U.S. Department of the Interior 

U.S. Geological Survey 

USGS Fact Sheet 092-02 

August 2002 

U.S. GEOLOGICAL SURVEY and the NATIONAL PARK SERVICE—OUR VOLCANIC PUBLIC LANDS

Mount Mazama and Crater Lake: Growth and

Destruction of a Cascade Volcano

For more than 100 years,

scientists have sought to

unravel the remarkable story of

Crater Lake’s formation. Before

Crater Lake came into existence,

a cluster of volcanoes dominated

the landscape. This cluster,

called Mount Mazama (for the

Portland, Oregon, climbing club

the Mazamas), was destroyed

during an enormous explosive

eruption 7,700 years ago. So

much molten rock was expelled

that the summit area collapsed

during the eruption to form a large

volcanic depression, or caldera.

Subsequent smaller eruptions

occurred as water began to fill

the caldera to eventually form the

deepest lake in the United States.

Decades of detailed scientific

studies of Mount Mazama and

new maps of the floor of Crater

Lake reveal stunning details of

the volcano’s eruptive history and

identify potential hazards from

future eruptions and earthquakes.

The cataclysmic eruption 

of Mount Mazama 7,700 

years ago began with a 

towering column of pumice 

and ash, as depicted in this 

painting by Paul Rockwood 

(image courtesy of Crater 

Lake National Park Museum

 and Archive Collections). 

 After the collapse of the 

 summit of the volcano, the 

 caldera filled with water to 

 form Crater Lake. (Photo by 

 Willie Scott, USGS) 

When Clarence Dutton of the U.S.

Geological Survey (USGS) visited Crater

Lake, Oregon in 1886, he was awestruck by its

amazing beauty: “as the visitor reaches the

brink of the cliff, he suddenly sees below him

an expanse of ultramarine blue of a richness

and intensity which he has probably never

seen before, and will not be likely to see

again.” He spent nearly a month sounding the

depths of the lake from a rowboat, wondering

at the forces that could create such a

magnificent landscape. In the more than 100

years since Dutton’s first survey, scientists

have steadily unraveled the mystery of the

formation of Crater Lake and with it the

demise of Mount Mazama.

Mount Mazama

Mount Mazama was formed over a period

of nearly half a million years by a succession

of overlapping volcanoes. The first eruptions

about 420,000 years ago built Mount Scott,

located just east of Crater Lake. Over the next

several hundred thousand years, Mount Scott

and other nearby volcanoes became extinct,

while new volcanoes grew to the west. Layers

of lava flows from these volcanoes are visible

in the caldera walls and in landmarks along

the south rim of Crater Lake, including

Applegate and Garfield Peaks. During the

growth of Mount Mazama, glaciers repeatedly

carved out classic U-shaped valleys. Some

were filled with lava from later eruptions,

while others, such as Kerr Notch and Sun

Notch, were not.

By about 30,000 years ago, Mount

Mazama began to generate increasingly

explosive eruptions that were followed by

thick flows of silica-rich lava, an outward sign

of the slow accumulation of a large volume of

highly explosive magma deep beneath the

volcano. Grouse Hill and Redcloud Cliff are

thick lava flows erupted between 30,000 and

25,000 years ago. Another such eruption
Caldera Walls Tell the Story of Mount Mazama

Beginning about 30,000 years 

ago, eruptions of a silica-rich 

magma called rhyodacite 

signaled the beginning of 

explosive eruptions that would 

lead to the cataclysmic 

eruption. During one such 

eruption about 7,900 years ago, 

the white layer of pumice and 

ash beneath Llao Rock was 

formed. After the explosive 

phase ended, the thick lava 

flow of Llao Rock oozed over 

the pumice and ash. 

A thick layer of 

pumice and 

ash from the 

eruption that 

formed Crater 

Lake caldera 

blankets much 

of the caldera 

rim. Other 

layers of 

pumice and 

ash, like the 

orange Pumice 

Castle 

(circled), are 

deposits from 

older, smaller 

explosive 

eruptions. 

The earliest eruptions about 

420,000 years ago built 

Mount Scott. 

Glaciers carved classic U-shaped 

valleys on the flanks of Mount 

Mazama during ice ages, including 

the valleys of Sun Notch and Kerr 

Notch. 

After the eruptions at Mount Scott, a series of 

successively younger overlapping volcanoes 

grew to the west. Landmarks along the south 

rim of Crater Lake caldera, such as Garfield 

Peak, are made up of lava flows erupted from 

these volcanoes over several hundred 

thousand years. 

400,000 year-old lava flows make up 

Phantom Ship, the oldest rocks 

exposed above lake level within the 

caldera. 

Photos by Ed Klimasauskas, USGS 
about 7,900 years ago formed a white layer of

pumice and ash and the thick lava flow of

Llao Rock. Within 200 years of the eruption

at Llao Rock, another thick lava flow erupted

near present-day Cleetwood Cove. These

eruptions reached a peak 7,700 years ago in

the largest explosive eruption in the Cascades

during the past 1 million years.

Mount Mazama’s Final Eruption

The cataclysmic eruption of Mount

Mazama 7,700 years ago started from a single

vent on the northeast side of the volcano as a

towering column of pumice and ash that

reached some 30 miles (50 km) high. Winds

carried the ash across much of the Pacific

Northwest and parts of southern Canada. So

much magma erupted that the volcano began

to collapse in on itself. As the summit

collapsed, circular cracks opened up around

the peak. More magma erupted through these

cracks to race down the slopes as pyroclastic

flows. Deposits from these flows partially

filled the valleys around Mount Mazama with

up to 300 feet (100 m) of pumice and ash. As

more magma was erupted, the collapse

progressed until the dust settled to reveal a

volcanic depression, called a caldera, 5 miles

(8 km) in diameter and one mile (1.6 km)

deep.

Crater Lake

Soon after the cataclysmic eruption,

eruptions from new vents within the caldera

built the base of Wizard Island and the central

platform. As eruptions continued, rain and

snowmelt also began to fill the caldera. For

the next few hundred years, eruptions from

these new vents kept pace above the rising

water level. Lava flowed into the deepening

lake, creating benches on the flanks of the

growing cones that tell scientists how deep the

lake was during these eruptions. One set of

eruptions from a crater on the west edge of the

central platform formed lava tubes or channels

that sent lava far out onto the caldera floor.

Later eruptions from a vent in the northern

part of the caldera, just south of present-day

Cleetwood Cove, built Merriam Cone. The

erupting Merriam Cone probably never

reached the lake surface. The ever-deepening

lake eventually drowned the central platform

volcano as well. Only Wizard Island managed

to grow high enough to stay above the

waterline. The last eruptions at Wizard Island

took place when the lake was about 260 feet

(80 m) lower than today. All of this activity

occurred within 750 years after the

cataclysmic eruption. The water level

continued to rise until reaching near presentday levels, where it encountered a thick layer

of porous deposits in the northeast caldera

wall. These deposits stabilize lake levels like

an overflow drain in a bathtub.

Sometime after these eruptions, the Chaski

Bay landslide, the top of which is still visible

above the southern shore of Crater Lake,

slumped into the caldera and ran up onto the

edge of the central platform. Other landslides

within the caldera were deflected by the base

of Wizard Island and the central platform,

burying explosion craters and other features on

the lake floor with even more debris.

The last known eruption at Crater Lake

occurred when a small lava dome erupted

under water on the east flank of the base of

Wizard Island about 4,800 years ago. Since

that time, the volcano has remained quiet,

allowing as much as 100 feet (30 m) of

sediment to accumulate on the lake bottom.

The Future

The long history of volcanic activity at

Crater Lake suggests strongly that this

volcanic center will erupt again. The most

recent eruptions occurred on the lake floor in

the western part of the caldera. Future

eruptions are more likely to occur in the same

area than farther east. Interaction of magma

and lake water at shallow levels (less than 100

feet or a few tens of meters) could generate

explosions that throw large rocks and ash out

beyond the caldera walls. The largest

explosions could produce pyroclastic surges,

hot, rapidly moving clouds of gas and ash,

which could move out a few miles from vents

along the margin of the lake. Eruptions in

deeper water are less likely to be explosive or

affect areas around the rim. An eruption

similar to the one 7,700 years ago is unlikely

because large volumes of magma are not

available to cause such an eruption.

Landslides from the caldera walls can

cause waves that flood shoreline areas, but one

that could cause overtopping or failure of the

caldera walls and catastrophic draining of

Crater Lake is unlikely. At its lowest point,

the caldera rim is more than 500 feet (165 m)

above the lake level. Only major volcanic

activity could cause the caldera walls to fail

and result in flooding of lowland areas below

Crater Lake.

Crater Lake lies within a region where the

Earth’s crust is being gradually stretched. The

faults that accommodate this extension could

produce damaging earthquakes in Crater Lake

National Park today. The West Klamath Lake

fault zone consists of numerous faults, one of

which passes 1/2 mile west of Rim Village.

Although earthquakes as large as magnitude

7.0 are possible, scientists believe that they

occur only about once every 3,000 to 10,000

years. Such earthquakes could cause landslides

large enough to generate dangerous waves on

Crater Lake. Smaller earthquakes are much

more likely, but they are unlikely to trigger

large landslides.

Cataclysmic eruption to present. Eruptions of ash and

pumice: The cataclysmic eruption started from a vent on the 

northeast side of the volcano as a towering column of ash, 

with pyroclastic flows spreading to the northeast. Caldera

collapse: As more magma was erupted, cracks opened up 

around the summit, which began to collapse. Fountains of 

pumice and ash surrounded the collapsing summit, and 

pyroclastic flows raced down all sides of the volcano. Steam

explosions: When the dust had settled, the new caldera was 

5 miles (8 km) in diameter and 1 mile (1.6 km) deep. Ground 

water interacted with hot deposits causing explosions of 

steam and ash. Today: In the first few hundred years after 

the cataclysmic eruption, renewed eruptions built Wizard 

Island, Merriam Cone, and the central platform. Water filled 

the new caldera to form the deepest lake in the United 

States. Figure modified from diagrams on back of 1988 

USGS map “Crater Lake National Park and Vicinity, Oregon.” 
Probing the Depths

Details of features beneath the 

surface of Crater Lake constructed 

using data from the 2000 bathymetry 

survey. Colors range from orange 

to blue with increasing water depth. 

For more than 100 years, scientists 

have marveled at Crater Lake’s great depth 

and sought to unravel the mysteries hidden 

beneath its surface. Clarence Dutton of the 

USGS led the first expedition to determine 

the depth of the lake in 1886. With a lead 

weight and piano wire, Dutton’s party made 

168 soundings from a rowboat and reported 

a maximum depth of 1,996 feet (608 m). The 

National Park Service made additional 

soundings of this type between 1938 and 

1940. 

It wasn’t until the advent of sonar that 

the nature of the caldera beneath the lake 

became clearer. In 1959, the US Coast and 

Geodetic Survey obtained more than 4,000 

echo soundings and provided a more 

accurate estimate of the maximum depth of 

Crater Lake 1,932 feet (589 m). Contours 

of these data revealed the principal 

features on the floor of Crater Lake – the 

central platform, Merriam Cone, a small 

lava dome on the east side of Wizard Island, 

and the Chaski Bay landslide – for the first 

time. About the same time, Crater Lake 

ranger-naturalist C. Hans Nelson collected 

dredge samples that showed a variety of 

post-7,700-year eruptive activity. In 1979, 

Nelson returned to the lake as a USGS 

marine geologist and used acoustic imaging 

techniques (like making a CAT scan) of the 

lake floor to discover that as much as 250 

feet (75 m) of sediment had accumulated 

since caldera collapse. Nelson’s study 

found that beneath the sediment were many 

depressions located along the calderacollapse ring fractures. These crater-like 

structures were probably formed by steam 

explosions as water began to accumulate 

within the still-hot debris shortly after the 

cataclysmic eruption. 

A later USGS party mapped areas 

where large amounts of heat escaped from 

the lake floor. In 1988 and 1989, scientists 

from Oregon State University, the National 

Park Service, and the USGS explored parts 

of the lake floor with a manned 

submersible, paying special attention to the 

warm areas identified earlier. Several dives 

were devoted to exploring the volcanic 

features on the lake floor and to sampling 

caldera-wall outcrops. The exploration with 

the submersible suggested the need for 

remapping the lake floor with modern 

techniques to provide a bathymetric (depth) 

map for interpreting the postcaldera 

geologic history and as a base for further 

study of the lake itself.

The new bathymetric mapping was 

completed in the summer of 2000 by 

scientists from the USGS, the National Park 

Service, the University of New Hampshire, 

andC&C Technologies. Using a highresolution acoustic mapping system 

mounted on a special boat, they collected 

millions of accurately located echo 

soundings during a 5-day survey. The 

information gained from this latest survey 

not only provided a new maximum depth for 

Crater Lake – 1,949 feet (594 m) – but also 

resulted in a detailed map of features on the 

lake floor. By combining the new 

bathymetric data with past decades of other 

research, scientists now have the clearest 

picture yet of events that happened since 

the massive eruption 7,700 years ago that 

destroyed Mount Mazama and created 

Crater Lake. 

– 

Ed Klimasauskas, Charles Bacon, 

and Jim Alexander 

Graphics and layout by

Ed Klimasauskas, Bobbie Myers,

and Peter Dartnell

COOPERATING ORGANIZATIONS

National Park Service

Oregon State University

University of New Hampshire

Crater Lake Natural History Association

For more information contact: 

U.S. Geological Survey

Cascades Volcano Observatory 

1300 SE Cardinal Court, Bldg. 10, Suite 100 

Vancouver, WA 98683-9589 

Tel: (360) 993-8900, Fax: (360) 993-8980 

http://vulcan.wr.usgs.gov/ 

or 

USGS Volcano Hazards Program 

http://volcanoes.usgs.gov/ 

or 

USGS Crater Lake Data Clearing House 

http://craterlake.wr.usgs.gov/ 

or 

Crater Lake National Park 

http://www.nps.gov/crla/ 

See also What Are Volcano Hazards? 

(USGS Fact Sheet 002-97), 

Volcanic Ash Fall—A “Hard Rain” of 

Abrasive Particles (USGS Fact Sheet 027-00) 

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