Volcanic Landforms
Pictures and Descriptions
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The volcanoes themselves, of course, are landforms. Some (shields and cones) are
much more obvious than others (fissures and calderas) unless you know what to look
for.
Mauna Loa – a shield volcano
Capulin, NM – a cinder cone
(photo courtesy US Geological Survey.
J.D. Griggs, photographer)
Sunset Crater, AZ – a collapse caldera
Mt. Ranier, WA –
a composite cone
Valles Caldera, NM – an explosive caldera
Black Mesa, NM – a flow from a fissure eruption
There is a variety of small secondary volcanoes as well.
A resurgence dome in the Valles Caldera, NM.
Nothing actually erupts but pressure from beneath
arches the interior of the caldera upward after the
main explosion.
There are small secondary volcanos in the explosive caldera of Mt. St Helens, WA.
These are “plug domes”. The lava is so thick it barely flows.
(Mt. St. Helens, WA, courtesy of US Geological Survey. Panorama by Willie Scott of the USGS.)
Wizard Island in Crater Lake, OR is another secondary cone.
A small spatter cone near Sunset Crater, AZ.
Forms when the chilled top of a flow breaks open
and lava erupts through it.
(Image from Google Maps)
Forms of Lava Flows
This photograph, from the top of Capulin Volcano, NM, shows several lava flows.
Because there is no lava in the cone itself, they must have erupted from cracks
around the base of the mountain.
A couple of the flow margins are outlined, and you can probably recognize at least one other flow. The front of the
flow always bulges outward.
You can determine the margins of the flows because the loose material that accumulates there holds water and
allows plants to root and survive better in this desert climate.
You can also tell which flow occurred first by seeing which covers the other.
The leading margins of flows
always bow outward because
the flow is thickest and
fastest near the middle of
the flow. The advancing
lateral edges move relatively
slowly and cool relatively
quickly; the middle outruns
them.
1984 lava flows on Mauna Loa, HI. Photo courtesy US Geological Survey, ‘RBM” photographer.
The convex margins are
obvious on these flows near
Capulin, NM.
The advancing front is also convex in cross section.
The pressure of the flowing magma is greater near
the bottom of the flow because of the weight of
the thickness (dimensionally, not in terms of
viscosity) of the lava, so that outruns the top.
The front moves in fits and starts, not smoothly and continuously. It advances a little and
the margin chills. The next time it advances the previous margin is not necessarily evenly
driven forward by the pressure. In the diagram, former fronts of the flow are shown by the
dotted lines. The solid red line shows the eventual surface of the flow, including the front
of the last surge of the flow.
The upper surface of the flow thus has ridges and swales on it.
Remember that all these fronts bulged forward in map view too. The ridges and swales are
therefore curved across the upper surface, convex in the direction of flow.
These highs are called pressure ridges.
Over time, weathered material from the ridges washes into and accumulates in the swales. This loose material serves as a soil for plant roots
and holds water to sustain them, even in this desert climate. The swales then become very obvious when viewed from above and afar. Some
are indicated below, and you can easily spot others.
The surfaces of flows exhibit interesting small scale textures as well. Closer to the vent the lava is
still relatively hot, and even after the surface crystallizes and crusts over it behaves plastically. The
pressure of the underlying lava creates bends and buckles in the surface not unlike tiny pressure
ridges. This surface is said to be “ropy” and is called pahoehoe. (This is a native Hawai’ian word
for this lava texture. It is pronounced something like “pa-hoy-hoy”.)
Pahoehoe in HI, courtesy US Geological Survey. Photo by Alicia Burtner of the USGS.
Farther from the vent the lava is cooler, and the
solidified surface behaves more brittly. The
pressure of the underlying lava breaks the crust
into angular blocks and rotates them randomly
away from the surface, exposing the sharp
corners. This surface is said to wreck boots very
quickly. Aa (pronounced as two short a’s) is the
name for this type of surface – another native
Hawai’ian word.
Pahoehoe in HI, courtesy US Geological Survey. Photo by Tim Orr of the USGS.
Aa near Sunset Crater, AZ.
Once the lava hardens and stops it begins to
contract. Initially, while it is still relatively plastic, it
can shrink as a single body.
Eventually though the lava is cool enough that the
stresses associated with the shrinkage become
more and more localized. Cracks form within the
body and penetrate through it. This works in lava
flows and also in small plutons (intrusions.)
The Giant’s Causeway on the northern coast of Ireland shows the results beautifully. You can see the cracks in the
surface of this ancient basalt flow at the bottom of the picture, and how they extend through the flow where the
waves have eroded into them near the top of the photo.
These are called columnar basalts.
Giant’s Causeway, County Antrim, Ireland. Photo by “Code Poet”, posted on Wikipedia. Used under Creative Commons License.
The Bear Lodge in WY (more commonly, and officially,
called by the incorrect translation of the Native
American name, “Devil’s Tower”), is a small intrusion (a
laccolith). The fractures and columns are obvious in this
photograph. They are still called columns, even though
this is neither a flow nor a basalt.
There are columnar basalts in
Shenandoah National Park, VA that are
very ancient. They formed on a seafloor
that existed in the region before the
Appalachians were raised.
One of the column ends is outlined, and
you should easily spot others.
Bear Lodge (Devils Tower), WY, courtesy US Geological Survey. Photo by Alex Demas of the USGS.
When lavas erupt underwater the margins chill and harden very quickly. While they are still plastic the margins
inflate like water balloons, but once they are sufficiently brittle they crack. The lava escaping from the crack starts
the process over in an adjacent spot, and so on. These are called pillow lavas.
There are metamorphic rocks in the Georgia piedmont near Carrollton that may be metamorphosed pillow lavas
from the same seafloor as the columnar basalts in Virginia.
Waters off Hawaii. Photograph courtesy US National Oceanic and Atmospheric Administration (NOAA). Posted on Wikipedia.
Craters and Calderas
When a volcano has a small opening on a high summit that opening is called the crater.
This term is generally used for the opening from which eruptive material leaves the
volcano, and in this sense it is also called the vent.
Mt. Fujiyama, Japan. Photograph is in the public domain according to copyright law of Japan. Downloaded from Wikipedia.
In some cases the opening at the top of the volcano is much larger than necessary for a
normal lava eruption. These are called calderas. They can form in two ways.
Sunset Crater, AZ is a collapse caldera.
< ¼ mile
After a particularly large eruption brings
A great deal of lava (and maybe some pyroclastics)
to the surface. All of this material was formerly
helping to support the volcano, but now that
support is gone
The top of the mountain falls
in to fill the now empty space.
The volcano that erupted here (near Crede, CO) was explosive. It is surrounded by a huge volume of pyroclastics
(some of which, I believe, reached south Georgia). When it exploded it blew a hole in the Earth over ten miles across.
If there was a mountain before, it was obliterated. Compare the scale to Sunset Crater on the previous slide.
The mountainous region in the middle of the explosive caldera (including Snowshoe Mountain) is a resurgence dome.
Image from Google Maps
2 miles
Just for comparison, the Yellowstone Caldera in Wyoming and Idaho is
about 34 x 45 miles in size. It is so large, and the explosion was so
destructive, that the edges of it are hard or impossible to spot in satellite
images, even though it is far younger than the Crede Caldera in Colorado.
The rock beneath is still hot enough to produce hot springs and geysers.