This image shows the volcano Olympus Mons. With a diameter of more than 600 kilometers (the size of Arizona) and a height of nearly 25 kilometers above the surrounding plains, it is the tallest volcano known to exist in the solar system. When clouds are present, it is sometimes even visible above the clouds (see slide #18).
The relative ages of the surface in various parts of Mars can be estimated from the number of impact craters present in a given area, with young regions having fewer craters than old regions. Only two craters are visible here, indicating that Olympus Mons is young, probably the youngest volcanic feature on Mars. By some estimates, the most recent large volcanic eruption at Olympus Mons occurred only 25 million years ago. The oldest activity at Olympus Mons could be much older than this and would have been buried by younger lava flows.
The caldera of Olympus Mons is the depression near the top center of the image. The caldera is about 65 × 80 kilometers across (the size of Rhode Island) and occurs near the maximum elevation of the volcano. It formed when magma within the volcano either erupted out of vents on the side of the volcano or temporarily drained deeper into the planet. In either case, the removal of this magma allowed part of the overlying surface to collapse, producing a topographic depression that is termed a caldera. The overlapping series of structures in the Olympus Mons caldera demonstrates that this magma withdrawal occurred a number of different times. Similar calderas are seen on other volcanos both on Mars and on Earth.
Viking Orbiter image 641A52