7.10: Detailed Figure Descriptions

Figure 7.1 Province Map

The Cascade Range topography is characterized by several conical peaks in a narrow band between the Klamath Mountains and the Modoc Plateau. The Cascade Range extends into Oregon, but the California province stops at the Oregon border. It is bounded by the Sierra Nevada and Great Valley to the South. The Modoc Plateau is located directly east of the Cascade Range province and is bounded by the Basin and Range to the east and south. The Modoc Plateau province also reaches the Oregon border. It is characterized by relatively flat topography compared to the surrounding provinces.

Figure 7.1.1 Volcanoes and Subduction

This map depicts the relationships between the major volcanoes of the Cascade Mountains and the tectonic setting of the Pacific Northwest where the Pacific plate, Juan de Fuca plate and North American plate meet. The Pacific plate and Juan de Fuca plate are separated by the spreading center (bold red lines) of the Juan de Fuca ridge. The Juan de Fuca plate and the North American plate are separated by the Cascadia subduction zone (thin red line with teeth). The major Cascade Volcanoes (red triangles) are located approximately 200 to 300 km inland from the Cascadia subduction zone. Only the major volcanoes are pictured, including Mount Shasta, Lassen Peak and Medicine Lake Volcano in California. Volcanoes in other states and provinces include: eight volcanoes in Oregon, with Crater Lake and Mount Hood labeled; five volcanoes in Washington, with Mount St. Helens and Mount Rainier labeled; and three volcanoes in British Columbia.

Figure 7.1.2 Ring of Fire

This figure is included here to show the location of the Cascade Range in the broader context of global plate tectonics. Many of Earth's volcanoes occur at suduction zones and many of these subduction zones surround the Pacific Ocean, creating a ring of volcanoes known as the Ring of Fire. The Ring of Fire is produced by the subduction of the westward moving Pacific Plate along its western margin as well as some smaller eastern moving oceanic plates subducting beneath the west coasts of the Americas. The Juan de Fuca Plate is one of these smaller eastward moving pates, and the Cascade volcanoes are shown as part of the Ring of Fire, although neither is labeled. It is worth noting that not all volcanoes are located in the Ring of Fire. Some of the volcanoes outside the Ring of Fire are not formed by subduction, but there is also subduction related volcanism elsewhere in the world. One example is the Sunda Arc of the Indian Ocean, which is very near the Ring of Fire, but is not shown as part of it in this figure. This figure also depicts the arrangement of continents, oceans, tectonic plate boundaries and plate motions for the whole earth with labels for the continents, oceans and other features. Global Plate Tectonics and global geography are not specific learning outcomes of this chapter, but if a review of this material is desired, helpful tactile graphics can be found in the Geological Tactile Image Repository.

Figure 7.1.3 Subduction of the Juan de Fuca Plate Beneath the North American Plate

The process pictured here is described in detail in the text. However, some learners may benefit from the tactile graphic, “Continental Subduction Zone,” which can be found in the Geological Tactile Image Repository. The descriptions within this chapter will help students relate the generalized labels in the tactile graphic to the specifics of the Cascadia subduction zone.

Figure 7.1.4 Crystal Settling

A magma chamber is depicted in all three stages of crystal settling and the formation of a zoned magma. The magma chamber may be 100s of meters to several km in diameter. Magma generally moves from depth to the surface, entering the magma chamber from a conduit at the bottom and leaving from a conduit at the top. In the first stage early-formed olivine crystals begin to form near the top of the magma chamber, In the second stage, these denser crystals settle to the bottom through the non-viscous magma and the loss of olivine to the bottom of the magma chamber makes the upper magma more felsic. In the third stage, the crystals re-melt in the hotter, lower magma, making it more mafic. This leaves a zoned magma chamber with more felsic magma at the top and more mafic magma at the bottom.

Figure 7.1.5 Rising Magma Diapirs

The origin of magma is partial melting of the mantle, near the top of the asthenosphere. Some magma then rises as diapirs through the lithospheric mantle into the crust, forming intrusions. Intrusion at the base of the crust is called “underplating”. Stagnation and crystallization of the diapirs form plutons of intrusive igneous rocks in the crust. Magma differentiation and assimilation in magma reservoirs also occurs in the crust. Some magma continues rising through the crust causing volcanic eruptions and extrusion of lava. Volcanic gasses mix with the atmosphere in both the troposphere and the stratosphere above.

Query 7.2.1 Rock Identification: Chaos Jumbles

This activity requires interpretation of visual information, including the recognition of color. It may not be appropriate for all learners.

Query 7.3.1 Anatomy of a Volcano

Although this activity can be completed with a screen reader, some learners may achieve a better conceptual framework with addition of the tactile graphic, Magmatic Environments in the Geological Tactile Image Repository.

Figure 7.3.1 Composite Volcano Cut-Away

A diagram of the different ways a composite volcano may erupt throughout time and the different features that may form. A cut-away of the volcano interior shows magma rising through a conduit and intruding into cracks. At the main summit vent, an eruption column is shown rising into the atmosphere and an eruption cloud has formed downwind in the prevailing wind direction. Ash (tephra) is shown falling from the eruption cloud. Pyroclastic flows appear as clouds that hug the flank of the volcano. One of these is associated with a lava dome part-way up the volcano flank. A landslide (debris avalanche) is also shown on the flank of the volcano. Near this there are gas clouds indicating fumaroles. An active, molten lava flow reaches from the summit to the base of the volcano. A lahar (volcanic mudflow) is shown surging down a river valley.

Figure 7.3.2 Shield Volcano Cut-Away

At the summit of the shield volcano a fire fountain emerges from the main vent. A small amount of tephra also forms a cloud above the fire fountain and haze of vog (volcanic smog) has collected downwind in the prevailing wind direction. Multiple lava flows issue from the fire fountain and flow down the volcano flanks. Active flows are surrounded by cooled lava flow fields. Some lava flows through a lava tube, which is covered by dark, hardened lava except in a few places. In one place a lava flow emerges from a tube at a breakout. Laze plumes emanate from locations where flows enter the ocean.

Figure 7.3.3 Cinder Cone Cut-Away

A cut-away of the volcano interior shows magma rising through a conduit and intruding into cracks. One intrusion emerges from the volcano at a vent on the lower flank, above a rafted cinder cone. A red lava flow also originates from this smaller flank vent. From the main summit vent, a plume of tephra and volcanic bombs emerges. The plume shape is slightly extended downwind in the direction of the prevailing winds. Spatter has accumulated around the crater rim and layers of cinders and ash have accumulated to form the edifice of the cone.

Figure 7.3.4 Lava Dome Cut-Away

Magma rises through a conduit and forms a lava dome within an existing crater. The dome is surrounded by rubbly breccia. The dome exhibits flow banding within and a rough, rubbly surface with spines.

Video: How to Classify Volcanoes

Portions of this video require viewers to practice visual identification of images, which may not be appropriate for all learners. Some learners may benefit from the Volcano Types tactile graphic available in the Geological Tactile Image Repository.

Query 7.3.2 Classifying California Cascade Volcanoes

This activity requires interpretation of images and may not be appropriate for all learners. Some learners may benefit from the Volcano Types tactile graphic available in the Geological Tactile Image Repository.

Query 7.4.1 Volcanic Ash Comparison

Areas that received 2 inches to 5 inches of ash during the May 18, 1980 eruption of Mount St. Helens (red) are limited to the area surrounding the volcano and a small patch near Ritzville, in eastern Washington. The area that received between a half inch and 2 inches (orange) is an elongated oval extending to the east of Mount St. Helens as far as the Idaho/Montana border. The area that received trace amounts less than a half inch (yellow) is much larger, extending east from the volcano and fanning outwards. Trace ash traveled as far as Minnesota, New Mexico and presumably into Canada (though Canada is not shown). A small isolated patch of trace ash also exists in Oklahoma.

Mount Shasta is in Northern California. Eureka, CA, which is west of Mount Shasta on the coast, is the closest of the three cities listed in the question to Mount Shasta. Sacramento, CA is further and is nearly directly due south. Boise, ID is the furthest of the three cities and is located to the east and slightly north of Mount Shasta.

Query 7.5.1 Rock Identification: Mount Shasta

This activity requires interpretation of visual information, including the recognition of color. It may not be appropriate for all learners.

Figure 7.5.4 Mount Shasta Hazard Map

The near-volcano hazard zone includes lava and pyroclastic flows, thick tephra, lahars, ballistic ejecta and rock fall. This area surrounds Mount Shasta, which has an elevation of 14,179 feet (4,322 meters), to a distance of around 15 to 25 km (about 10 to 17 miles). The near-volcano zone includes the parts of Interstate 5 as well as Highways 89 and 97. The towns, Weed, Mount Shasta and McCloud are all in this zone.

An additional Lahar (volcanic mudflows) hazard zone includes river valleys outside the near-volcano hazard zone to show that lahars can be potentially far-traveled in valleys draining the volcano. These valleys include the Sacramento River Valley and two tributaries of the McCloud River to the South of Mount Shasta and the Shasta River Valley to the northwest.

A zone of Regional lava flows lies to the east of Mount Shasta, between the near-volcano hazard zone of Mount Shasta and the Medicine Lake Hazard Zone. This zone is susceptible to lava flows from vents dispersed between major volcanoes.

Volcanic Ash is not shown on the map, but is a hazard associated with a potential eruption. Fine fragments of volcanic rock are carried downwind.

Figure 7.6.1 Map of Lassen Volcanic National Park

Lassen Volcanic National Park is approximately 25 km across east to west and 20 km north to south. Highway 89 weaves its way north-south through the west side of the park, while the eastern part of the park has fewer major roads. A road with a separate park entrance goes to Butte Lake near Cinder Cone. There are several lakes in the eastern part of the park, including Snag Lake, Juniper Lake and Horseshoe Lake. Manzanita Lake is in the northwest part of the park. Mount Diller, Brokeoff Mountain and Mount Conard are all located within the circle representing Brokeoff Volcano and are remnants of Brokeoff Volcano, which is located in the southwest corner of the park. Lassen Peak lies to the North of Brokeoff Volcano and Chaos Crags is further north. Red Cinder is located at the far eastern edge of the park. Most of the hydrothermal features are in the southern part of the park, including Bumpass Hell, which is within the circle of Brokeoff Volcano. Growler Hot Springs and Morgan Hot Springs are south of the park boundary.

Figure 7.6.2 Lassen Volcano Map

Volcanic features labeled as lava domes include: Chaos Crags, Lassen Peak, Crescent Center, and Reading Peak, all in the western part of the park. Broke-off Volcano in the southwest corner is labeled as a composite volcano. The cinder cones are Hat Mountain, Crater Butte, Red Cinder Cone, Fairfield Peak, and Cinder Cone; all in the central or eastern part of the park. Two shield volcanoes, Sifford Mountain and Mount Harkness, are near the southern park boundary and one, Prospect Peak is near the northern park boundary.

Figure 7.6.4 Projected Profile of Brokeoff Volcano

A dotted line shows the outline of the eroded Brokeoff Composite Volcano. Lassen Peak and Bumpass Mountain are in the background. The dotted line connects to Brokeoff Mountain.

Figure 7.6.7 Generalized Geologic Map of Lassen Peak

27,000 year old Lassen Peak Lava Dome (pale peach) erupted multiple times in May, 1915.. Deposits from the May 19-20 eruptions can be found near the summit (darker green lava flows and dome), near Emigrant Pass (lighter green avalanche and lahar), and near the location of “Hot Rock” (Figure 7.6.8) on the northeast flank of the mountain. Deposits from the pyroclastic flow and related lahars (light brown) of May 22, 1915 are located mainly in the Lost Creek and Hat Creek drainages and reach multiple 10s of kilometers from the volcano. The devastated area is on the northeast flank of the volcano and reaches as far as Emigrant Pass and “Hot Rock”.

Figure 7.6.9 Lassen Hydrothermal System

Precipitation falls mosty to the north of Lassen Peak, where it enters the ground as groundwater recharge. This meteoric water percolates to depths of about 5 kilometers (or 3 miles) where it is heated by the magma chamber. The actual magma is deeper, at depths of more than 8 kilometers (or 5 miles), but the solid rock surrounding the magma is also hot. It is more than 350 degrees Celsius (or 660 degrees Fahrenheit). This heat also causes thermal cracking, which can induce earthquakes. When the water is heated to 235 to 270 degrees Celsius (or 455 to 520 degrees Fahrenheit), it rises convectively first through the liquid dominated zone. When the water moves up into low enough pressure, some of the water boils, creating a vapor-dominated zone. This vapor rises and emerges at the surface as fumeroles in places like Bumpass Hell, located between Bumpass Mountain and Mount Conard. Hot water also produces acid sulfate hot springs in this area. Some of the gas depleted thermal water that did not boil flows southward underground as an outflow plume, emerging at the Growler and Morgan Hot Springs.

Figure 7.7.2 Shaded Relief Map of Medicine Lake Volcano

Medicine Lake Volcano is east of Mount Shasta. A caldera at its center is marked with a dotted line. There are many eruptive vents (stars) in the area surrounding the caldera. Regional faults have a general southeast-northwest strike, accommodating east-west extension.

Figure 7.8.2 Glaciers of Mount Shasta

By the early 2000s, a pattern of glacial retreat was evident throughout the American West and the world. However, researchers found that this pattern did not hold true for the glaciers of Mt. Shasta. Both Whitney and Hotlum Glaciers were, at the time, actually growing. Whether a glacier grows or shrinks depends primarily on two climatic factors, temperature and precipitation. The researchers reasoned that, although the climate surrounding Mt. Shasta was getting warmer each year, the winters were also getting wetter during the same period. The increased snowpack in the winter was making up for the increase in melting during the summer months. They also hypothesized, in 2006, that this trend was not likely to hold in the future. In the subsequent decades this prediction has been shown to be correct. California was struck with a prolonged drought from 2012 to 2016, and the warming trend continued, with spikes of record breaking summer heat in the early 2020s. Between 2020 and 2022, Whitney Glacier lost over a quarter of its volume. By the end of the 21st Century, it’s unlikely that any glaciers will remain in the California Cascades.