I have climbed a few volcano around the world and observed different
type of volcanic activity. I was always puzzled by the scale (mass
and energy) of volcanic phenomena. The scale is large compared to
human scale... but minuscule compared to the earth size. Everyone is
amazed when a volcano delivers 10 cubic Km of hot lava or blows up 10
cubic Km of rocks... but the earth has 3.4E11 cubic Km! I also suspect
that eruptions have a minuscule part in the thermal exchange between
the earth interior and the surface. In planetary terms I have the
(wrong?) impression that volcanism is unimportant and could be
ignored. Can you provide a "back of the envelop" computation to
explain why our planet is so tame?
I am also puzzled by all the cross sections of volcanoes found in
different books: the hot lava raises through a long vertical tube
surrounded by solid rock, the tube forks close to the surface of the
volcano. In my experience materials under pressure get cracks, not
narrow channels. Most flows (e.g. rivers) do not fork as you move
down stream. Why are volcanoes different?
Jacques Rutschmann
Dear Jacques,
I agree that some eruptions, even fairly large eruptions, don't seem that
big on a global scale. But you and I are too grounded in our thinking
and limited in our temporal perspective. A glance over to the world map
on my wall reminds me that most of the Earth's surface is covered by the
oceans. The ocean sits on volcanic rock made at mid-ocean ridges. This
layer of rock (oceanic lithosphere) averages about 50 km (30 miles) in
thickness. It is all less than 200 million years old. So volcanism and
tectonics remove great volumes of magma (and heat) from the mantle and
bring it to (or near) the surface in a relatively short amount of
geologic time. Perhaps the oceanic plates have been recycled more than
20 times in Earth history. This seems like a significant volume of
volcanic rock and a significant amount of thermal exchange.
Some quick and dirty numbers:
If we multiply the surface area of the Earth (5.1E10 square m) times the
average thickness of an oceanic plate (50 km) and divide by the age of
the oldest rocks in the ocean basin (200 million years), we get an
average rate for the production of oceanic plates of 125 cubic km per
year. Not too shabby.
Here's another one:
If we divide the volume of the mantle (8.6E11 cubic km) by the average
eruption rate (125 cubic km per year) we get 6.9E9 years. So, after 7
billion years of Earth history all of the mantle will potentially be
cycled through mid-ocean ridges. After 4.5 billion years of Earth
history roughly two-thirds of the mantle has been cycled through the
mid-ocean ridges.
I think the cross sections are giving you the false impression that lava
is moving from a single conduit into two conduits at the same time. This
does happen with some eruptions. Probably what they are trying to show
is different pathways by eruptions at different times. In general, the
magma will follow the path of least resistance to the surface. This path
can vary within a volcano over time (due to stress within the volcano,
location of the magma body, size of the magma body, etc.) If, as an
eruption progresses, pressure in the magma increases it may find a second
weak area to fracture and migrate into.
Jacques, I don't know if I convinced you of anything but I enjoyed
thinking about your questions. If you'd like a different answer rephrase
your question and send it in again.
Steve Mattox, University of North Dakota
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