VOLCANOES
Both earthquakes and volcanoes are surface expressions of heat within the Earth's interior. The lava
that erupts from a volcano has its origin within the Earth's mantle. At depths of approximately 75250 km mantle rocks can melt, generating magmas that rise to the surface and erupt.
A quick tour of volcanoes of the world would show us that there are two fundamentally different
behaviors that occur when volcanoes erupt. We'll tour two volcanic mountain ranges, Hawaii and the
Cascades to explore these differences. Click along the menu on the left side of this screen to tour the
different volcanic mountain ranges.
Hawaii is a chain of volcanoes that rise from the floor of the Pacific Ocean. There are five volcanoes
visible in this photograph. The two large ones are Mauna Kea (left) and Mauna Loa (right). On the
right-hand flank of Mauna Loa is Hualalai, and in front of Mauna Kea is Kohala. These four mountains
are all on the Big Island of Hawaii. The black slope in the foreground is Haleakala volcano, on the
island of Maui, and the vantage point from which this photo was taken.
Notice that the shape of these volcanoes is not what you might think of when you imagine a "typical"
volcano. The Hawaiian volcanoes are all very broad with gently sloping sides. These are referred to as
Shield Volcanoes. They are very large in comparison with other volcanoes; the summit of Mauna Kea
is 4205 meters (13,796 feet) above sea level, and its base is another 5000 meters below sea level.
The lavas that erupt from Hawaiian volcanoes are very hot (1300°C) and very fluid. This produces
beautiful lava fountains, lava rivers, and lava ponds, rather than enormous explosions. The lavas also
spread and flow across the surface. These fluid lavas have a low viscosity, meaning that they flow
easily (viscosity is defined as resistance to flow). The speed of erupting Hawaiian lavas is 10's of miles
an hour, and lava fountains can reach a height of 600 meters. Because the eruptions are nonexplosive it is possible to get very close to the flows--close enough to sample the molten lava with a
rock hammer, or even to toast marshmallows!
Although Hawaiian eruptions are not explosive they can still cause damage to property. A number of
villages on the Big Island of Hawaii have been destroyed by recent eruptions of Kilauea volcano. The
I-beams in this photo are all that is left of
a Visitor Center in Hawaii Volcanoes
National Park.
The low viscosity lavas of Hawaii are capable of flowing very long distances. Some lava flows from the
summit of Mauna Loa have traveled 60 kilometers to the city of Hilo. Here lavas from Kilauea flow
into the ocean. Although the flows can be long, they are confined to narrow channels, and thus do
not cause widespread damage.
Hawaiian lavas cool to form a rock called basalt. Basalt is black or dark gray in color. It is a low-silica
rock, containing 45-55% SiO2. Typical minerals in a basalt include olivine, pyroxene and plagioclase
feldspar. The fluid nature of Hawaiian basalts is evident even after the rock has cooled. This photo
shows a typical "pahoehoe" flow. Pahoehoe is a Hawaiian word that means "ropy."
CASCADES VOLCANOES
Here is a photo of Mt. Rainier, taken from the ferry that crosses Puget Sound to the city of Seattle,
Washington. Mt. Rainier is a stratovolcano, located in the Cascade Range of the Pacific Northwest. It
shows the classic cone shape characteristic of this type of volcano. It is 4390 meters (14,410 feet) tall,
but is much smaller in total volume and mass than shield volcanoes such as Mauna Loa or Mauna Kea.
When stratovolcanoes erupt there are few lava flows. Rather, the eruptions consist largely of ash and
"pyroclastic" flows. Pyroclastic material consists of fragments of rock, ash, or magma ejected at high
velocity from the volcano. The eruption temperature of these materials is usually 600-800°C. They
leave the volcano at supersonic speeds and can reach heights of 70,000 feet (21 km) or more. The
lava that does flow from stratovolcanoes has a very high viscosity and moves extremely slowly,
forming thick, short flows and domes.
Click on the image mosaic below to see larger versions of the photos.
Because of the explosive nature of stratovolcanoes, an eruption can cause rapid and widespread
damage. These trees were knocked down in the blast from Mt. St. Helens, in 1980. If you look
carefully at the red box drawn on this image, you'll see two people standing at the base of the hillside,
which gives you a sense of how large these trees are. The pyroclastic surge traveled at 450 miles per
hour with a temperature greater than 600°F. Ashfall from Mt. St. Helens, also in the Cascade Range,
reached as far away as Colorado and Minnesota.
Crater Lake sits inside the remains of Mt. Mazama, another Cascade volcano. Ash deposits from the
prehistoric eruption of Mt. Mazama are widespread. Here ash entombs tree trunks from a former
forest. Ash is very highly fragmented lava. When lava flows do occur in the Cascades, they form the
rocks Andesite and Dacite. These are light gray to white in color (notice the white ash), and contain
55-70% silica (SiO2). Typical minerals include quartz, sanidine feldspar, biotite mica and amphibole.
Click on the image mosaic below to see larger versions of the photos.
Both the Hawaiian volcanoes and those of the Cascade Range form a linear chain. This is true of
virtually all volcanoes around the world. Indeed, the chain of volcanoes that nearly circles the Pacific
Ocean is so striking it is referred to as the Ring of Fire. The details provided here about Hawaii and
the Cascades should convince you that these two chains of volcanoes have very different origins. As
mentioned at the beginning of this section, both earthquakes and volcanoes are surface expressions
of heat within the Earth's interior, so let's examine volcanoes and earthquakes together in order to
better understand why the volcanoes are so different.
Series of interactive exercises follow…