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—acpanied by swarms of earthquakes and plumes of abyssal smoke and hot water—that we a 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 bees 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 puter models to predict future climate change。 Volcanic particles injected into the upper air are also an additional cause of thinning of the ozone layer。
* Even with its mountains and submarine trenches； our planet is astonishingly smooth。 If the Earth were the size of a billiard ball； the largest protuberances would be less than a tenth of a millimeter in size—on the threshold of being too small to see or feel。
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 the； 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 counter…argument was that we know there are asteroids and ets 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 pletely 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 paratively 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 than the one Mars holds today。 What would the place have looked like if we had visited it then？
Some volcanic flows on Mars （for example； in Cerberus） formed as recently as 200 million years ago。 It is； I suppose； even possible—although there is no evidence either way—that Olympus Mons； the largest volcano we know about for certain in the Solar System； will be active again。 Volcanologists； a patient sort； would doubtless wele the event。
In 1990…93 the Magellan spacecraft returned surprising radar data about the landforms of Venus。 Cartographers prepared maps of almost the entire planet； with fine detail down to about 100 meters； the goal…line…to…goal…line distance in an American fo