ISNS 4359 EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 15. Calderas, The Big Ones: produce the largest, most violent eruptions
A caldera (Spanish for cauldron or kettle) is a large (>2 to ~100 km diameter), circular or elliptical
depression formed when magma is withdrawn or erupted from a shallow underground magma reservoir.
The removal of large volumes of magma forming pumice-rich sheets may result in loss of structural
support for the overlying rock, leading to development of a ring fracture, collapse of the ground and
formation of a large depression. Crater-basin on the top of a cinder cone or stratovolcano <2 km diameter.
Caldera Analog-Piston subsiding in a cylinder
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Calderas are “negative” volcanoes
Typical calderas are 5-25 km across, with steep, 1 km high walls, and a flat floor.
During historical times, >1,300 episodes of ‘unrest’ have occurred at ~140 calderas >5 km diameter.
Each year, some form of unrest (earthquakes, ground deformation, change in fumarole activity, or
eruptions) occurs at ~20 large calderas worldwide, and eruptions occur within >5 of them; on average,
only 2 or 3 calderas form each century.
Five Main Caldera Types:
Basaltic shield calderas are small (~0.1-0.2 km deep, <~5 km diam.; VEI<~5), circular depressions at the
summit caused by quiet magma withdrawl (eg. Mauna Loa, Galapagos, Reunion, Galapagos).
Stratovolcano collapse calderas are the most abundant, (~0.1-1 km deep, <~10 km diam.; VEI~4-7),
caused by shallow magma chamber withdrawl (eg. Tambora 1815 Indonesia; Pinatubo 1991 Philippines; Azores).
Volcanic field calderas (2~30 km diam., VEI~6-7) are caused by collapse with violent explosions and eruptions
encompassing several individual volcanoes (eg. Krakatau 1883 Indonesia; Crater Lake, OR; Santorini).
Debris avalanche calderas (2~10 km, VEI~5-6) occur in stratovolcanoes when a flank avalanche causes a violent
eruption, creating a hole without collapse (eg. Mt. St. Helens 1980; Tanerife, Canary Is; Bezymianny).
Ash-flow tuff calderas are the largest and rarest (~30-100 km across, but shallow, VEI~7-8), usually
associated with extremely large (>50-100 km3) violent rhyolite eruptions (eg. Taupo 186 AD NZ, Yellowstone 0.6 Ma WY;
Toba 73,500 BP Sumatra).
Four Selected examples:
Yellowstone National Park, WY consists of 3 calderas that erupted ~2.2 million years ago (Ma) (volume =
2500 km3), 1.3 Ma (V=280 km3), and 0.63 Ma (V=1000 km3). The youngest caldera is 45 x 75 km across.
Lake Toba, (Sumatra, one of the largest calderas on Earth ~30 x70 km), erupted 2000 km3 of rhyolite
ash ~73,500 years ago, blanketing 25,000 km2 of land, and 5 million km2 of Indian Ocean floor.
It may have caused/contributed to the last ice age (decreasing N Canada surface temp. by 10-15°C).
Thera, Santorini Aegean Sea (Greece) (~10 km diameter, 0.4 km deep) ~30 km3 of magma erupted
1628 BC; destroying the Minoan Bronze Age city of Akrotiri (Plato’s lost city of Atlantis). Activity
began ~ 650 thousand years ago (ka); since 360 ka, over a dozen large explosive eruption cycles have
occurred every 3010 ka (basalt, andesite, dacite, rhyolite); ~11 recent eruptions: 197 BC-1950.
Crater Lake (Mount Mazama, OR) (~10 km diameter, 0.6 km deep) ~40 km3 of magma erupted 5677
BC, ash preserved in Greenland Ice Sheet.
Caldera-forming ash flow tuff eruptions are the largest eruptions on Earth!
For example, the Fish CanyonTuff eruption in SW Colorado (~28 Ma) erupted >10,000 km3 of magma
from La Garita caldera, enough magma to bury the entire state of California to a depth of nearly 20 m!
Caldera cycle: 1) pre-caldera volcanism, 2) eruption, 3) collapse, 4) depression fills with a lake,
5) sedimentation, 6) resurgence (uplift), 7) post-caldera lavas leak out along ring fractures, repeat 1-7.
One cycle may require hundreds of thousands to millions of years and requires a shallow magma chamber.