There's one thing you can say about the planet Mars, it does everything big. There's a valley there that would stretch across most of the United States. Then there's Olympus Mons, which is the biggest volcano known to exist in the Solar System:
Olympus Mons is a shield volcano 624 km (374 mi) in diameter (approximately the same size as the state of Arizona), 25 km (16 mi) high, and is rimmed by a 6 km (4 mi) high scarp. A caldera 80 km (50 mi) wide is located at the summit of Olympus Mons. To compare, the largest volcano on Earth is Mauna Loa. Mauna Loa is a shield volcano 10 km (6.3 mi) high and 120 km (75 mi) across. The volume of Olympus Mons is about 100 times larger than that of Mauna Loa. In fact, the entire chain of Hawaiian islands (from Kauai to Hawaii) would fit inside Olympus Mons!
Olympus Mons (NASA)
After seeing this photo at the Astronomy Picture Of The Day (APOD), I naturally wondered how some of Earth's other volcanoes stacked up against it. As the NASA site says, the caldera is about 50 miles in diameter. How would Mt. St. Helens look in relation to it? The answer appears to be "fairly unimpressive":
Image credit: All composites were composed by Cujo359 based on Googlesat images displayed in FoxtrotGPS and a NASA photo from APOD
[All pictures are reduced in size on this page. Click on them to see them full size.]
Mt. St. Helens, of course, is well known to us because it caused so much trouble back in 1980 when it erupted. It's barely noticeable in this picture of Olympus Mons. I think that the cone would fit comfortably inside that little caldera in the lower left.
A bigger eruption that many of us are familiar with occurred at Mt. Mazama a few thousand years ago. That one threw debris at least thirty miles away. It was almost certainly noticed by anyone who was then living in the Pacific Northwest. Here's how it looks next to Olympus Mons:
A bit more noticeable, but it's still not what I'd call a serious competitor. It turns out, though, that we earthlings don't have to hang our heads in shame. On the island of Sumatra, there is one big, honkin' hole in the ground that was formed roughly 75,000 years ago:
That's more like it. I knew someplace would come through in the name of planetary pride.
Much like Crater Lake, which was formed after Mt. Mazama exploded, this lake occupies a caldera. As you can see from the scale in the lower left hand corner, though, this is a much larger lake than Crater Lake.
What was it like when this volcano erupted? It very nearly made humans extinct:
The Toba supereruption (Youngest Toba Tuff or simply YTT) was a supervolcanic eruption that is believed to have occurred some time between 69,000 and 77,000 years ago at Lake Toba (Sumatra, Indonesia). It is recognized as one of the Earth's largest known eruptions. The related catastrophe hypothesis holds that this event plunged the planet into a 6-to-10-year volcanic winter and possibly an additional 1,000-year cooling episode. This change in temperature is hypothesized to have resulted in the world's human population being reduced to 10,000 or even a mere 1,000 breeding pairs, creating a bottleneck in human evolution.
Wikipedia: Toba Catastrophe Theory
Of course, even the Lake Toba volcano was probably much smaller than Olympus Mons, though its caldera rivals it in size. How did Olympus Mons, on a planet with a quarter of the surface area of Earth, and roughly an eighth the volume, create such a tremendous volcano? Turns out there's an explanation:
Shield volcanoes have very shallow slopes and gentle eruptions. The Hawaiian volcanoes form when a plate of crust moves over a hot spot. The hot spot produces magma that gradually forms the volcanoes. Since Earth has plate tectonics, the crustal plate moves over the hot spot producing a chain of volcanoes.
Mars does not have plate tectonics, which causes the magma to build a volcano in one location making Olympus Mons so large.
Layers in a Scarp on Olympus Mons
What does this mean? To illustrate, let's consider the Yellowstone Caldera, which I wrote about a couple of years ago. Like the Lake Toba volcano, its eruption was huge, producing at least 1,000 cubic kilometers of debris. It wasn't the first, though, nor is it likely to be the last. Why isn't there a huge mountain in the northwestern corner of Wyoming? Here's why:
Image credit: Metrodyne/Wikimedia
Each of those colored spots is the site of a past eruption of the same hotspot that caused the Yellowstone eruption. The numbers represent how many millions of years ago an eruption happened there. Because the chunks of Earth's crust move, though, this means that the eruption burst through a different patch of crust each time. On Mars, this doesn't happen anymore.
The reason the Earth's tectonic plates move around so much is because of our planet's thin crust and molten core:
We are all adrift, the ground beneath us in constant, if imperceptible, motion. Huge slabs of Earth's crust slide over the partially molten mantle, pulled at one end by the slab dipping into the mantle at subduction zones, pushed at the other by new crust welling up at mid-ocean ridges. That, anyway, is classical plate tectonic theory, and it works fine for oceanic crust.
Now [Carnegie geophysicist David] James and his Carnegie colleague John VanDecar, along with Marcelo Assumpcao of the University of Sao Paulo in Brazil, claim to have found a major clue to what powers continental motion. The continents, they say, do not so much ride over the underlying mantle as drift along with it, on currents of rock that extend to depths of 300 miles.
The Mantle Moves Us; 1996
Whether it's pushing them along, or just providing a lift, the molten core of the Earth is what makes those plates move. Mars, it seems, has more crust and less core:
[Mars'] iron sulfide core is partially fluid, and has twice the concentration of the lighter elements that exist at Earth's core. The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but now appears to be dormant. Besides silicon and oxygen, the most abundant elements in the martian crust are iron, magnesium, aluminum, calcium, and potassium. The average thickness of the planet's crust is about 50 km, with a maximum thickness of 125 km. Earth's crust, averaging 40 km, is only one third as thick as Mars's crust, relative to the sizes of the two planets. The InSight lander planned for 2016 will use a seismometer to better constrain the models of the interior.
So, there's more crust to carry around on Mars, and less mass to move it with.
I guess we'll just have to live with being second best.