Large Silicic Volcanic Flows

Joyce Cheatham, Pawnee Middle School, Oklahoma
Sharon Densler, Dover Central Middle School, Delaware
Stephanie Hart, Kealing Middle School, Texas
Sherri Jackson, Albuquerque Academy, New Mexico
Duke Johnson, Clark Planetarium, Utah
Debbie Krygiel, Amy Belle Elementary School, Wisconsin
Mary Matthes, Rehoboth Elementary School, Delaware

The structure and eruptive style of volcanoes is dependent on several characteristics including chemical composition and gas content of the magma.

The silicate tetrahedra in highly silicic magma tend to form long silicate chains, causing the magma to be extremely viscous. Silicate magmas often contain large amounts of dissolved gases, primarily water vapor. As the magma reaches the surface and the pressure drops, the volume of dissolved gases can expand as much as 900 times (like an explosion), causing overlying rock and the magma to fragment. It is this combination of thick viscous magma and trapped gases that results in the explosive eruptive style of high silica magmas. 

These violent eruptions can send a column of ash into the stratosphere where it can be distributed over a very wide area. Much of this material returns to Earth as air fall. The column of ash can collapse and form a dense flow — a pyroclastic flow — of hot ash, pumice and water vapor that moves across the landscape at speeds of 100 to 160 km per hour (60 to100 miles per hour) with temperatures ranging between 300 – 400˚ C. Their particles become more rounded through their interactions with each other and vast areas can be buried in seconds. Due to the extreme heat and weight, the whole mixture may deform and weld together. Any organic matter under the ash is likely to carbonize.

Three rock types that are common in large silicic volcanic features include andesite, dacite, and rhyolite.

Igneous Rocks with High Silica Content


Silica content

Common characteristics



Less fluid than basalt
Flows shorter distances and forms steep-sided domes
Typically gray or brown in color



Produces short, thick flows that build up into steep-sided domes
Light colored


72% or more

Produces short, thick flows that build up into steep-sided domes
Usually light in color (shades of gray, beige or pink)

*Silica content helps determine viscosity, color and explosive potential.

We will examine how these different magmas contribute to the creation of a large silicic feature — Mount Mazama. Mount Mazama was a high, steep-sided stratovolcano that formed over the last 400,000 years. The volcano was believed to be about 3,700 m (between 11,000 and 12,000 feet) above sea level before the cataclysmic eruption blew approximately 50 km3 of debris into the stratosphere. The entire volcano covers over 400 km2 (155 miles2) in southern Oregon.

Approximately 6,900 years ago, Mount Mazama experienced a climactic eruption that blew much of the upper part of the volcano away, leaving a caldera. After the explosion the caldera sank due to depletion of the magma chambers. Water slowly filled the depression, ultimately forming the present day Crater Lake. Subsequent smaller events pumped more magma back into the caldera, forming Wizard island cinder cone and other features that occur below the water’s surface. Thousands of years of glaciation have transformed its rim. 

The lower part of Mount Mazama contains basalt flows, formed from relatively low silica, low viscosity, non-explosive magma.  As Mount Mazama grew, the flows became more and more silicic. Stratovolcanos form from hundreds and hundreds of individual eruption events.

Devil's BackboneDevil’s Backbone is a dike that occurs in the volcano walls around Crater Lake directly north of Wizard Island. Devil’s Backbone varies from 1.5 to almost 8 m (5 to 25 feet) in thickness and extends from the lake surface to the top of the caldera rim.  It is comprised of andesite and dacite. It has been radiometrically dated to be about 50,000 years old.  Volcanic rocks the dike cuts through in the image are as old as 190,000 years.

A dike is a unit of rock that cuts through other rocks, in this case, the more horizontal stacked volcanic units of Mount Mazama. Dikes are common features within volcanos and are fed by the magma chamber(s) that feed the main conduit of the volcano.  A dike forms as magma moves through fractures in the rock. The pressurized magma can cause fractures or the moving magma can take advantage of existing fractures.  Sometimes the magma reaches the surface and breaks through the flanks of the volcano. 

Devil’s Backbone dike is more resistant to erosion than the surrounding rocks, which makes it stand out. 



Llao Rock

Named after a Native American god, Llao Rock forms a 365 m (1,200 foot) high cliff on the northwest side of Crater Lake 575 m (1,884 feet) above the surface of the lake. 

Llao Rock

Llao Rock is comprised of rhyodacite and represents a single lava flow that occurred not long before the caldera-forming eruption. The photograph shows several layers below Llao Rock, including a light, eroded pumice unit and darker andesitic units. The light unit on top of Llao Rock is the ash that fell just at the beginning of the final explosive event of Mount Mazama. Here we see more evidence that Mount Mazama was built over time by a succession of different events and that the composition of the magma changed through time. Lower, older units are less silicic (basaltic and andesitic) than the overlying units (andesitic and rhyolitic).


Cleetwood Flow

The Cleetwood Flow is a dark blocky layer of rhyodacite observed in the cliff face of the photograph below. This layer erupted a short time before the final eruption of Mount Mazama. A thick layer of reddish-orange pumice, associated with the final eruption, covers the Cleetwood Flow. This layer can be seen above the Cleetwood Flow in the photograph. The lower boundary of the pumice has been fused to the Cleetwood Flow, suggesting the Cleetwood flow was hot as the climactic eruption started, depositing pumice on its surface.

Cleetwood Flow


Wineglass Tuff

The Wineglass Tuff is present on the north and east flanks of the crater. It has an ryodacite composition (between dacite and rhyolite) and contains several individual flows that appear to have welded into one unit, indicating that they cooled together. The unit fills topographic depressions, suggesting it was a ground-hugging, dense pyroclastic flow. 

This unit represents the first stage of the cataclysmic eruption of Mount Mazama 7,700 years ago. As the eruption of Mount Mazama began, a massive column of ash developed on the northeast side. The ejected materials were blown high into the atmosphere and carried to the northeast by prevailing winds. The region covered by the Mazama ash includes areas in Oregon, California, Nevada, Washington, Idaho, Montana, and north into British Columbia. When the eruption column collapsed it generated ash flows. These ash flows resulted in the deposition of the Wineglass Welded Tuff. 


Pumice Desert

As you travel along the north entrance road to Crater Lake National Park you will pass through the Pumice Desert. It is about 14 km2 (5.5 miles2) in size and contains deposits about 15 m (50 feet) thick. 

Pumice Desert

Pumice Desert was formed during the climactic eruption of Mt. Mazama. Ejecta were carried across the surface. Larger, heavier materials dropped out, while fine materials continued to move. Pumice, volcanic bombs and other materials scattered and formed the pumice desert in the northern portion of the park. This flat treeless area is in contrast with the surrounding lodge pole pine forest. The pumice soil here has been found to be very nutrient deficient as compared with pumice soils elsewhere in Oregon. The porous, mineral deficient soil and harsh environmental conditions have prevented most plant and animal species from utilizing this area as habitat.    

Wizard Island

Wizard Island

Rising approximately 230 m (764 feet) above the surface of the lake stands Wizard Island, a small, steep-sided volcano that formed in the lake after the eruption of Mount Mazama.

This cinder cone, made of andesite, has a rhyodacite dome on its eastern side. The crater at the top of this cinder cone is approximately 30 m (100 feet) deep and 150 m (500 feet) in diameter. Due to the age of the oldest tree on the island, it is thought that the last volcanic activity occurring from Wizard Island was about 800 years ago. This volcano within a volcano is one of the most easily recognized features of Crater Lake.


Planetary Connections

There is evidence of volcanic features on other planets in our solar system, however, most of these appear to be comprised of basalt, based on:

  • Images from orbiters and probes that show rocks that are dark color, fine grained, and weathered to a reddish color (due to iron content)
  • The low slopes and large sizes of volcanos that are more similar to basaltic shield volcanos.
  • The presence of lava channels suggesting a low-viscosity silica poor lava rather than a high-viscosity silicic lava.

There are a few volcanic features that may have formed from a higher viscosity silicic lava.  Venus has “Pancake Domes,” steep sided volcanic domes.  These, however, are flat topped and much smaller — about 20 km across — than Mount Mazama.

Pancake domes on Venus

Pancake domes on Venus.  These domes are approximately 20 km across.
Unusual Volcanoes on Venus



Crater Lake Gem of the Cascades 
Cranson, K.C. 2005, KRC Press Lansing, MI., ISBN 0-9770880-0-6
Geological Story of Crater Lake National Park, Oregon. [3rd edition] 

Volcanoes[4th edition] 
Decker, Robert and Barbara, 2006,  W.H. Freeman,  New York, ISBN 0-167-8929-9

Roadside Geology of Oregon
Alt, David D. and Hyndman, Donald W. 1978,  Mountain Press Publishing Company, Missoula, Montana, ISBN 0-87842-063-0

Fire Mountains of the West
Harris, Stephen L., 2005 Mountain Press Publishing Company  Missoula, Montana, ISBN 0-87842-511-x
The Cascade and Mono Lake Volcanoes, [3rd edition] 

Geology of Oregon 
Orr, Elizabeth and William, 200 Kendal/Hunt Publishing Company, ISBN 0-7872-6608-6


The Heat From Within Home Page