Olympus Mons

We're pretty familiar with hotspot volcanoes on earth. A rising plume of magma reaches to the crust, creating a volcano. The magma plumes can that cause the hotspots stay in the same spot for tens of millions of years, but plate tectonics works to keep the crust moving above the plume. The result is a series of volcanoes, with a small number of active volcanoes over the hotspot, at the end of a line of extinct volcanoes that trace the plate's movement.

The Hawaiian Islands are the classic example of this process on earth.

In this Google Earth view, the Big Island of Hawaii (at the lower right) is active. Moving toward the upper left, Haleakala has erupted in historic times, but the West Maui volcano, Lanai, and Molokai have not. Oahu and Kauai are older yet. A string of atolls stretches to the northwest from Kauai to Kure, with the Emperor Seamounts to the north of Kure providing us with the oldest traces of this hotspot's activities.

That's what a hotspot can do on Earth, where the crust is in constant motion. But what would happen if there was a hotspot somewhere without continental drift?

This is Olympus Mons, on Mars. It's the largest mountain in the solar system, at least as far as we know. The Google Mars picture above is at more or less the same scale as the picture of the Hawaiian Islands. But if we want to get a better idea of the scale of Olympus, Photoshop can help:

That's the Big Island, added for scale. The Island of Hawaii is the largest shield volcano on Earth. It's also the largest volcano on the planet, and if you measure it from sea floor to summit, it's the tallest mountain. I included the undersea parts of Hawaii when I cropped the island, and it's still insignificant next to Olympus Mons. Even if we add the rest of the main Hawaiian Islands to the picture, they're still dwarfed by the Martian volcano's bulk:

There's still room to add another island to the picture without covering up the entire mountain. In fact, we can add a pretty big island - like Ireland:

It's not just Olympus Mons itself that's enormous.

Shield volcanoes - like the volcanoes of Hawaii and Olympus Mons - frequently develop a features known as collapse calderas. As the volcano's magma chamber drains, it stops providing support to the rocks above. They collapse downward into the chamber, forming a large, round, crater-like feature. If further volcanic activity occurs, the process can repeat, which can result in a whole complex of calderas forming at the top of the volcano.

The calderas can be really impressive features. I took this picture looking out across Kilauea Caldera a couple of years ago:

It's an impressive view - the caldera is about 3 by 5 km, and more or less oval shaped. That's Hawaii. This is Mars:

It's a bit hard to get a feel for the size in that shot, so here's another view, with a scale marker:

That's Maui, for scale.

This dose of perspective was brought to you by Google Earth, Google Mars, and my copious free time.