Medicine Lake Glass Flow Geologic Area

Northern front of Medicine Lake Glass Flow. Here, the dacitic glass is approximately 60 feet thick. Medicine Lake Glass Flow Geologic Area is about one square mile in area and was created about 5,000 years ago by molten lava. It is a pancake–shaped landform situated within Medicine Lake Volcano's caldera. The majority of the flow is located on the Modoc NF, with about 30 acres on the Klamath NF. The best access is via NF–49, along the northern margin of the flow. 


Geologic Background

Rocks formed by cooling of a molten source are called igneous rocks. Above the earth's surface, molten rocks are known as lava; below earth's surface, it is called magma. As the melt cools, minerals begin to crystalize. The longer the cooling time, the larger the crystals become. Volcanic rocks cool quickly after lava erupts Flow banding in the obsidian of Medicine Lake Glass Flow is a feature commonly seen. from vents, leaving very little time for crystal formation; as a result, the rocks at this lava flow appear to be glassy, and lack well–developed crystals. The rocks found at Medicine Lake Glass Flow are obsidian, based on their glassy texture, and dacite based on their chemical composition.


Geologic Processes at Work

Volcanic rocks are separated into several categies by their chemical composition. Silica (SiO2) is the dominant chemical component in volcanic rocks. Silica also happens to be the major component of glass windows and glass bottles. In terms of percentage of silica, chemical classification for igneous rocks is as follows: Basalt 45–52%, Basaltic–andesite 53–56%, Andesite 57–62%, Dacite 63–69%, Rhyolite 70% or more silica.

Pancakes made with thick batter produce thick pancakes, whereas pancakes made with thin or “runny” batter yield thin pancakes. Similarly, the amount of silica present in lava influences its viscosity, and also controls the thickness of the lava flow. For example, silica–rich flows of rhyolitic lava produce very thick glass flows, such as Little Glass Mountain Geologic Area which almost 300 feet high. In contrast, silica–poor basaltic lavas, like the flow at Fourmile Hill Tree Molds Geologic Area, form rather thin flows, 3 to 30 feet high. Medicine Lake Glass Flow, which is composed of dacite, is as high as 90 feet high in some locations.


Cultural Significance

Obsidian within the Medicine Lake Highlands has been used for thousands of years by numerous American Indian tribes. With a cutting edge sharper than a surgical scalpel, obsidian was formed into a variety of everyday tools. Often these tools, and the flakes from their manufacture and maintenance, are the only traces left of past human activities on the landscape. Obsidian from the Medicine Lake Highlands was a principal trade item and has been found as far away as California 's northwestern coast and central valley. Obsidian and obsidian source locations have spiritual as well as functional value for American Indians; as such these areas continue to be of special interest to today's American Indians.

Collection of obsidian from Medicine Lake Glass Flow Geologic Area is prohibited. Natural features and archeological and historical objects are protected by federal law. You can help preserve America's past by leaving archaeological and historic remains undisturbed, encouraging others to do the same, and reporting information about disturbances of these remains to National Forest personnel.

 Generalized map of Medicine Lake Glass Flow.


Donnelley–Nolan, J.M., in press. Geologic Map of Medicine Lake Volcano, northern California: USGS Scientific Investigations Map 2927, scale 1:50,000.