The Formation of the Cascade Range and Mt. Shasta

Mt. Shasta is a double-peaked dormant stratovolcano that lies in the Northern corner of California about 60 miles south of the Oregon border. (College of Siskiyous Library, 2005)

The mountain is the second tallest in the Range, the first being Mt, Rainier and is expected to be 590,000 years old. (USGS,2023)

Located in the Cascade Range of Siskiyou County, Mt. Shasta dominates the region standing at 14,163 ft. (4,317 km) and 36,981 acres. (Shasta-Trinity National Forest, 2024)

The volcano is on the southern end of the Cascade Range and terminates around Lassen Peak. (USGS, 2023). It is comprised of four cones all of which, were constructed during separate eruptions and events.

Mt. Shasta formed during the Pleistocene period, <10,000 years, but since has undergone many volcanic eruptions and deformations.

The cones Shastina and Hotlum were both formed during the Holocene geologic era making them the youngest. (USGS, 2023)

Misery Cone is the second oldest and approx. 130,000 years old. (Geolibretexts, 2023)

The oldest of the cones is Sargents Ridge which is approx. 250,000 years old. (Geolibretexts, 2023)

• Originated 36 million years ago (Sherrod, 2023)
• Arrangement of dormant and active volcanos that extend from British Columbia, Canada to Northern California
• Composed of igneous volcanic rock that arose due to subduction with the underlying Pacific plate
• Contains landmark features such as:
  • Mt. Shasta
  • Mt. St. Helen
  • Mt. Adams
  • Crater Lake
  • The Three Sisters
  • Mt. Rainier

The Cascade Range lies on the Cascadia subduction zone. The zone is comprised of the Juan de Fuca Ridge on the western edge, the North American plate to the south, and subduction occurs at the Cascadia Trench. (USGS, 1905)

Subduction occurs when tectonic plates come into contact and one slide beneath the other. Subduction is a very gradual process as plates move at rates of 1-3 inches(2-8 centimeters) a year. (USGS, 2020)

The subducting plates form trenches, and as they descended with the subducting plates. In the Cascadian, the Juan de Fuca Plate subducts thick layers of andesite and basalt that primarily accumulate in the crust.

Other lithologies are found at this site as well such as rhyolite, mudflows, tuffaceous sediments, pumice, and pyroclastics. (USDAFS, 2024)

As sediment and materials accumulate at these zones, pressure and temperature increase, enabling melting and magma formation.

The process of plate subduction leads to a build-up of sediments, basaltic lava flows, and volcanic ash which have slowly formed the Cascade Range we see today.

• Definition: Cone-shaped volcanos are made of accumulated layers of ash, lava flows, and breccia from previous eruptions (Wilkerson, 2017)
• Often characterized by their violent eruptions occur at subduction zones
• Typically a symmetric cone shape before eruption

As depicted in the Mt. St. Helen timeline above, a new peak may form indicating, that tectonic activity and subduction are still occurring.

The Cascade Range is still active today and one of its most hazardous volcanos is Mt. Shasta as it has not erupted in 350,000 years

Mt. Shasta was formed by alpine glaciation which is when large masses of moving ice are usually confined to valleys that have already experienced drainage (Wilkerson, 2025)

Alpine glaciers are typically cooled temperate areas of high elevation making them ideal zones for snowfall accumulation.

The relief on the valleys and walls allows for potential energy to melt and move snow and materials down the fastest mode of transport. Seven glaciers cover Mt. Shasta with a mass area of 10.8km

1. Whitney
2. Bolam
3. Hotlum
4. Konwakiton
5. Watkins
6. Mud Creek
7. Wintum

The Whitney Glacier is the longest in California located on the Northern side and spanning over 3 km long.

As depicted in the image to the upper left, the Northern side of the mountain contains notably more glaciers than the South.

The uneven glacial placement is due to the eruptional history of the volcano as that sector collapsed leaving room for glacial and debris deposits.

• Definition: Flat-bottomed, broad valleys with steep walls (examples pinpointed in the slider above)(Wilkerson, 2017)
• Result of erosion or deposition of glacial ice in alpine feature

1. Snow, ice, and/or water build up in the zone of accumulation at high elevations
2. Gravitational energy drives the material in the direction of transport
3. Transport and halt of material will be visible through lateral and recessional moraines
4. Materials flow to the ablation area and either continue till terminal moraine or will deposit sediments.
5. The remaining material will flow to join the main valley or show arrest at the terminal moraine

• Cirque
  • round amphitheater-shaped basin that lies under zones of accumulation (Wilkerson, 2017)
• Hanging Valley
  • The tributary valley above the main valley before joining (Wilkerson, 2017)

• Arete
  • A sharp and narrow ridge eroded on two sides
  • Typically lying at high elevations (Wilkerson, 2017)

• Horn/Peak
  • Peak lying at the highest point of a landform and eroded on three or more of its sides
  • Mt. Shasta has two peaks following its eruption in the Northern front: the high peak in the west and a low elevated peak in the east (Wilkerson, 2017)

• Tarn
  • Located in cirques, lakes filled with alpine glaciation, or lake(Wilkerson, 2017)
• Paternoster Lakes
  • Connection of tarns in a series creating a stream(Wilkerson, 2017)
• Truncated Spur
  • Highly angled ridges that stop abruptly
  • Typically in a triangular cross-section (Wilkerson, 2017)
• Lateral Moraines
  • Ridges and mounds that lie parallel to the valley walls can scrape material off them (Wilkerson, 2017)
• Terminal Moraines (end moraines)
  • Ridges that lie at high angles to the valley axis mark glacial movement's end or terminal points. Indicated by arrest lines often surround the landform. (example in upright image) (Wilkerson, 2017)
• Recessional Moraines
  • Ridges indicate a pause in the glacial movement for a prolonged period of time (Wilkerson, 2017)

Mt. Shasta may be 590,000 years old but the structures deformation indicates a landslide and collapse nearly 300,000, marking its last major evolution.

The collapse occurred in the volcano's Northern part, leaving a trail of ash and debris.

While the landform has been relatively quiet in recent decades, the U. S. Geologic Survey (USGS) has marked Mt. Shasta as potentially hazardous due to:

1. Recently erupted centers and cones
2. Flank vents yet to erupt (USGS, 2023)

The mountain's steep slopes and valleys make for highly elevated rivers that transport sediment.

Mt. Shasta has a radial drainage pattern beginning near the peak of the structure.

Radial drainage diverges from a central area typically, in conically shaped areas like volcanoes, laccoliths, and mountains (refer to image to the right). The streams and rivers will flow away from the center and radiate in all directions. (Wilkerson, 2017)

Mt. Shasta has many tributaries but, three stand out. All of which are sourced from alpine glaciation and accumulation. These rivers are crucial to California's population as they serve as water sources/reserves.

Major Rivers