The largest volcanoes in the solar system tower above the surface of Mars. The Earth and Venus both have many volcanoes—as does Jupiter's moon lo—but these are puny compared to those of Mars, which are giants in both height and spread. Even though the largest, Olympus Mons, stands 17 miles high—three times the height of the Earth's tallest volcano, Mauna Loa, in Hawaii—it was known only as a dot on the surface until the Mariner 9 space probe visited in 1971. And Olympus Mons is just one of 20 huge volcanoes that dominate the Martian landscape.

The surface of Mars bears witness to an active geological history. Wind and water have scoured and sculpted the Martian crust, leaving meandering river beds, broad deltas and smoothed-out rock formations like those familiar to us on Earth. But the most spectacular relics of Mars' past are its huge volcanoes—the biggest in the solar system. Even though Mars is only a little more than half the size of the Earth, these extinct giants dwarf any on our planet. So why did Mars breed such monsters?

Despite the dramatic evidence of eruptions on its surface. Mars' geology has been much more stable than the Earth's. The thick Martian crust stayed immobile above the geological "hot spots" that fueled its volcanoes, and so while the planet's interior was hot enough to drive lava up to the surface, the mountains of lava and ash that piled up in eruption after eruption kept growing.

The same forces are building volcanoes on Earth today. But our planet's crust is fractured into continent-sized plates that are constantly on the move. Because of this, volcano-firing hot spots must find new outlets for their lava every few million years. The Hawaiian Island chain shows this phenomenon in action. The islands are a line of mostly extinct volcanoes that map the progress of the Pacific plate over a hot spot. The Earth's biggest volcano, Mauna Loa, is simply the youngest of the chain to appear above sea level—which is why it is at the end of the line, dominating the southern part of the island of Hawaii.

All that we know of Mars 7 volcanoes has come from photographs taken by orbiting spacecraft, starting with Mariner 9 in 1971. Since then, the two Viking probes (1976) and the Mars Global Surveyor (1997-9) have returned more detailed images that tell the life histories of Mars' volcanoes. From studies of impact craters on volcano slopes, astronomers have calculated that the oldest volcano still visible on the surface rose up over 3 billion years ago and the youngest was active only 30 million years ago. The oldest type, called Patera, are shaped like upside-down saucers, rising only a few miles above the surrounding terrain, and were probably formed from a mixture of lava and ash. The youngest and tallest are shield volcanoes—the same type as the Earth's Mauna Loa—with gentle slopes spreading out over huge areas. The lava that made these was relatively thin, and flowed huge distances before cooling and solidifying. A third type of Martian volcano is the dome, with steeper sides made from stickier lava or fewer eruptions.

But not all volcanoes resemble the popular image of a cone-shaped mountain topped by a smoking crater. Lava can also spread out over a wide area, filling up valleys and submerging impact craters to leave smooth plains. When Mars was young, eruptions covered most of the planet's northern hemisphere in just this way—the same process that gave our Moon its so-called "seas."

The great days of Martian volcano-building are long gone. It must have been a very different planet then, with massive outgassings into the atmosphere along with the ejections of lava. But though we may never see Olympus Mons blow its top again, there may be small-scale volcanic activity still disturbing Mars' crust. Only further exploration of the surface will tell.