Pale Blue Dot - Carl Sagan [68]
A volcano is, in fact, an aperture to an underground realm much vaster than the thin surface layer that humans inhabit, and far more hostile. The lava that erupts from a volcano is liquid rock—rock raised to its melting point, generally around 1000°C. The lava emerges from a hole in the Earth; as it cools and solidifies, it generates and later remakes the flanks of a volcanic mountain.
The most volcanically active locales on Earth tend to be along ridges on the ocean floor and island arcs—at the junction of two great plates of oceanic crust—either separating from each other, or one slipping under the other. On the seafloor there are long zones of volcanic eruptions—accompanied by swarms of earthquakes and plumes of abyssal smoke and hot water—that we are just beginning to observe with robot and manned submersible vehicles.
Eruptions of lava must mean that the Earth’s interior is extremely hot. Indeed, seismic evidence shows that, only a few hundred kilometers beneath the surface, nearly the entire body of the Earth is at least slightly molten. The interior of the Earth is hot, in part, because radioactive elements there, such as uranium, give off heat as they decay; and in part because the Earth retains some of the original heat released in its formation, when many small worlds fell together by their mutual gravity to make the Earth, and when iron drifted down to form our planet’s core.
The molten rock, or magma, rises through fissures in the surrounding heavier solid rocks. We can imagine vast subterranean caverns filled with glowing, red, bubbling, viscous liquids that shoot up toward the surface if a suitable channel is by chance provided. The magma, called lava as it pours out of the summit caldera, does indeed arise from the underworld. The souls of the damned have so far eluded detection.
Once the volcano is fully built from successive outpourings, and the lava is no longer spewing up into the caldera, then it becomes just like any other mountain—slowly eroding because of rainfall and windblown debris and, eventually, the movement of continental plates across the Earth’s surface. “How many years can a mountain exist before it is washed to the sea?” asked Bob Dylan in the ballad “Blowing in the Wind.” The answer depends on which planet we’re talking about. For the Earth, it’s typically about ten million years. So mountains, volcanic and otherwise, must be built on the same timescale; otherwise the Earth would be everywhere smooth as Kansas.*
Volcanic explosions can punch vast quantities of matter—mainly fine droplets of sulfuric acid—into the stratosphere. There, for a year or two, they reflect sunlight back to space and cool the Earth. This happened recently with the Philippine volcano, Mt. Pinatubo, and disastrously in 1815–16 after the eruption of the Indonesian volcano Mt. Tambora, which resulted in the famine-ridden “year without a summer.” A volcanic eruption in Taupo, New Zealand, in the year 177 cooled the climate of the Mediterranean, half a world away, and dropped fine particles onto the Greenland ice cap. The explosion of Mt. Mazama in Oregon (which left the caldera now called Crater Lake) in 4803 B.C. had climatic consequences throughout the northern hemisphere. Studies of volcanic effects on the climate were on the investigative path that eventually led to the discovery of nuclear winter. They provide important tests of our use of computer models to predict future climate change. Volcanic particles injected into the upper air are also an additional cause of thinning