Under Mount Shasta

Mount Shasta is as beautiful and dramatic underground as above, hosting numerous natural caves and intricate lava tubes under its expansive environment, some of which can be explored today.

While caves can be created in a variety of ways, including rainwater, wind erosion, and sand erosion, for all their exoticism, they are formed from just two commonplace ingredients: rock and water. Not just any rock will do, however, as caves are generally formed from gypsum, limestone, dolomite, or salt.

Caves around Mount Shasta were formed when rainwater percolated through the soil, picking up carbon dioxide (from both the air and surrounding soil), to form a weak acid. This acidic water was squeezed between the cracks and layers of limestone, dissolving out a channel in the bedrock for the water to flow — a process that can take anywhere from around 50,000 years or so to millions of years.

More notable in this area are lava tubes, which are natural underground tunnels that are rapidly formed by the flow of fiery hot molten lava during volcanic eruptions.

When a volcano erupts, lava flows can create channels through which the molten rock passes. As the outer layer of the lava flow cools and solidifies, it forms a crust, insulating the still-flowing lava inside. This can lead to the development of a tunnel-like structure as the lava continues to flow and eventually drains out, leaving behind a hollow tube, not unlike a drinking straw.

These lava tubes can be enormous and run for miles in length, forming elaborate networks, stacked layer upon layer, throughout multiple eruptions, often occurring over many, many years.

Sometimes the roof of a hidden lava tube will collapse, exposing it to the surface. Sometimes the roof and/or floor of a lava tube will collapse, revealing the levels above or below. The fact of these collapses should serve as a warning to those who would explore the depths of Mount Shasta’s lava tubes, even as they beckon the way by providing a passage into the mountain’s mysterious netherworld.

Lava Tube Habitation

Lava tubes offer unique environments with relatively stable temperatures and protection from the elements, making them attractive habitats for various organisms, including humans. Lava tubes provide shelter for a variety of animals, including bats, insects, and other small creatures — as well as for deadly rattlesnakes seeking to escape the heat in the entranceways, and the occasional mountain lion lurking in the depths — as well as other living organisms that are adapted to the dark, humid, and cool conditions found deeper within the lava tubes.

While lava tubes are not as abundant around Mount Shasta as they are in some other volcanic areas, there are a few notable locations where they can be found. The most prominent of these is the fabulous Lava Beds National Monument. Located northeast of Mount Shasta, Lava Beds National Monument is known for its extensive system of lava tubes. This area was formed by the eruption of the Medicine Lake Volcano around 10,000 to 30,000 years ago and boasts more than 700 lava tube caves, with some being accessible to visitors. Notable lava tubes in this area include Mushpot Cave and Skull Cave.

In addition to its lava tubes, Mount Shasta Caverns, a popular tourist attraction that is easily accessible south of the mountain along the shores of Shasta Lake, offers visitors a tour of limestone caves that have formed in the marble rock near Mount Shasta. These more traditional caves were not created by volcanic activity, but by the dissolution of limestone rock over countless years, creating the beautiful formations underground.

While less commonly explored, there are also reports of small caves and lava tube-like structures on the slopes of Mount Shasta itself. These features are not as extensively studied or accessible as the ones at Lava Beds National Monument, however.

It is important to note that while caves, mines, and lava tubes can provide interesting opportunities for exploration and scientific study, they should be entered with caution and proper equipment, as they can pose many risks due to uneven terrain, potential collapses, the darkness of the environment, and more.

In summary, lava tubes are formed by the flow of molten lava during volcanic eruptions, creating underground tunnels. While there are not as many lava tubes directly under Mount Shasta, the nearby Lava Beds National Monument is known for its very extensive system of lava tubes. These underground environments offer unique habitats for various life forms and provide opportunities for exploration and scientific study for those who are properly equipped and prepared.

Mount Shasta and its surrounding region feature a diverse geologic landscape composed of a variety of rocks, minerals, and gemstones.

Here are 10 of the most prevalent examples found in the area:

Basalt
Basalt is a common volcanic rock found around Mount Shasta. It is formed from the solidified lava flows that have erupted from the volcano over time. Basalt is typically dark-colored, fine-grained, and rich in minerals such as pyroxene and plagioclase feldspar.

Andesite
Andesite is another volcanic rock commonly found in the Mount Shasta area. It is intermediate in composition between basalt and rhyolite and often forms during volcanic eruptions. Andesite is characterized by its fine-grained texture and typically light to dark gray color.

Rhyolite
Rhyolite is a less common volcanic rock in the region, but it does occur. It is typically light-colored, fine-grained, and composed of minerals such as quartz, feldspar, and mica. Rhyolite forms during explosive volcanic eruptions.

Dacite
Dacite is an intermediate volcanic rock that is occasionally found around Mount Shasta. It is similar in composition to andesite but can be slightly more silica-rich. Dacite typically has a fine-grained texture and ranges in color from light gray to tan.

Quartz
Quartz is a common mineral in the Mount Shasta area. It can be found in various forms, including clear quartz crystals and amethyst. Quartz is often used in jewelry and can be found in veins within rocks and in sedimentary deposits.

Feldspar
Feldspar is a group of minerals that are abundant in many rock types, including granite and granodiorite. These minerals can have various colors, including pink, white, and gray.

Garnet
Garnet is a semi-precious gemstone that can be found in some metamorphic rocks around the region and is typically red to reddish-brown but can also be green, yellow, or other colors. Garnet is sometimes used in jewelry.

Serpentine
Serpentine is a greenish mineral often found in metamorphic rocks. It is known for its smooth, waxy texture and can occasionally contain veins of other minerals, such as asbestos.

Limestone & Marble
Some areas around Mount Shasta contain limestone and marble, which are both sedimentary rocks formed from the remains of marine organisms. Marble is a type of limestone that has undergone metamorphism and can be used for decorative purposes.

Obsidian
Obsidian is a volcanic glass that can be found in the region. It is formed when lava cools quickly and lacks crystal structure. Obsidian is often used for making sharp-edged tools and is known for its glassy luster and dark color.

It is important to note that collecting rocks, minerals, and gemstones in the Mount Shasta area may be subject to federal regulations and land management rules. Always obtain the necessary permits and follow guidelines when collecting minerals and gemstones in natural areas.

Soaring high above its surrounding area to a peak of 14,179’ (4,322 m), Mount Shasta is a stunning stratovolcano that dominates the northern California landscape.

Mount Shasta boasts a rich and complex geologic history, with a fascinating formation story that is intricately tied to the tectonic processes that continue to shape the western United States and beyond, and which continue to define this rugged landscape and its environment today.

Mount Shasta is part of the Cascade Volcanic Arc, an extensive line of volcanoes that stretches from northern California to southern British Columbia — the result of the ongoing subduction of the oceanic Pacific Plate beneath the continental North American Plate. In this process, the melting of the subducted plate as it relentlessly forces its way beneath the continental crust generates blazing-hot magma bubbles that rise to the earth’s surface, leading to volcanic activity — and the making of mountains.

A Volcano Is Born

The story of today’s Mount Shasta begins more than 600,000 years ago during the Pleistocene epoch, when the ancestral peak of Mount Shasta, which was likely a composite volcano, began forming.

Over the next several hundred thousand years, Mount Shasta continued to grow through effusive and explosive volcanic activity, with numerous lava flows, ash deposits, and pyroclastic flows contributing to its increasing size and majesty. During this ancient time, the broader region also experienced multiple glaciations, further shaping the landscape around the volcano by creating beautiful valleys and cirques.

Around 300,000 years ago, Mount Shasta slowly began taking on its modern form as a stratovolcano with a pronounced conical shape. Stratovolcanoes such as Mount Shasta are characterized by alternating layers of lava flows, ash, and volcanic rocks, which result from various eruptive events, leaving a unique geologic fingerprint that provides insights into this volcano’s dramatic formation and history.

Just west of Mount Shasta is its scenic little sister, Shastina, a subsidiary cone reaching 12,303’ (3,758 m), and featuring a very distinctive “volcano-like” appearance. The peaks are located so closely together that many casual observers and locals alike refer to the twins as one single, massive entity — “Mount Shasta.”

Recent Volcanic Activity on Mount Shasta

Mount Shasta remains an active volcano, with its most recent eruption occurring in the mid-18th century — an event reported in 1786 by French Explorer Lapérouse, who claimed to witness a volcanic eruption as he sailed along California’s Pacific coast — in what may be the first documented European explorers’ sightings of Mount Shasta.

Although it hasn’t erupted since then, the mountain is still closely monitored due to the potential hazards associated with volcanic activity, which need not be limited to the huge volcano’s spewing of potentially toxic gases and molten lava for it to be dangerous.

For example, the scenic lumber town of Weed, California, located at the base of Mount Shasta, serves as a stark reminder of the overwhelming and varied destructive powers of volcanoes, as Weed was severely impacted by a dangerous debris flow resulting from a massive landslide triggered by rapid snowmelt and heavy rains in 1978 — providing a warning from the past regarding the area’s future.

Due to the severity of threats posed by this imposing mountain, the United States Geological Survey (USGS), part of the Department of the Interior, constantly monitors Mount Shasta’s slumber for any signs of awakening. The current status of the mountain’s volcanic activity can be found on the USGS website.

Mount Shasta’s geologic history is ongoing, revealing a beautiful and imposing testament to the dynamic processes that have shaped and continue to shape this remarkable region. From its early beginnings as an ancestral volcano to its present-day prominence as a dramatic stratovolcano and iconic landmark for millions of travelers along the U.S. West Coast, Mount Shasta’s fiery formation and ongoing evolution remain intimately tied to the unrelenting tectonic forces and volcanic activity that have yet to end for this majestic mountain and the region that lies in its shadow.