Download Lecture #12 – Volcanic landforms – Part II – super volcanoes and

Lecture #11 note; Geology 3950 2006; CR Stern
Volcanic landforms – Part II –
super volcanoes and giant calderas (pages 184-224 in the 5th edition)
Explosive eruptions of andesite and rhyolite with volumes >1 to 100 km3 typically
generate caldera, which then are reconstructed into stratovolcanoes by subsequent dome
building eruptions of relatively dry, volatile free magmas. Many such calderas have been
formed by this size eruption during the last few thousand years (see lecture #11)
However, much bigger explosive eruptions, in the range 500 to 5,000 km3 in volume of
new rock erupted, generate much larger calderas, often so large (100 x 50 km in size) that
they can not simple be seen or assessed as calderas without geologic mapping and/or
satellite photos. These giant calderas are typically related to eruption of rhyolitic magma
and they occur in areas of continental crust, above subduction zones (such as the Toba
caldera in Sumartra – see below), in areas of continental rifts (such as the Long valley
and Valles calderas in the western US), or above hot spots (such as the Yellowstone
caldera). Giant calderas do not form in oceans because rhyolites do not form in oceans.
No such giant caldera is less than 74,000 years old (Toba) and civilization would
certainly be devastated by such an eruption anywhere on the earth.
Giant calderas form in the same way as smaller caldera, by collapse of the roof into the
magma chamber after an explosive eruption, but typically go through more than one
cycle of eruption. As they prepare for a second or third eruption, new magma coming into
the very large magma chamber below the caldera causes them to uplift, or resurge, and
they are called resurgent calderas (see figure 1 below). Smaller calderas like Crater Lake
rebuild themselves into stratovolcanoes by new eruptions, but they are not “resurgent”.
Three giant calderas in the western US, all formed in the last 1 million years include the
Yellowstone hot-spot and two rift related supervolcanoes, Long valley caldera in eastern
California and Valles caldera along the edge of the Rio Grande river in New Mexico (see
figure 2 below) Each covered large areas of the US with volcanic ash when they last
erupted.
Yellowstone is the currently active volcanic center above a hot spot that breeched the
surface about 15 million years ago to create the Columbia plateau basalts (fig 3 below)
As North America drifts to the west pushed by the widening of the Atlantic ocean, this
hot-spot appears to have drifted to the east. Yellowstone park occurs at the eastern end of
the Snake river plain which is a depression resulting from the formation of numerous
calderas followed by thermal subsidence as the hot-spot migrated more to the east.
Calderas inside the plain have produce giant rhyolite extrusions dated as 12 Ma near the
Oregon, Idaho, Nevada border, and decreasing in age from 8 million, to 5 million, to
finally 2 million years in age inside the area of Yellowstone park
There are three nested calderas inside Yellowstone park (figure 4 below); the oldest and
largest was formed 2 million years ago by erupting 2,000 km3 of rhyolite mapped in
green in the figure, the next and smallest was formed 1.3 million years ago by erupting
300 km3 of rhyolite mapped as grey in the figure, and the youngest was formed 640,000
years ago by erupting 1,000 km3 of rhyolite, so that the cycles of giant caldera formation
have been approximately every 600,000 to 700,000 years, with the last at 640,000, so – if
the past is the key to the future - we are approaching another giant event. The continued
activity in this giant volcanic system is indicated by the thermal features in Yellowstone,
such as Old Faithful and other geysers, and the resurgence of two small domes inside the
youngest caldera. Also many minor eruptions have occurred inside the youngest caldera,
including the formation of obsidian cliffs at 120,000 and the formation of west thumb of
Yellowstone lake which is actually a caldera itself but not as large as the giant resurgent
caldera within which it occurs.
Standing on the inner wall of the youngest caldera (near the point marked wit the number
3) and looking south, one can barely see the other side of the caldera wall as a low lines
of hills in the far background (see figure 5 below)
Long Valley caldera, which erupted 1,000 km3 of rhyolite about 700,000 years ago,
occurs in eastern California occurs on the western edge of the Basin and Range
extensional tectonic province. It has a resurgent dome, a magma chamber at about 5-6 km
depth below the crust as identified by geophysical techniques (see figure 6 below), and
could erupt again any time in the future, but there is no information concerning the timing
of previous cycles at Long Valley
Valles caldera occurs on the western edge of the Rio Grande rift valley near Los Alamos
New Mexico. It erupted 900 km3 of rhyolite 1 million years ago. Canyons cut in this
rhyolite 300 m thick rhyolite pyroclastic flow, called the Bandelier tuff (see figure 7
below) were inhabited by the Anastazi Indians and are the center of the Bandelier
National Monument
Approximately 20 other such giant calderas have formed world wide in the last 1 million
years, which implies, on the average, one forming every 50,000 years. Some examples
are the Kayseri caldera in the region of Cappodacia central Anatolia Turkey, where the
rhyolitic pyrolcastic material has been eroded into cones and spires inhabited by the
locals (see figure 8 below)
Another, the Maipo caldera, one among 8-10 giant calderas in the Andes, erupted 500
km3 of rhyolite 450,000 years ago. This material flowed both east into Argentina, and
west into Chile, over 300 km to the Pacific coast, covering an area now occupied by the
city of Santiago and 6-7 million people. Currently a large andesitic stratovolcano occurs
in the center of the Maipo volcano. (see figures below)
The youngest giant caldera know is one of the largest, the Toba caldera in Sumartra
Indonesia, just south of where the Banda Aceh earthquake occurred, which erupted 2,500
km3 of rhyolite 73,500 years ago (see figure below) This eruption was though to have
lowered global temperatures by 3-6 degrees for a number a years during the middle of the
last ice age and reduce human population to as little as 5-10,000 individuals worldwide.
The largest giant caldera in the world, the Garita caldera, which erupted the 5,000 km3
rhyolitic Fish Canyon tuff, formed in the San Juan Mountains of Colorado, but not in the
last 1 million years – it is 28 million years old. This was during the early stages of rifting
related to basin and range extension in the western US.
Numerous other now extinct calderas occur in the San Juan Mountains, Utah, Nevada,
Arizona and New Mexico, all related to extension just as the still active Long Valley and
Valles caldera are today.
Summary of the tectonic environments of volcanism in the western US
Volcanoes in the western US occur in the subduction related cascade volcanic range, the
snake rive plain-Yellowstone hot-spot and the basin and Range and Rio Grande rift
extensional areas. The map of volcanic risks in the western US puts most significance on
the Cascade stratovolcanoes because of their relatively frequent eruptions, height and
large volume of snow pack, but it is clear that the really largest volcanoes, with the
greatest potential for producing high catastrophes, occur above the Yellowstone hot-spot
and in the rifting portion of the west at Long valley and Valles caldera.
No volcanism occurs east of Colorado. No volcanism occurs along the San Andreas fault
or near Los Angeles in California, but small basaltic cones again begin to appear in
southernmost California near the Salton Sea as the San Andreas fault merges with the
oceanic spreading ridge-rift system inside the Gulf of California (see figure 2 in lecture 9)
The giant Columbian river basaltic plateau lava field formed in the northwest 15-16
million years ago with the first appearance of the Yellowstone hot spot, and numerous
giant calderas formed between 16 Ma and the present as this hot-spot migrated east, and
also from 30 million years ago to recent times as the Basin and Range extensional
province developed, including the world’s largest know Garita caldera in the San Juan
Mountains of Colorado. These created the rich gold deposits in the San Juans.
Prior to 30 million years ago, between 80- 40 Ma, volcanism occurred in Colorado in
association with plate convergence and low angle subduction that produced the thrust
faults uplifting the Colorado Rocky Mountains (see figure 7 in lecture #5). The basalt
lava flows in Golden are 65 Ma and belong to this episode of volcanism.