Visitors to this national park marvel
at the spectacle of a large volcano that collapsed nearly eight thousand
years ago to produce a basin now filled with indescribably blue water.
Indeed, the geological story of what happened to produce the Crater Lake
basin that we see today may be the best description of a "young" caldera
in the world. There are, however, numerous other interesting volcanic
features in the park. At least 20 cinder cones have been identified
within the boundaries of Crater Lake National Park. And, of course,
Wizard Island, the most famous of these, rises more than 700 feet above
the lake's surface near the western shoreline and is well known to all
Cinder cones as landforms
Figure 1: Location of the major cinder cones
in Crater Lake National Park, Oregon. The dashed line inside the
caldera represents the location of Merriam Cone and the ring
fracture zone where post caldera eruptions reached the surface.
The Central Platform is outlined by the dotted line. Sketch by
One simple method of classifying
volcanoes groups them into three categories: shield volcanoes, strato-volcanoes,
and cinder cones. Shield volcanoes are massive, low, dome-shaped
features produced primarily by fluid lava flows, and usually composed of
basalt. The islands of Hawaii are examples of these huge volcanic
features but only a small portion is visible above sea level. A strato-volcano
is constructed layer by layer of more or less alternating lava flows and
pyroclastics (loose fragments erupted during explosive activity) that
tend to be large features on land with relatively steep slopes. They are
the subjects of picture postcards and calendars. In the Cascade Range,
Mount Rainier and Mount Shasta are good examples of strato-volcanoes.
Cinder cones are the "baby" members of
the volcano family listed above. They seldom exceed a mile across at the
base and a thousand feet high. Unlike the shield or strato-volcanoes,
this smallest member of the volcano family have an extremely short life
span—being active for just days to a few months in most cases. Only in
rare occasions does their activity extend for a year or more. From a
geological perspective, this is like an instant or a snapshot in the
volcanic record of an area. Due to their origins, cinder cones are
called "monogenetic," with the entire feature built during one eruption
episode from one source of magma.
A typical cinder cone eruption begins
by venting magma rich in gas that expands, producing solid fragmental
volcanic products like blocks, bombs, scoria, ash and dust. This ejected
material, the pyroclastics noted above, accumulates at a single location
to build an inverted cone around the vent. The larger and heavier
particles collect closest to the vent, while smaller and lighter ones
drift farther—or are carried away by the wind. As the cone grows, a
central region may develop composed of the larger pieces (blocks, bombs
and scoria) mixed with more fluid magma that hardens into a rigid core.
On a young cone this core is seldom seen as it is usually buried by
additional pyroclastic material blown out of the vent later.
Figure 2: Schematic cross section of magma
sources for cinder cones and the climatic eruptions at Crater
Lake National Park. The outer margins of the Climatic Eruptive
Magma Source defines the shadow zone, that portion of Mount
Mazama that collapsed to form the caldera. Sketch by the author.
In time, the gas dissolved in the magma
that powered the explosive initial eruption is exhausted and magma
reaches the surface as lava. When this happens, lava flows may break out
from the base of the cinder cone and flow away from the volcano. Magma
forming cinder cones is typically basaltic or basaltic-andesite in
composition. Lavas with this composition tend to be relatively hot and
flow readily, sometimes for great distances. Although the most prominent
part of these small volcanic features is the cone that develops,
associated lava flows may actually contain up to ten times more volcanic
material. Since cinder cones are relatively small features, they are
often overlooked. They are, however, the most abundant kind of volcanic
cone—being common worldwide and numbering in the thousands. One report
suggests that at least 400 cinder cones have formed in the Oregon
portion of the Cascade Range alone. Most of those in Crater Lake
National Park are associated with the construction of Mount Mazama.
Wizard Island, of course, was formed in the caldera following the
collapse of Mazama's summit.
Cinder cones generally develop in
volcanic areas that do not have larger volcanoes, or are associated with
more massive volcanic cones. Those related to larger volcanic features,
like shield and strato-volcanoes, usually develop along weak areas in
existing rock. Such zones are often referred to as basement fractures
and tend to be radial to the larger volcano—something like the spokes of
a wheel. Although this is not obvious for the cinder cones at Crater
Lake, connecting certain pairs does produce a crude radial pattern.
Examples are Desert Cone and Red Cone, or Scoria Cone and the adjacent
Cinder cones in the park
The southern flank of Williams Crater along
Rim Drive on the west rim of the caldera. Basaltic magma rising
to the surface vent here appears to have "tapped" some magma
from the Climatic Eruptive Magma Source to produce "mixed magma"
rocks. Photo by the author.
The cinder cones in Crater Lake
National Park fit into two categories: those associated with the small
basaltic shield cones of Union Peak and Timber Crater, and all the
others that are related to Mount Mazama. Hill 6902 and the cinder cone
on its summit appear to be part of the Timber Crater shield volcano in
the northern portion of the park. In a like manner, the southwestern
quadrant holds Castle Point and its summit cones, along with several
smaller cinder cones belonging to the Union Peak shield volcano.
Red Cone and Desert Cone, clustered in
the Pumice Desert in the northwestern section of the park, are good
examples of cones related to Mount Mazama. Both appear symmetrical when
viewed from the North Entrance road, a distance of about a mile. As with
most other cinder cones throughout the park, both have a symmetrical
appearance when viewed from above. Other prominent Mount Mazama cinder
cones include Bald Crater, Crater Peak, Scoria Cone and Maklaks
Crater—the latter is called Diller Cone on older maps.
Howell Williams at Crater Lake in 1965. NPS
photo by Ed Paine.
The cinder cones associated with Mount
Mazama exhibit a large range in volume. Of the eight more prominent
cones for which a volume has been determined, Crater Peak is the largest
with a volume of just under a tenth of a cubic kilometer. Hill 6545,
near Scoria Cone, is the smallest measuring somewhat less than a tenth
the size of Crater Peak. Wizard Island's volume falls about midway
between these extremes.
Unlike the typical cinder cones
described above, cinder cones located away from the caldera exhibit few
lava flows. Since all of these cones predate Mazama's climactic
eruption, it could be that the flows are buried by eruptive material.
The tendency to erode easily is another factor since the edifice of
cinder cones is composed of loose debris. Older cinder cones are thus
more rounded with lower slopes that result from erosional effects. In a
like manner, summit craters are typical in younger cinder cones, like
the crater nearly 100 feet deep on Wizard Island. For the other Crater
Lake cones, however, only shallow depressions remain of any craters that
may once have been present.
Red Cone illustrating pyroclastic debris and
forested northern slope. This cinder cone is typical of the
small monogenetic volcanoes related to Mount Mazama outside the
caldera. Photo by the author.
All of the Crater Lake cinder cones are
similar in composition. One method of describing and comparing the
composition of volcanic rocks is expressed by how much silicon (Si) and
oxygen their rocks contain. Reports of chemical analysis combine these
two elements and express them as the percentage of SiO2. The
great majority of cinder cones found in the park have a SiO2
range of between 52% and 58%, thus producing igneous rocks called basalt
or basaltic andesite.
Based on the amount of weathering, soil
cover, erosion and a few radiometric dates (the measurement of
geological time by means of the rate of disintegration of certain
radioactive elements), most cinder cones in Crater Lake National Park
appear to be less than 50,000 years old. Radio-metric dates (using the
potassium-argon method of dating, or K-Ar) have been made on rocks from
four cones: Timber Crater, Red Cone, Scoria Cone and Desert Cone. Only
the results for Desert Cone indicate an earlier formation with a date of
about 200,000 years. It should be noted, however, that the ages
resulting from such techniques have a very large range. For example, the
age for Red Cone rocks is recorded as 36,000 years before present, plus
or minus 12,000 years.
Williams Crater & Wizard Island
Figure 3: This east-west profile of Red Cone
illustrates the typical shape of cinder cones at Crater Lake.
Slopes for cones outside the caldera suggest some decrease from
the original shape due to erosion. Angles for young, fresh
cinder cones tend to be about 30°, similar to that of Wizard
Island which has a slope of 29°. Sketch by the author.
Just outside the west caldera rim a
small cinder cone can be found called Williams Crater, previously
labeled as "Forgotten Crater" on older maps. This feature may provide an
interesting insight into the nature of the magma sources that produced
Mount Mazama and the climatic eruptions. Some erupted materials appear
to be formed from "mixed magmas" and contain both low and high SiO2
compositions. These have been interpreted as a mixing of the two
different magma sources associated with the Mount Mazama region - the
deep magmas and the climatic eruption magma source. Based on glacial
erosion and other evidence, Williams Crater appears to have been active
between 22,000 and 30,000 years ago.
All the volcanic features on the floor
of the Crater Lake caldera developed after the collapse of Mount Mazama.
Soon after the climatic eruption and collapse that created the caldera,
renewed volcanic activity formed the central platform, Merriam Cone,
Wizard Island, and other features. An estimated 3 km3 of post
caldera volcanic material was erupted through a ring fracture system -
most of it part of the Wizard Island edifice. Merriam Cone, rising some
four hundred meters above the caldera floor, has the general appearance
of a small cinder cone. Recent evidence, however, suggests it was formed
below water. Wizard Island's cinder cone rests on top of a pile of lava
flows that extend eastward over the central platform.
Several research devices and techniques
were used to map the topography of the caldera floor and investigate the
nature of the rocks and sediments that occur in the Crater Lake basin.
Rock samples collected from some 250 feet below lake level, on the
flanks of Wizard Island, appear to have been in placed under water. The
composition of these samples is identical to the youngest sub-aerial
flows of the island's cinder cone located above them. All this suggests
that Wizard Island was built on top of lavas erupted to create the
central platform in the rising waters of Crater Lake.
Timber Crater across Pumice Desert as viewed
from the North Entrance road. This cone, resting on the summit
of a small shield cone, is the largest cinder cone in Crater
Lake National Park. Photo by the author.
The maximum age of Wizard Island and
Merriam Cone is constrained by the date of Mount Mazama's collapse a
little less than eight thousand years ago. There are various estimates
of how long it took for the lake to fill to its current level. Other
evidence suggests that Wizard Island formed very early in the history of
the basin. Using radiocarbon (C-14) dates for the collapse, and allowing
a few hundred years for the lake to fill, places Wizard Island's age at
about 7,000 years before the present. Its conical shape gives Wizard
Island the look of a young feature, having suffered little erosion.
Since there are no "hot" areas outside the caldera to suggest remnant
volcanism, the lava flows at the western base of Wizard Island may
represent the most recent volcanic activity in the park.
Over the past 20 years, a large, number
of publications have resulted from intensive geological fieldwork. These
efforts have concentrated on the construction, climatic eruptions, and
ultimate collapse of Mount Mazama, resulting in the Crater Lake caldera.
This is, of course, the geological story to be presented at Crater Lake
National Park—the very reason the park was established. Other geologic
features, such as the cinder cones, however, are also worthy of further
K.R. "Rod" Cranson is a geologist who worked at the park twice,
1967-68 and 1978-82. He is currrently in the midst of preparing a third
edition of his book, Crater Lake: Gem of the Cascades.