Geography of Castle Lake and the Mt. Shasta Region
The uplift of the Cascade Mountains, northwestern United States, is a result of plate tectonics. The Juan de Fuca oceanic plate collides with the North American continental plate forming a convergent plate boundary, creating a subduction zone. The denser oceanic plate descends under the continental plate, and the oceanic crust heats, melts, and forms magma underneath the earth's surface. The Cascade Mountains are a series of volcanoes ranging from Mt. Lassen (California) in the south up to Meager Mountain (British Columbia) in the north.
Castle Lake Geography
Castle Lake and the Limnological Research Laboratory is located southwest of Mt. Shasta City in the Shasta/Trinity National Forest of northern California.
Castle Lake was formed during the Pleistocene Era over 10,000 years ago when much of North America was covered with glaciers. A glacier carved out the basin in which Castle Lake lies today. Castle Lake is a typical glacier 'cirque' lake. Cirque is a French word meaning semicircle or amphitheater. Cirque lakes are the deepest against the steep rock wall--also known as the cirque face-- where the glacier did most of the eroding. The northeastern shore of the lake is a terminal moraine of boulders and gravel that form a natural dam.
The rock composition within the Castle Lake basin is generally very poor in the essential plant nutrients nitrogen and phosphorus, which partially explains the great clarity of Castle Lake and why it is only moderately productive.
Against the cirque face of Castle Lake, the waters are up to 110 feet deep (35 M). At the other end of the lake is the outlet where the lake's depth ranges from 10 to 15 feet (3 to 5 M).
Bedrock Geology
The Castle Lake basin primarily contains dense mafic and ultramafic rocks that solidified from magma underneath the oceanic plate 400 to 500 million years ago. Usually, as dense oceanic plates move towards lighter continental plates, they sink and re-melt. Surprisingly, these oceanic rocks were moved on top of the continental plate and were preserved. Oceanic rocks that are thrust on top of continental rocks instead of subducted are called ophiolites. After the ophiolite was in place on the continent, magma intruded into these oceanic rocks 250 to 300 million years ago forming diorite and granodiorite plutons. These plutons are more resistant to erosion than the oceanic gabbros. This explains why the diorite and granodiorite rocks are found on the ridges surrounding the Castle Lake basin.
Castle Lake Mapping
Google Earth
The following file is the geological layer exported for Google Earth. Click on the link below to launch Google Earth and automatically pan to Castle Lake with the geological layer overlaying the terrain. Click on the labels for a descriptions of the symbol, or scroll down toward the bottom of this page for more details.
Shape Files
The following are shape files (.shp) which can be brought into ArcGIS, QGIS, or other GIS system which can read this format. GIS Mapping of Castle Lake is a work in progress, and we hope to add and improve these files over time.
cal_lake : Lake Layer
cal_outflow : Stream Outflow
cal_geol : Geology Layer
cal_geom : Geomorphology Layer
cal_dikes : Dikes
cal_faults : Faults
Coordinate System / Projection
NAD83 California Teale Albers
Download Shapefiles
Geology Map Unit Descriptions
- Qu - Quaternary deposits - glacial and stream deposits
- Pcc - Permian Castle Crags granodiorite
- Ph - Permian diorite dikes and plugs
- Oig - Ordovician, intrusive hornblende clinopyroxene gabbro with flow layering
- Olg - Ordovician, layered gabbro hornblende-bearing clinopyroxene gabbro and orthopyroxene-clinopyroxene gabbro
- Ou - Ordovician, cumulus ultramafic rock of the Castle Lake layered mass; includes dunite, wehrlite, olivine clinopyroxene, and clinopyroxene
- pOu - pre-Ordovician, serpentinized, metamorphic peridotite; the peridotite is the basal member of the Trinity ophiolite sequence
Credits
This GIS work was done by Brooke Eustis, with some assistance from Ryan Boynton and Dave Waetjen.
Mt. Shasta
Mt. Shasta is a majestic mountain. It rises 14,179 ft. (4,322 meters) above sea level, and is the second tallest peak in the Cascade range (behind Rainier) and the fifth tallest in California.
Mt. Shasta is the said home of the ancient lemurians. For more information on this lore, visit the following website:
Medicine Lake
Introduction
The Medicine Lake Shield volcano is located in the rain-shadow north-east-east of Mt. Shasta (Latitude: 41.58 N, Longitude: 121.57 W). While it is still considered part of the Cascade chain of volcanoes, most of the others are stratovolcanoes, having steeper slopes and a higher elevation. The region is filled with remnants of past eruptions, which include Lava Beds National Monument, Glass Mountain, numerous smaller cinder cones, and Medicine Lake itself, from which Medicine Lake volcano derives its name.
Medicine Lake Shield Volcano
The Medicine Lake shield volcano is the largest volume volcano in the Cascade range (Ritter and Evans, 1997). This is a bit surprising since it practically sits in the shadow of Mt. Shasta and is significantly smaller in elevation; Mt. Shasta is 4,317 meters above sea level while the Medicine Lake shield volcano is only 2,412 meters. But the Medicine Lake shield volcano has broad, gentle slopes built from the eruption of basalt that flows easily across the ground while Mt. Shasta is a stratovolcano, also called a composite volcano, formed by alternating layers of pyroclastics and lava that have accumulated over thousands of years to form a tall cone. The total area of the flows from the Medicine Lake shield volcano is much greater than those of the stratovolcanoes of the Cascade range, including Mt. Shasta.
Lava flows from the Medicine Lake volcano cover an area of approximately 2000 km2, approximately 40 km east to west and 50 north to south (Ritter and Evans, 1997). The flows are approximately one kilometer thick. The flows are composed chiefly of rhyolitic, basaltic, basaltic andesitic, and andisitic rock.
Most eruptions initially occurred during the Pleistocene (1.8 million years ago to 10,000 years ago) and later in the the Holocene (10,000 years ago to present) with the most recent eruptions, forming Glass Mountain, occurring approximately 1000 years ago (Ritter and Evans, 1997). The volcano rests on top of a Tertiary and early Quaternary volcanic plateau (Condie and Hayslip, 1974).
Looking at a geologic map of the region, one can see the vast flows emanating from the caldera rim, made up of the chief compounds described above. The most notable features when looking at the region are the 100+ Quaternary and Pliocene cinder cones that formed in the region. A cinder cone is a steep sided volcano (sometimes 40 degrees) and usually not more than a few 100 ft high. These cinder cones were formed from pyroclastics (rock fragments or congealed lava) spewed from a single vent in the earth. The fragments fall around the base of the vent, forming the cone shaped mound. Many of these cinder cones are now covered in vegetation, but the shape and size provide a clue to their identity.
Medicine Lake
Medicine Lake sits in the caldera, the depression near the eruption site. The lakes has an area of 1.65 square kilometers, a volume of 10,860 acre feet, a maximum depth of 46 meters and a median depth of 5 meters. The lake bed is composed of relatively course rhyolitic material producing a high pitch scraping sound when trampled.
The rim of the caldera is roughly 7 x 12 kilometers. It is speculated that no single eruption caused the formation of the caldera, but rather it collapsed as a result of a series of lava extrusions early in the volcano's history.
The lake has cultural significance to several Native American tribes. For example, the Pit River people believe the Creator's spirit is in the lake, providing powers to heal and renew oneself. For this reason, the area has been a training ground for medicine men. Although it has been designated as a Traditional Cultural District, this is being threatened by the construction of a geothermal plant that is to be constructed one mile from the lake, an initiative approved by the Bush administration.
Little Glass Mountain
The eruption that created Little Glass Mountain occurred less than 1000 years ago, just west of the caldera rim of the Medicine Lake shield volcano. The flows are composed mainly of rhyolite and dacite, which include extremely large obsidian, pumice, and olivine boulders. Many of the olivine specimens have many pore spaces that are produced by gasses that escape as the lava cools. Obsidian has a a glassy texture, and it has cooled quickly preventing the formation of crystals. Pumice, an ultra light porous glass which can sometimes float in water (Wyckoff, 1999).
Because of the composition of rocks in the area, one needs to be careful while hiking around this flow. Obsidian can be very sharp. One slip could quite easily produce a knife like cut. Because of this property, obsidian is one of the main rock types used by Native Americans in the creation of arrow heads and stone tools. Little Glass Mountain is a rich source of obsidian, and it was likely exploited by many groups in the region.
Sources
- Condie, Kent C., Hayslip, Dave L. (1975). Young bimodal volcanism at Medicine Lake volcanic center, northern California, Geochimica et Cosmochimica, Vol. 39, pp. 1165-1178, 1975.
- Dzurisin, et.al., 1991, Crustal Subsidence, Seismicity, and Structure Near Medicine Lake Volcano, California: Journal of Geophysical Research, v.96, no.B10.
- Ritter, Joachim R.R., Evans, John R. (1997). Deep Structures of Medicine Lake volcano, California, Tectonophysics, Vol. 275, pp. 221-241, 1997.
- Wicander, Reed, Monroe, James S. (1989). Historical Geology: Evolution of the Earth and Life Through Time, Revised Edition. West Publishing Company.
- Wyckoff, Jerome (1999). Reading the Earth: Landforms in the Making. Adastra West Inc. Publishers.
Websites
Lava Beds National Monument
The Lava Beds National Monument is located on the north eastern flank of the Medicine Lake volcano. Like most of the other flows of the region, the lava compounds are mostly basaltic and andesitic. In addition to the cinder cones (described above), the region also boasts spatter cones and maar volcanoes. Spatter cones erupt through minor vents where the ejected materials fly through the air and solidify before hitting the ground, forming "clinkery" rocks. Maar volcanoes are eruptions that form flat bottom craters and are without a cone (Wyckoff, 1999).
According to the National Parks Service, there are over 500 lava tubes. Non-claustrophobic visitors to this national monument can explore these tunnels, some of which go for more than a mile under the earth. A light source is essential, and two backup light sources are recommended. While these tubes have their share of lava-based stalactites, most are smooth surfaced.
The semi-arid desert environment of Lava Beds receives an annual average rainfall of 15 inches. Because of the relatively high altitude, the winters are cold, and the average annual snowfall is 44 inches. The area supports are high diversity of desert plant life, including desert sage (Salvia dorrii carnosa) and desert sweet (Chamaebatiaria millefolium). Fern species are present at the entrances to some of the the caves including the spreading wood fern (Dryopteris expansa) and the western swordfern (Polystichum munitum).
Official Website: http://www.nps.gov/labe/