
  2. The number of people in your group.
  3. 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?
  4. As you travel north, does the height of the hills increase or decrease? Why?
  5. In the road cut, what type of avalanche facies does the freeway go through?
  6. 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

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