The blue of Crater Lake shows through even when partly obscured by the smoke of forest fires in the Cascade Range of Oregon. Crater Lake occupies a caldera in a composite cone called Mt. Mazama. Mt. Mazama was once a tall, cone-shaped volcano like Mt. Rainier. About 7,000 years ago Mt. Mazama literally blew its guts out and collapsed, leaving behind a gaping hole that gradually filled with water from rain and melted snow to become Crater Lake.

The caldera-forming explosion of Mt. Mazama is one of the largest eruptions in the geologic record of the Cascades volcanic arc. Native Americans who lived in the Pacific Northwest at the time witnessed the eruption.

The ash from the caldera-forming eruption of Mt. Mazama spread across a huge area. It can be seen today in many parts of the Pacific Northwest, from British Columbia to California, preserved as a white layer in the soil.

The island in Crater Lake is a cinder cone called Wizard Island. Wizard Island grew within the caldera of Mt. Mazama soon after the caldera had formed.

Llao Rock

This image shows Llao Rock, which is set into the rim of Crater Lake. It is the darker gray rock with the steep face on which no trees grow, across the center of the photograph. Llao Rock is a dacite dome, formed by the eruption of lava that piled up into a dome shape. Dacite is felsic volcanic rock with a composition between andesite and rhyolite. The high silica content of felsic lava makes it sticky and viscous. As the lava that became Llao Rock erupted, it would have flowed with about the same consistency as toothpaste.Select the image to see a larger view. Use your browser's back button to return to this page.

When Mt. Mazama erupted and collapsed to form the Crater Lake caldera, the eruption removed about half of Llao Rock, leaving behind this well-exposed profile of a dacite dome in the wall of Crater Lake.

Dacite domes are a type of lava dome that is common on composite cone volcanoes. Since Mt. St. Helens erupted in 1980, a dacite dome has grown on the floor of its crater. Mt. Hood, near Portland, Oregon, has a dacite dome that has filled its most recent crater.

The eruption that created Llao Rock probably filled in part of a u-shaped glacial trough on the flank of Mt. Mazama. The shape of the base of Llao Rock is consistent with this idea.

Pinnacles

The spires in this photo are called the Pinnacles. They have been exposed by stream erosion along Wheeler Creek in a valley 12 km (8 mi.) SE of Crater Lake. Select the image to see a larger view. Use your browser's back button to return to this page.

The Pinnacles are made of ash from the gigantic eruption of Mt. Mazama 7,000 years ago. Following that cataclysmic, explosive eruption, the remaining upper parts of Mt. Mazama collapsed into the evacuated magma chamber, inverting Mt. Mazama from a high volcanic peak into a giant hole in the ground. Eventually, the giant hole filled in with water and became Crater Lake.

The Pinnacles show that the magma that erupted 7,000 years ago consisted of two chemical compositions, or two types of magma. The two magmas were layered, one on top of another. The first type of magma to erupt represents the top layer of the magma chamber. It is the lighter-colored, rhyolitic (felsic) ash that forms the bottom layer of the Pinnacles. The second type of magma to erupt represents the lower layer in the magma chamber. It formed the darker-colored, upper layer of the Pinnacles. It has an andesitic (intermediate) composition.

The spires called the Pinnacles are apparently due to differential erosion. The theory is that where gas from the pile of hot ash rose up through the ash, it formed fumaroles where the gas vented out into the atmosphere. These fumaroles and their natural underground plumbing systems left behind a mineralized, cylindrical zone beneath the vent of each fumarole. These mineralized zones are more resistant to erosion than the rest of the ash pile, so they now stand out as pinnacles.

Castle

This picture shows part of the wall of Crater Lake. It is a view inside a composite cone volcano, exposed by the eruption and collapse of Mt. Mazama 7, 000 years ago. The walls of Crater Lake are a cross-section of the interior of a composite cone volcano. They reveal the types of materials and structures that are common inside many composite cone volcanoes. Select the image to see a larger view. Use your browser's back button to return to this page.

Lava flows make up most of the layers of solid rock visible in the image. Ash flows and ash falls make up the rest of Mt. Mazama, as seen in the walls of Crater Lake. The ash layers tend to be lighter gray or tan in color, and to not be solid layers but rather layers of loose material.

In the center of the picture is the Fairy Castle. The Fairy Castle was sculpted by natural erosion from a slightly "welded" ash flow. A welded ash flow comes from an eruption of hot, semi-molten ash in which bits of still-molten magma fall to the ground with enough heat left to weld themselves together. Gas coming out of welded ash as it cools may oxidize the upper part of the ash flow into a reddish color.

The lava flows and volcanic ash layers seen in this picture show two styles of eruption: effusive lava flows and explosive ash eruptions. Composite cone volcanoes tend to switch back and forth between these two styles of eruption, at irregular intervals.

Pumice Field

The pile of pumice in this photo is located just outside the rim of Crater Lake, alongside Crater Rim Road. Pumice is frothy volcanic glass. Pumice is commonly so full of holes that pieces of it can float in water. Not very many types of rock can float in water. Select the image to see a larger view. Use your browser's back button to return to this page.

This pumice was erupted 7,000 years ago, in the gigantic eruption that formed the Crater Lake caldera. As you travel farther from the volcano and look at the layer of material erupted 7,000 years ago, the pieces of pumice grow smaller and smaller. Once you get more than 50 miles from Mt. Mazama, volcanic ash is all that you see in the layer. Volcanic ash consists of fine shards of glass which are broken bits of pumice, and other fine-grained material thrown far and wide by the explosive eruption.

Glossary terms that appear on this page: caldera; composite cone; cinder cone; lava dome; felsic; intermediate; dacite; andesite; rhyolite; magma; pumice