The Volcanic Legacy Scenic Byway now has a new map with an itinerary of the many geologic sites. Please click on the map image for a larger scale map.
Starting at Lassen Volcanic National Park in Northern California and ending at Crater Lake in Oregon, learn about the history of fire and ice that shaped this land. You can download this geology tour by clicking on link below.
The Geology Tour (pdf) lists 26 sites with a description of the volcanic history and origins of each site.
The Volcanic Legacy Scenic Byway traverses the southern section of the great Cascade range, a chain of active volcanoes that stretch from the Canadian border to northern California.
Of the 13 potentially active volcanoes in the Cascade Range of the Pacific Northwest, 11 have erupted in the past 4,000 years and 7 in just the past 200 years. Cascade volcanoes tend to erupt explosively, and on average two eruptions occur per century--the most recent were at Mount St. Helens, Washington (1980-86), and Lassen Peak, California (1914-17).
While not the tallest, the Medicine Lake volcano is actually the largest volcano along the Byway. In fact, with a volume of more than 130 cubic miles, it is the largest volcano in the entire Cascade range. Medicine Lake Volcano, a broad shield capped by a 4- by 7-mile caldera, has erupted at least 8 times in the past 4,000 years, most recently about 900 years ago. This photo from the USGS shows the Medicine Lake caldera with Mount Shasta in the distance.
The major volcanoes you will explore along the Byway are Crater Lake, Medicine Lake, Mount Shasta and Lassen Peak. Crater Lake, or Mount Mazama, is the least active of these peaks...it has not been active for over 4000 years. The 6 mile wide caldera, which Crater Lake now fills, formed when a massive ash eruption 6,950 years ago emptied the magma chamber underneath Mount Mazama, causing it to collapse. The four thousand foot deep hole has filled with rain and snowmelt to a depth of about 1,900 feet. Seepage and evaporation now balance the incoming precipitation, and the level of the lake remains nearly constant.
Although a small peak, Black Butte is a unique volcano--and not a small part of its nearby neighbor Mount Shasta--known as a plug dome. A plug dome is a steep-sided, rounded mound formed when viscous lava wells up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged. In the middle of the valley to the west of Mount Shasta, and often mistaken for it by visitors when the bigger mountain is obscured by clouds, the tiny Black Butte volcano is made of four overlapping dacite lava domes that erupted about 9,500 years ago. A Forest Service fire lookout cabin once covered the top of Black Butte.Today, this peak provides one of the finest hikes of the entire Byway region.
At 14, 179 feet above seal level, Mount Shasta, a composite or strato volcano is the tallest volcano on the Byway. A composite volcano is typically a steep-sided, symmetrical cone of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases. Mount Shasta actually consists of the remains of at least four different cones, two of which were destroyed by erosion or explosion. Mount Shasta has been the most active volcano in California during the past 4,000 years, and is second tallest in the entire Cascade Range, next to Mount Rainier. During that time, Mount Shasta erupted on average about once every 300 years, producing many pyroclastic flows. It probably last erupted in 1786. Shastina, the northernmost peak of Mount Shasta, is a large subsidiary cone on the west flank of this volcano.
Lassen Volcanic Field, including Lassen Peak, is the southernmost volcanic center in the Cascades. Lassen Peak is a plug dome and its most recent volcanic eruptions in California occurred between 1914 to 1917. An explosive eruption on May 22, 1915, produced a large pyroclastic flow, lahars, and ash that fell as far away as Elko, Nevada, 300 miles to the east.
You will pass many more small volcanoes as you drive the Byway. The Cascades are actually made up mostly of hundreds (even thousands) of small shield volcanoes that lie between the large stratovolcanoes like Mount Shasta and Crater Lake (the former Mount Mazama). Deer Mountain, for example, on the road between Weed and Dorris in northern California, is a fairly small shield volcano separate from both Mount Shasta and the Medicine Lake volcano.
Please follow the links below to discover information on the volcanic features found along the Volcanic Legacy Scenic Byway.