NSS Convention ~ July 25-30, 2021

Deer Mountain Cinder Cone

Pre-Convention Geology Field Trip

Volcanic Geology of the Southern Cascades

  • Presented by Bill Hirt, College of the Siskiyous
  • Departs from the registration tent at CoS
  • Sunday, June 27, 2021 (Time TBD)

Field Trip Overview

Enter the Volcano

Cinder cone cross-cut

While most stops on this field trip only require limited walking over short and level distances, one stop requires a 1/4 mile round-trip hike on uneven ground. The convention team has arranged for a few ATVs to assist our mobility-impaired cavers that wish to participate.

At this location, you will actually hike into a cross-cut of a volcanic cinder cone and be able to look up at where the eruption hit the surface. You'll definitely want a camera for this stop!

Weed lies along the boundary between two of California's northernmost geologic provinces: the Klamath Mountains and the High Cascades. Although underlain by different rock types (ancient crystalline rocks in the Klamath and young volcanic rocks in the Cascades) both provinces were formed by the same process: subduction.

As a dense oceanic plate sank beneath the western margin of North America, island chains riding on this plate collided with the continent and were sutured onto its margin. Seven or eight such collisions assembled the range during the past 200-300 million years and subsequent uplift and erosion by streams and glaciers carved the rugged landscape we see today. At Castle Lake we will visit some of the oldest rocks in the Klamath Mountains: the roots of a volcanic arc that grew on ocean crust about 400 million years ago.

Subduction continues today as the Gorda and Juan de Fuca plates sink eastward beneath northern California Oregon, and Washington. Water carried into Earth's mantle by the sinking plates triggers melting of the mantle at a depth of about 100 km. This partially molten rock then rises through the overlying North American plate and some of it erupts to build the High Cascade volcanoes. In the Weed area, two of the most prominent are Mount Shasta and the Medicine Lake volcano.

Although they are only 50 km apart, Mount Shasta and the Medicine Lake Volcano are two of the least similar volcanic centers in the southern High Cascades. Mount Shasta is a steep, glacier-clad stratovolcano that has been built by thick andesite and dacite lava flows which are interlayered with fragmental deposits produced by searing pyroclastic flows and watery debris flows. The Medicine Lake Volcano, on the other hand, is a broad shield volcano whose flanks are covered mostly by basalt lava flows and dotted with cinder cones. Its summit is occupied by a shallow caldera and studded with rhyolite domes.

Although both volcanoes have been built by magmas rising from the Cascadia subduction zone, their different shapes, products, and eruptive behaviors reflect variations in the amount of fluid released from the sinking Gorda plate and the asthenospheric circulation above it as one moves east from the arc axis to the rear arc.

This field trip will begin with a brief visit to study the bedrock of the Klamath Mountains and then explore the differences between the eruptive characters of Mount Shasta and the Medicine Lake Volcano and explore how these differences affect the hazards the volcanoes pose to both nearby communities and larger populations throughout the western United States.

Trailhead to Mount Shasta

Field Trip Details

Field Trip Leader

Bill Hirt, PhD

College of the Siskiyous


This event has a group limit of 140 participants maximum. Once that limit is reached, a wait-list will be established.
Age limits are 18 and above (or 14 and above with adult accompanier). Lunch will be served at mid-day during the trip.

Field Trip Contact:

Gail McCoy, Convention Registration
Email: secretary@nsswest.org


To register for this event, please select "Pre-Convention Geology Trip" on your convention registration form. There is an additional fee for this event.


Most stops on this trip only require limited walking over short and level distances. One stop, however, requires a 1/4 mile round-trip hike on uneven ground. The convention team has arranged for a few ATVs to assist our mobility-impaired cavers that wish to participate. All other facilities meet ADA requirements.

This trip will return to campus in mid-afternoon to early evening.

Online Webinar Geology of the Klamath Mountains

On June 18, 2020, Siskiyou Land Trust hosted an online webinar with geologist Bill Hirt exploring the geology of the Klamath Mountains. The video is 70 minutes long - 5 minute introduction by SLT's executive director Renee Casterline, 45 minutes for Dr. Hirt's presentation, then an additional 20 minutes answering questions that were posted during the talk.

Today's Weather: Mount Shasta, CA

  • Updated: Saturday, June 12 at 6:17 am
  • Reporting Station: Mount Shasta (KMHS)
  • Elevation: 3,540 ft.
  • GPS: 41.31494;-122.31702
  • Data provided by: National Weather Service
51° F
Mostly Sunny

Regional Map Forecast Wind: 6 to 18 mph, SSE
Humidity: 65%
Dewpoint: 40°F
Barometer: 30.06 in.
Visibility: 10.0 miles

Today: Mostly sunny. High near 78, with temperatures falling to around 76 in the afternoon. South southeast wind 6 to 18 mph, with gusts as high as 28 mph.
Tonight: Partly cloudy, with a low around 56. South southeast wind 2 to 12 mph.