Overview

The giant shield volcanoes on Mars are truly huge. The largest are three times as high as the biggest Earth volcanoes. They also are bigger in diameter. Thus, the biggest volcano on Mars is comparable to a pile of nearly 100 Hawaiian volcanoes. Despite this difference in size, the Mars shields look a lot like shield volcanoes on Earth. Both have the same broad flat profiles, large central calderas, and similar lava flow features. The giant martian shields are also much older than any Earth volcano. The youngest lavas on the martian shields are about 20 to 200 million years old. The oldest lavas are near 2.5 billion years old. Thus, these giant volcanoes were active for billions of years. This may explain their large size. On Earth, plate tectonics is always moving volcanoes away from their magma sources. Such movements are very slow, but they mean that most Earth volcanoes have distinct lifetimes. In the Hawaiian islands, for instance, volcanism lasts fo only a few million years on any given island. In contrast, the lack of plate tectonics on Mars allowed volcanoes to just keep growing. The only limit on their final size was the volume of lavas available.
 

Size Comparison

 

The giant shield volcanoes on Mars are much larger than any feature on Earth. Shown here is a graphic comparison of Olympus Mons to the two largest mountains on Earth. (Note: Mount Everest is not a volcano.) Olympus Mons is the largest and youngest volcano on Mars. It is nearly 27 km high, and about 700 km across. Its summit is nearly 72 km wide, which is close to the size of Mount Everest and Mauna Kea. Mauna Kea is the largest Earth volcano. It rises nearly 10 km from the ocean floor, and is about 120 km wide. Mount Everest is a plate tectonic feature, created by uplift and erosion in the Himalayan Mountains. It is about 9 km above sea level, and it sits in a mountain range roughly 200 km wide. (figure is from NASA SP-4212, p. 366, and has a 2x vertical exaggeration.)
 

 

Olympus Mons

 

This picture clearly shows how large and flat Olympus Mons is. Although the volcano is nearly 27 km high, it is over 20 times wider than it is tall. Thus, most of the volcano has a fairly gentle surface slope. The image also shows the distinct cliff which marks the base of Olympus Mons. In places, this scarp is up to 6 km high. In other places, it is hidden under lava flows cascading out into the surrounding lava plains. This cliff is unique among the giant shield volcanoes on Mars. The rough, crinkly patches around Olympus Mons are also unusual. They form the Olympus Mons Aureole.

 

Both the Aureole and the basal cliff are poorly understood. However, their origins may be related. In one theory, the basal cliff was formed by many large landslides. The Aureole marks material piled up at the bottom of these landslides.
 

Olympus Mons (from Orbit)

 

This picture clearly shows how large and flat Olympus Mons is. Although the volcano is nearly 27 km high, it is over 20 times wider than it is tall. Thus, most of the volcano has a fairly gentle surface slope. The image also shows the distinct cliff which marks the base of Olympus Mons. In places, this scarp is up to 6 km high. In other places, it is hidden under lava flows cascading out into the surrounding lava plains. This cliff is unique among the giant shield volcanoes on Mars. The rough, crinkly areas around Olympus Mons are also unusual. They form the Olympus Mons Aureole. Both the Aureole and the basal cliff are poorly understood. However, their origins may be related. In one theory, the basal cliff was formed by many large landslides. The Aureole marks material piled up at the bottom of these landslides. (Viking image mosaic from Carr et al, 1977, J. Geophys. Res., vol. 82, p. 3996.)
 

Olympus Mons (3D)

 

This image provides a perspective view of Olympus Mons. North is to the left of the image. Note the clear basal cliff and the gentle rolling slopes higher up the volcano. The faint radial texture above the basal cliff marks the traces of thousands of separate lava flows. To the left and lower right, we can also see places where lavas have flowed over the basal cliff.
 

Olympus Mons Caldera

 

This image shows the summit caldera on Olympus Mons. It is nearly 90 km long by 60 km wide, and it contains 6 overlapping pit craters. These craters resemble the calderas found on shield volcanoes on Earth. Thus, they are probably not eruption vents. Rather, they are probably collapse pits that formed in the roof of a deeper magma chamber. The size of these pits suggests that the magma chambers in Olympus Mons were fairly large. Likely diameters range from about 20 km to over 60 km. For comparison, calder as in the Hawaiian Islands range in diameter from ~3-5 km up to ~18 km. (Viking Orbiter image 890A68, from Lunar & Planetary Institute slide set Volcanic Features of Hawaii and Other Planets.)
 

Tharsis Montes

 

This image shows the three giant volcanoes known as the Tharsis Montes. Arsia Mons is in the lower left, Pavonis Mons is in the middle, and Ascreus Mons is in the upper right. Olympus Mons lies off the image to the upper left. These volcanoes lie in the center of the Tharsis region, and they form a line nearly 1500 km long. They are nearly 700 km apart, and each reaches nearly the same height as Olympus Mons (~25-27 km). These volcanoes are located on a large pile of lava flows , however, which is nearly 10 km high. Thus, the volcanoes are really only about 15 km tall. (Note: This is still more than half again the height of the Hawaiian volcanoes on Earth.) All three volcanoes seem to have formed together, and they were active for a very long time. Still, Arsia Mons appears to be slightly older than Pavonis Mons, and Ascreus Mons seems to be slightly younger. Therefore, volcanism in the Tharsis region may have slowly shifted north over time. A similar progression of volcanism is found in the Hawaiian Islands. Thus, the giant shield volcanoes on Mars may have formed over a mantle hotspot like that in Hawaii. (from digital mosiac of Viking 1 images, prepared for NASA by the U.S. Geologic Survey, published on the Mars CD-ROM VO_2014.)
 

Elysium Mons

 

This image shows Elysium Mons. This volcano is much smaller than the Tharsis volcanoes. It is only 9 km tall, and is about 240 km in diameter. Thus, it is nearly the same size as the largest Hawaiian volcanoes. Like the Tharsis Montes, however, Elysium Mons sits on a large pile of lava flows. This lets it rise 12 km above the mean planetary elevation. It grades so smoothly into the surrounding lava plains that its base is hard to see. A smaller volcano, Albor Tholus, can also be seen. It is partly buried by the lava plains surrounding Elysium Mons. Note the number of channels in this image. In places, these features look a lot like lunar sinuous rilles. They are all large flat-bottomed valleys. They begin abruptly in broad depressions. And their sources seem to form a ring centered on Elysium Mons. Like sinuous rilles on the Moon, these valleys might be lava channels. However, water is a more likely cause for their formation. Specifically, ground ice appears to have been widespread in the Elysium region. Such ice is easily melted near hot magmas. Thus, melt water provides a ready source for erosion in the Elysium region. Further, the loss of a lot of ground ice can cause collapse depressions near the channel sources. (from digital mosaic of Viking 1 images, prepared for NASA by the U.S. Geologic Survey, published on the Mars CD-ROM VO_2014.)