7: Cascade Range and Modoc Plateau
- Page ID
- 20340
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Introduction to the Cascade Range and Modoc Plateau
The Cascade Range geomorphic province of California represents the southern end of the larger, North American Cascade Mountain Range, which extends north into Oregon, Washington and British Columbia and is the result of subduction at the Cascadia subduction zone. This narrow province is bounded by the Klamath Mountains to the west, the Modoc Plateau to the east, and the Sierra Nevada to the south (Figure \(\PageIndex{1}\)). It is characterized by high mountainous terrain of volcanic origin.
The Modoc Plateau lies to the East of the Cascades and west of the Basin and Range and can be thought of as the transition between the two (Figure \(\PageIndex{1}\)). The geology is influenced by the tectonics of Cascadia subduction as well as Basin and Range extension. As the name ‘Plateau’ implies, this geologic province is high in elevation and relatively flat. The volcanic features here differ from the iconic snow-covered peaks of the Cascade Range and consist mainly of smaller volcanoes and lava flows. In the rain shadow of the Cascades, the climate of the Modoc Plateau is dry. This same rain shadow effect exists throughout the Cascade Range and creates a dramatic change in climate from the rainy cities of Portland and Seattle to the deserts of eastern Oregon and Washington. The rain shadow effect is described in 8.1: Regional Extent and Overview of the Basin and Range in the context of the Sierra Nevada Mountain Range.
While both the Cascades and the Modoc Plateau contain varied geology, this chapter is focused primarily on the volcanoes and volcanic processes in the two provinces. We first introduce the conceptual framework for understanding volcanoes and their tectonic origins, then dive into three important case study areas, Mount Shasta, Lassen Volcanic National Park and the Medicine Lake Volcano (including Lava Beds National Monument). There is considerable overlap between the geology of the Klamath Mountains, Cascade Range, Modoc Plateau and Basin Range provinces. Many of the non-volcanic rocks in the western part of the Cascade Range are described in Chapter 10: Klamath Mountains (for example, 10.2: The McCloud Limestone- An Ancient Coral Reef), and the nature of faulting within the southeastern Modoc Plateau is comparable to that of the Basin and Range province (see 8.3: Rift-Related Faulting in Eastern California).
By the end of this chapter, you should be able to:
- Explain how magma is produced and how volcanic arcs are formed at subduction zones.
- Classify volcanoes based on shape, size, and composition.
- Explain the relationship between magma viscosity and volcano type.
- Recognize common volcanic features.
- Describe the process of formation of hydrothermal systems in volcanic environments.
- Identify the major volcanic hazards associated with Cascade Volcanoes.
- Recognize evidence of past glaciation in a volcanic setting.
- Locate the Cascade Range and Modoc Plateau on a map of California.
- Explain how the Cascade Range and Modoc Plateau fit into the broader context of the tectonic and geologic history of California.
- 7.1: The Cascadia Subduction Zone and the Cascade Continental Volcanic Arc
- The Cascade Volcanoes are produced by the subduction of the Juan de Fuca plate beneath the North American plate. Water released from the subducted Juan de Fuca slab causes flux melting in the mantle beneath the North American plate. This continental volcanic arc mountain range extends from Northern California, through Oregon and Washington, and into British Columbia.
- 7.2: Rock Types of the Cascades
- As is typical of subduction zone volcanism, the California Cascades include a variety of volcanic rocks. As magma diapirs rise from the mantle, some have more time to evolve into more felsic compositions, while others are less changed from the ultramafic mantle. Rock types include basalt, andesite, dacite and rhyolite, which are all named based on their composition. Other rocks such as obsidian, pumice and volcanic tuff are named for their unique textures and appearance.
- 7.3: The Variety of Volcanoes in the Cascades and the Modoc Plateau
- Subduction zones are often associated with composite volcanoes, but other types of volcanoes are present as well. Composite volcanoes and shield volcanoes are regional volcanoes and both are present in the California Cascades. Mount Shasta is a classic example of a composite volcano. Medicine Lake Volcano is sometimes classed as a shield volcano based on its shape, but its composition is atypical of a shield volcano. Cinder Cones and Lava Domes can be found throughout the California Cascades.
- 7.4: Volcanic Hazards
- There are a variety of volcanic Hazards in the California Cascades. Lava flows form when molten rock reaches the surface of the earth and flows across the ground or down the slopes of a volcano. Pyroclastic flows are a mixture of gas, ash and other debris that flows along the ground surface. Lahars are water-based mudflows composed of volcanic material that can form with or without a volcanic eruption. Ash and tephra are ejected in the air and can rain down far from the volcano.
- 7.5: Mount Shasta
- Mount Shasta is California’s iconic composite volcano. The modern edifice has been built over the past 250,000 years, through four cone-building episodes. Mount Shasta is ranked fifth in the U.S. Geological Survey’s National Volcanic Threat Assessment, meaning that there is a significant potential for a future eruption. There is also a risk of lahars or debris flows that are not associated with an eruption, as hot summer temperatures result in rapid melting of glaciers.
- 7.6: Lassen Volcanic National Park
- Prior to the eruption of Mount Saint Helens in 1980, the 1915 Lassen Peak eruption was the most famous volcanic eruption in US History. Today, the park is known for a variety of volcanic features, not just for the evidence of the eruption. Four types of volcanoes can all be found in the park. The park also hosts hot springs and geysers that are the result of hydrothermal heating from the magma chamber at depth.
- 7.7: Medicine Lake Volcano
- Medicine Lake Volcano is shaped much like a classic shield volcano, but while most shield volcanoes are made of exclusively basalt, Medicine Lake volcano consists of felsic rocks as well. One example of this is Glass Mountain, which is an obsidian dome. Lava Beds National Monument, known for its caves formed by lava tubes, is located on the flank of Medicine Lake Volcano where there are many basalt lava flows.
- 7.8: Glaciers in the Cascades
- There are not many glaciers still present in the California Cascades, as they make up the southernmost part of the Cascade Range. However, during the Pleistocene ice ages, glaciers were extensive. Volcanic rock is often easily eroded by glaciers, and thus these glaciers significantly shaped the landscape of the Cascades. The only glaciers that remain in the California Cascades today are on Mount Shasta, but these too are threatened as the climate continues to warm.
- 7.9: Chapter Summary
- A summary of the Cascade Range and Modoc Plateau chapter.
- 7.10: Detailed Figure Descriptions
- Descriptions of complex images within this chapter, as well as additional guidance for users who have difficulty seeing and interpreting images.
Thumbnail: "Cascade Range and Modoc Plateau Provinces" is licensed under CC BY-NC 4.0 by Allison Jones.