Crater Lake (Mount Mazama)
http://www.hotstockmarket.com
Kristen Kenny
Intro to Geology 1010
TR 10 AM
When my friend asked me to go to Oregon with her to visit family I had asked
immediately if we could make a stop to see Crater Lake. At that time the only knowledge I knew
about the lake was the gorgeous blue of the water that I had to gaze upon. Arriving there and
learning more about its origin captivated me more than the color of the lake. I found myself
excited for this project and privileged to learn more about the volcano that I had visited.
The Formation:
Let’s start with the plate tectonics and how they were to form Mount
Mazama. The Juan de Fuca plate is an oceanic plate and is denser than the continental North
American plate which overrides it. As a result of this process, magma is forced to the surface
leading to the formation of volcanoes. Subduction zones are often referred to as destructive
zones because the oceanic lithosphere is destroyed in these areas where it is sub ducted (Bacon,
1997). Mount Mazama is part of a chain of stratovolcanoes that extends along the crest of the
Cascade Range from Lassen Peak in California to Mount Garibaldi in British Columbia
(Williams, 1972). It began to grow half a million years ago with the oldest rocks visible today
around 420,000 years old. Mount Mazama was constructed by episodic growth of many
overlapping shield and composite volcanoes, each of which probably was active for a
comparatively brief period (Bacon, 1997). Lava flowed from many volcanic vents overlapping
and building an ill-shaped mountain (Mount Mazama) about 8,000 years ago it may have stood
as much as 12,000 feet tall. Around Mazama are cinder cones, lava fields, and small shield
volcanoes that produced calc-alkaline basalt and andesite (Warfied, Juillerat, & Smith, 1999).
These range in age from around 600,000 years to perhaps 40,000 years ago and are similar to
monogenetic (one generation in the life cycle) volcanoes up and down the High Cascades.
The Eruption:
Crater Lake lies inside the ancient volcano Mt. Mazama the greatest eruption
of this volcano was 7,700 years ago and was the largest to occur in North America over half a
million years ago (Wood, 1990). The magma chamber was emptying and the underlying support
for the mountain was lost and the walls of the volcano began to collapse. The eruption spewed
50 cubic kilometers of magma (Bacon, 1997) and in just a matter of days the top of the mountain
was destroyed and left a caldera. A caldera is a word used by geologists to describe large basinshaped volcanic depressions (Williams,1972). The erupted magma was mainly andesite and a
column of hot gas with volcanic rock was ejected high into the air. The magma that happened to
fall to the earth was fragments of frothy white pumice (Klung, 2001).
http://www.nps.gov/crla
The Sequence of Events: How Crater Lake was formed.
Mount Mazama’s eruptive style, in the last 70,000 years saw more highly explosive
eruptions of silicic magma (dacite and rhyodacite). In the last 30,000 years the only record of
activity, prior to the caldera forming climactic eruption of 7,700 years ago, was limited to a small
number of preclimactic pyroclastic (fragments of volcanic origin) eruptions and ensuing lava
flows of rhyodacite (Warfied, 1999). With these kinds of eruptions Crater Lakes eruption style
happens to be strombolian which builds cinder cones (common volcanic landform), and
disperses incandescent fragments (Houghton, 1999). Subsequent to the climactic eruption all
volcanic activity has occurred within the caldera itself. Wizard Island is a cinder cone and lava
flows of post caldera andesite, erupted soon after the caldera formed. Several more post caldera
volcanoes are hidden by the 1,943 foot lake (Warfied, 1999). Filled by the rain and snow of
Oregon’s west coast, Crater Lake is one of the nation’s deepest lakes of today (Wood, 1990).
http://cacmp.org/cmpor/Crater%20Lake
Recent Activity:
The biggest threat from young calderas is of renewed volcanic activity.
As of now, no volcanic activity as big as the caldera forming one is likely to occur and has had
been dormant from volcanic activity for the last 5,000 years. Sediment studies show no evidence
of magma movement beneath the earth’s surface as well as no earthquakes showing volcanic
evidence. Although one thing geologists are worried about is of the caldera being filled with
water producing tremendous flooding if the caldera wall fails. The last major wall failure was
200 years after the cataclysmic eruption of Mount Mazama (NPS, 2012). For now, Crater Lake
shows no sign of wall failure and is safe to enjoy year round with weather permitting. When I
visiting it was an overwhelming site to take in, the massiveness of the lake and the tones of blue
of the water is something everyone should go and witness.
REFERENCES
Bacon, Mastin, Scott, and Nathenson. (1997) Volcano and Earthquake Hazards in the Crater
Lake Region, Oregon: USGS Open-File Report 97-487
Houghton, Bruce (1999). Encyclopedia of Volcanoes. Academic Press
Klung, C.; Cashmen, K & Baco, C. (2001). Structure and Physical Characteristics of Pumice
from the Climatic eruption of Mount Mazama (Crater Lake), Oregon. Springer - Verlag
Warfied, R; Juillerat, L. & Smith, L. (1999). Crater Lake: The Story Behind the Scenery, KC
Publishing.
Wood, C.A. & J. Kienle (eds) 1990. Volcanoes of North America (United States and Canada).
Cambridge: Cambridge University Press.
Williams, Howell. (1972) Crater Lake: The Story of its Origin, University of California Press.
http://www.nps.gov/crla
PICTURE REFERENCES
http://www.nps.gov/crla
http://www.hotstockmarket.com/t/76948/random-thread-is-random/craterlake
http://cacmp.org/cmpor/Crater%20Lake