Pale Blue Dot - Carl Sagan [69]
So a large volcanic explosion in some unfrequented and obscure part of the world can alter the environment on a global scale. Both in their origins and in their effects, volcanos remind us of how vulnerable we are to minor burps and sneezes in the Earth’s internal metabolism, and how important it is for us to understand how this subterranean heat engine works.
IN THE FINAL STAGES of formation of the Earth—as well as the Moon, Mars, and Venus—impacts by small worlds are thought to have generated global magma oceans. Molten rock flooded the pre-existing topography. Great floods, tidal waves kilometers high, of flowing, red-hot liquid magma welled up from the interior and poured over the surface of the planet, burying everything in their path: mountains, channels, craters, perhaps even the last evidence of much earlier, more clement times. The geological odometer was reset. All accessible records of surface geology begin with the last global magma flood. Before they cool and solidify, oceans of lava may be hundreds or even thousands of kilometers thick. In our time, billions of years later, the surface of such a world may be quiet, inactive, with no hint of current vulcanism. Or there may be—as on Earth—a few small-scale but active reminders of an epoch when the entire surface was flooded with liquid rock.
In the early days of planetary geology, ground-based telescopic observations were all the data we had. A fervent debate had been running for half a century on whether the craters of the Moon were due to impacts or volcanos. A few low mounds with summit calderas were found—almost certainly lunar volcanos. But the big craters—bowl- or pan-shaped and sitting on the flat ground and not the tops of mountains—were a different story. Some geologists saw in them similarities with certain highly eroded volcanos on Earth. Others did not. The best counterargument was that we know there are asteroids and comets that fly past the Moon; they must hit it sometimes; and the collisions must make craters. Over the history of the Moon a large number of such craters should have been punched out. So if the craters we see are not due to impacts, where then are the impact craters? We now know from direct laboratory examination of lunar craters that they are almost entirely of impact origin. But 4 billion years ago this little world, nearly dead today, was bubbling and churning away, driven by primeval vulcanism from sources of internal heat now long gone.
In November 1971, NASA’s Mariner 9 spacecraft arrived at Mars to find the planet completely obscured by a global dust storm. Almost the only features to be seen were four circular spots rising out of the reddish murk. But there was something peculiar about them: They had holes in their tops. As the storm cleared, we were able to see unmistakably that we had been viewing four huge volcanic mountains penetrating through the dust cloud, each capped by a great summit caldera.
After the storm dissipated, the true scale of these volcanos became clear. The largest—appropriately named Olympus Mons, or Mt. Olympus, after the home of the Greek gods—is more than 25 kilometers (roughly 15 miles) high, dwarfing not only the largest volcano on Earth but also the largest mountain of any sort, Mt. Everest, which stands 9 kilometers above the Tibetan plateau. There are some 20 large volcanos on Mars, but none so massive as Olympus Mons, which has a volume about 100 times that of the largest volcano on Earth, Mauna Loa in Hawaii.
By counting the accumulated impact craters (made by small impacting asteroids, and readily distinguished from summit calderas) on the flanks of the volcanos, estimates of their ages can be derived. Some Martian volcanos turn out to be a few billion years old, although none dates back to the very origin of Mars, about 4.5 billion years ago. Some, including Olympus Mons, are comparatively new—perhaps only a few hundred million years old. It is clear that enormous volcanic explosions occurred early in Martian history, perhaps providing an atmosphere much denser