Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Jaci Sieler
Remnants of Long Ago Giants
You may think of
rocks as materials or objects that are permanent and durable. Volcanoes being
made up of durable material, fall under this thought as well. They are massive
protrusions from the earth that spout, spray and hurl material from their magma
chambers to earth’s crust. Volcanoes have been around since the beginning of our
Volcanoes come in
different shapes and sizes as well. Basalt volcanoes, which have low viscosity,
include Shield Volcanoes most well-known from the
of Hawaii and the Galapagos Islands.
*Shield Volcano in the Galapagos
These volcanoes have long low profiles yet account for the largest volcano known to man. Olympus Mons on the Planet Mars. Which is 360 miles across and 16 miles high. Cinder Volcanoes, which are not large in stature, are volcanoes that are not made of densely compacted materials and therefore send a cloud of cinders into the air upon explosion. Haleakala Hawaii and Sanfransico Peak in Flagstaff Arizona are good examples of this type. The following are pictures of cinder cones.
*Cinder cone in the crater at the top of Haleakala-3(Hawaii)
loose cinders in foreground San Francisco Peak, Flagstaff Arizona
A Maar volcano is the last example of a basaltic volcano.
Maar volcanoes are
extremely explosive due to their high amounts of pressurized, dissolved gas
under the surface. These volcanoes erupt without warning leaving behind a huge
crater. The after math can be confused for the drop site of a hydrogen bomb. A
well-documented site and remnant of Maar volcanoes is found
in the Pinacates Mountains near Rocky Point Mexico.
*The Maar volcanoes literally blew themselves up!
Another type of Volcano that is not basaltic are Intermediate or Composite volcanoes. These volcanoes look like the drawing a child would create of a mountain, steep sides with snow on top. Intermediate/Composite volcanoes have alternating layers of erosion resistant material and cinders which are not resistant to erosion. Mt. Saint Helens and Mt. Rainer are visual examples of this type of volcano.
Mt. St. Helens before the eruption in 1980.
*Mt. Rainer in the background.
All of these volcanoes are massive in comparison to their surroundings. Yet erosion can, and will erase them from the landscape with time. How can this be done, one might ask?
This will be the
conversation of the remainder of this paper; the erosion of volcanoes. Erosion
takes place through wind, rain, and sun. Rain mixes with chemicals as it falls
making an acid that dissolves the rock with time. Water leeches into cracks and
crevices in the rock surface, as temperatures drop below freezing the water
turns to ice and expands. This leads to the rock cracking, splitting, and
crumbling. Weakening of rock can also take place by the expanding and shrinking
caused by heat. This action can cause rock in a dry desert environment to become
sand. Once this has taken place, wind blasts away loose material. These physical
phenomenon make short work of cinder cone, shield volcanoes and more.
A Cinder Cone
volcano, often called Cinders or
Scoria is made of glassy gas bubbles “frozen” into place when the magma explodes
into the air and suddenly cooled. This replaces the molten magma of other
volcanoes. Cinder Cones dot the land in North New Mexico and Eastern Arizona,
but are often hard to find and
identify due to their cinder nature ( they blow away). identify due to their cinder nature ( they blow away).
Cinder Cones have a wide range in size from tens to hundreds of
meters tall. Cinder cones are made of
pyroclastic (yellow/red hot material that is flung out of the open volcano
vent/mouth) material. Cinder Cones tend to have a bowl-shaped crater at
the summit. Erosion works on this bowl-shaped crater. Rain and wind erode the
rim of the crater until it breaks open and the contents of the bowl are able to
spill out during rain, wind, and rockslides. As this happens the cinder cone
erodes away becoming less and less recognizable until only a ring where the
original volcano is left.
*Aerial photos of cinder conesà
This erosion takes place with all volcanoes just in varying degrees and speeds due to differences in hardness, toughness, types of cementations (rocks glued together by other material acting as natural cement), and chemical makeup. This is called differential weathering, which works with erosion. Cinder Cones due to their physical makeup are the most rapid to show signs of erosion.
A Maar volcano due to its powerful explosion/eruption has already
given erosion a head start. The result is a crater, which can fill with water
and become a kettle lake in wet climates, or can be an empty bowl,
weather-beaten and grooved on the outside through erosion by heavy rains and
winds, as is visible in this photo below.
*Notice the erosion channels down the side of the volcano. Erosion is working
her magic through wind and rain.
Weathering although a different process aids in the process and break down of volcanoes. The Hawaiian Volcanoes being basal turn to hematite once weathering sets in. This is the process of the material turning a reddish rust color to the crumbly fertile soil of the Hawaiian Islands. During this process, the volcano becomes smaller as it breaks down from rock to soil. This process can readily be seen on the Big Island of Hawaii. The rich black lava flow after a few years oxidizes and turns to hematite, a rust colored material. This then breaks down due to sun, wind and rain into the fertile soil of Hawaii. As seen in the photo below
coastline, Maui. The red color is due to the presence of hematite.
As impressive as these past volcanoes have been the most impressive specimen of
erosion is of Ship rock in New Mexico. A Volcanic Neck reaching 600 meters into
the sky, 500 meters in diameter and dikes lengths of 2 miles. The sight of this
massive volcanic neck leaves the viewer mind boggled at the size of the volcano
that once covered it.
*The long ridge features in the photos are volcanic dikes.
photos show the expanse of the volcanic dikes that ran along the base of the
volcano 30 million years ago. The following illustration helps to put into
perspective what the volcano would have looked like, along with how extensive
the erosion is.
Although Shiprock is believed to have been created as much as 1,000
meters below the Earth's surface, millions of years of differential erosion have
uncovered the main volcanic neck,
This erosion took place over the last 30 million years. In geological time that
is quick. A whole volcano minus the neck and dikes eroded away. Erosion is
Differential Weathering is to blame for the outer part of the volcano eroding
away while the inner neck and dikes remain behind. The volcanic neck “Ship
rock”, and its dikes were more resistant to erosion and weathering and thus are
still present today. A prominent volcanic neck such as Shiprock is a very
As was stated at the beginning of the discussion volcanoes are huge and look
permanent this is not the case. Erosion with its most useful tools of water,
wind, and temperature flux will whittle away the most massive of Giants until
only remnants are left. These Giants in the sky will only live so long until
erosion claims them for her own. Let Shiprock, Cinder cones, Maars as well as
all extinct volcanoes remind you never to under estimate the persistence and
power of erosion.
*All Photos and diagram of Shiprock by Larry Kuss.