DIGITAL GEOLOGIC MAP DATABASE OF MEDICINE LAKE VOLCANO, CALIFORNIA

Geologic mapping of Medicine Lake volcano in the northern California Cascade Range has been digitally compiled as a spatial database in ArcInfo GIS. The digital database can be queried to determine the areal extents and spatial distributions of different rock types, geologic units, and other geologic and geomorphic features. These data, in turn, can be used to better understand the evolution and growth of the volcano.

The database reveals that the total area covered by Medicine Lake volcano is about 2200 km². Sixty percent of that area (1322 km²) is covered by basalt (<53% SiO₂). Basaltic andesite (270 km², 53.0-56.9% SiO₂) and andesite (305 km², 57.0-62.9% SiO₂) together comprise about 26% of the area. Silicic lavas cover 117 km², or about 5% of the total. Dacite (63.0-69.9% SiO₂) accounts for 66 km², but three-quarters of that area consists of the tuff of Antelope Well, the volcano’s only ash-flow tuff, which thinly covers (<1 m to a few m thick) a significant area low on the northwest flank of the volcano. Rhyolite (>69.9% SiO₂) covers 51 km². Slightly more than half of the exposed rhyolite (26 km²) was erupted in post-glacial time.

Approximately 250 geologic units have been mapped. These units represent eruptive events, each commonly including a lava flow and its vent (dome, cinder cone, spatter cone, etc.). Some cinder cones have not been matched to lava flows, as the corresponding flows are probably buried, and some flows cannot be correlated with vents. Only half a dozen of the units are thin surficial deposits such as alluvium, covering over 35 km², <2% of the total. Volcanic and sedimentary units older than Medicine Lake volcano, but within the mapped area account for 154 km², about 7% of the total area.

Plotted map colors indicate compositions. In addition, products of 16 post-glacial eruptions, covering 316 km², about 14% of the volcano’s area, are highlighted with a pattern. The largest individual units on the map are basaltic, including the post-glacial basalt of Giant Crater (198 km²). The single largest unit is the late Pleistocene basalt of Yellowjacket Butte (296 km² exposed but partly covered by a late Holocene andesite flow). Other features represented in the digital database include lava tubes, vents, faults (with direction of offset), dikes, and littoral cones.