One of the theories of the extinction of the dinosaurs is due to volcanic
activity.
One of the theories of the extinction of the dinosaurs is due to volcanic
activity. What does this theory say?
K. Gurley
Hi K. Gurley,
There are various variations on the main theory. In general it is proposed
that volcanic activity put so much ash and/or gas into the atmosphere that the
earth's temperature either got too hot for the dinosaurs or got too cold for
the dinosaurs. It sounds kind of funny that either can happen but it is true.
If the ash particles are really small (<2 microns) then they block out
incoming sunlight and the earth gets cool. If they are bigger than 2 microns
(but still pretty small) then they let sunlight in but don't let heat
radiation from the surface out, and the earth gets warm.
Anyway, if you have enough large explosive eruptions, then the theory says
that there will be enough ash in the stratosphere to have one of these
effects. You need an eruption (or series of eruptions) that is much bigger
than anything we have ever wittnessed. The reason that you need to put the
ash into the stratosphere is that if it is only in the troposphere (where
weather clouds are), then it will get rained out very quickly and it won't be
around long enough to have a climatic effect.
Of course the more famous idea is that a huge meteorite came in and hit the
earth, throwing up enough gas and dust into the stratosphere to have the same
heating or cooling effect. One line of support for this is that at the
geologic time boundary where the dinosaurs died out (the Cretaceous-Tertiary
boundary) there is a layer of clay that is rich in an element called iridium.
Iridium is not very common on Earth, but it is proposed to be more abundant in
asteroids and meteorites. One way to produce such a layer at the same instant
that the dinosaurs died out is therefore to have a meteorite bring it
in.
One major problem with the volcanic hypothesis is that volcanoes, especially
the explosive ones, don't produce much iridium. Basaltic volcanoes, such as
those here in Hawai'i produce more iridium but they are not very
explosive.
A more recent idea that tries to get around these problems is that instead of a
huge explosive eruption, you have a long-term basaltic eruption that mainly puts
SO2 gas into the troposphere. The gas will be converted into small droplets
of sulfuric acid which will block incoming sunlight. Because it is only in
the troposphere much of the acid may get rained out, but if you have an
eruption that continues long enough it can keep up with the rain to produce an
Earth-covering haze.
What kind of eruption might this be? There are places on Earth where huge
volumes of basaltic lavas are found. They are called flood basalts, and the
most famous are the Columbia River Basalts in Washington/Oregon, and the
Deccan Traps in India. The name "flood basalts" gives an indication of how
most people consider them to be erupted, namely as huge fast-moving floods of
basalt. However, recent work by a number of scientists here at the University
of Hawai'i (including Steve Self, George Walker, Thorvaldur Thordarson, and
Sarah Finnemore) have shown that these flood basalts look more like the
slow-moving type of basalt lava (pahoehoe) than the fast-moving type ('a'a).
This leads next to the conclusion that perhaps these flood basalts were not
emplaced as huge floods in short periods of time but rather as slower-moving
flows over a long period of time (such as 1-2 hundred years). The eruptions
would still have been much bigger than those we see here in Hawai'i, however.
This new idea thus provides a long-term sourse for lots of SO2, and a possible
explanation for the iridium (which, again, is found in basalt lava). It
wasn't the Columbia River Basalts that were the cause for the dinosaur
extinction because the dinosaurs disappeared about 65 million years ago, and
the Columbia River Basalts are only about 12 million years old. Perhaps it
was the eruption of the Deccan traps, since their age is about right.
This is definitely an area of volcanology where lots of work is continuing,
and I hope this answer has given you a taste of the things that have to be
taken into account.
Sincerely,
Scott Rowland, Unversity of Hawaii
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