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Plants are destroyed over a wide area, during an eruption. The good thing is that volcanic soil is very rich, so once everything cools off, plants can make a big comeback!
Livestock and other mammals have been killed by lava flows, pyroclastic flows, tephra falls, atmospheric effects, gases, and tsunami. They can also die from famine, forest fires, and earthquakes caused by or related to eruptions.
Mount St. Helens provides an example. The Washington Department of Game estimated that 11,000 hares, 6,000 deer, 5,200 elk, 1,400 coyotes, 300 bobcats, 200 black bears, and 15 mountain lions died from the pyroclastic flows of the 1980 eruption.
Aquatic life can be affected by an increase in acidity, increased turbidity, change in temperature, and/or change in food supply. These factors can damage or kill fish.
Eruptions can influence bird migration, roosting, flying ability, and feeding activity.
The impact of eruptions on insects depends on the size of the eruption and the stage of growth of the insect. For example, ash can be very abrasive to wings.
How quickly do plants begin to grow back? The answer is that it depends on how much rain falls in the particular area. For example, on the rainy side of the island of Hawai’i, flows that are only 2 years old already have ferns and small trees growing on them. Probably in 10 years they’ll be covered by a low forest. On the dry side of Hawai’i there are flows a couple hundred years old with hardly a tuft of grass in sight. This means that when you are looking at old lava flows and trying to determine how old they are based on the amount of vegetation, you have to take the climate into effect as well.
Image: Lava flows covering the Kamoamoa area of Hawai`i Volcanoes National Park. Photograph by Steve Mattox, November 14, 1992.
I think that actually the long-term effects of an eruption on wildlife are usually quite small. Certainly at Mt. St. Helens scientists saw that both plants and animals returned to the utterly devastated areas within only a year or so of the eruption.It is usually the short-term effects that are really bad. For example, there was a very big eruption of Santa Maria volcano (Guatemala) in 1902. The eruption itself killed a few hundred to perhaps 1500 people as well as thousands of birds. Pretty soon there were so many insects including disease-carrying mosquitoes that eventually 3000-6000 people died from malaria. (This information came from Volcanoes of the World, by Tom Simkin and Lee Seibert).
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 witnessed. 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.
Sources of Information:
Blong, R.J., 1984, Volcanic hazards: A source book on the effects of eruptions: Academic Press, Orlando, Florida, 424 p.
Del Moral, R., 1981, Life returns to Mount St. Helens, Natural History, v. 90, no. 5, p. 36-46.