Medicine Lake Volcano is a broad shield volcano in northeastern California and, by volume, the largest in the Cascade Range, built mainly by many relatively gentle basalt and basaltic andesite eruptions rather than steep, explosive activity. Spreading about 30 miles east to west and 50 miles north to south, it covers more than 800 square miles and contains a central caldera with the small lake that gave it its name. Active for about one million years, it has erupted in episodes over the past 11,000 years, most recently around 1,000 years ago at Glass Mountain. Its unusual size and long history have made the highlands scientifically important and protected.