Tutorial
Volcanic
Mars is blanketed in evidence of volcanic activity. Volcanic features form due to the eruption and/or flow of molten rock from beneath a planet's surface. Examples include lava tubes, volcanic vents, and volcanoes themselves. Volcanism is so important to Mars' geologic history that scientists have defined a geologic era 700 million years long to Mars' most volcanically active phase! This era is called the Hesperian and is thought to be when Mars' Olympus Mons, the largest volcano in our solar system, formed
Some volcanic features on Earth are thought to be related to the rise of early life on our planet. Hydrothermal vents are known to harbour extremophiles—microbes that thrive in conditions far hotter and more acidic than all other lifeforms on Earth. Microbial life has also been found living within ice inside lava tubes on Earth. Discoveries like these are exciting for planetary scientists, as these conditions may be similar to ancient environments on planets like Mars.
Cones
There are two types of cones related to volcanic activity: Rootless (also called “splatter) cones and cinder cones. Cinder cones are small volcanoes with steep slopes, formed by erupting material piling up around a volcanic vent. Sometimes lava flows are seen originating from them. Rootless cones on the other hand form from when hot lava flows across a wet surface such as a swamp or lake, causing explosions of steam that form cones of the splattered material. Both cinder cones and rootless cones tend to be circular when viewed from above, and can sometimes be confused with impact craters in satellite images.
Practice identifying volcanic cones
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Uranius Tholus and Cernauius Tholus shield volcanoes, viewed in daytime IR
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Volcanic cones
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Lobate debris apron in Protonilus Mensae
These are shield volcanoes.
This pair of shield volcanoes on Mars is Uranius Tholus (top) and Ceraunius Tholus (bottom). While they are volcanic features, they are much larger than cinder cones or rootless cones. The term "tholus" is used for smaller mountains or volcanoes on Mars, while large volcanoes bear the name “mons,” such as Olympus Mons, the largest volcano in the solar system.
Yes, these are cones!
They might look like impact craters at first glance, but the downward slopes beyond the rims of the “craters” show that these features are elevated and cone-shaped. These particular cones appear to have old lava flows surrounding them, suggesting that these are cinder cones (formed by erupted volcanic material piling up around a vent) as opposed to rootless cones (formed by hot lava moving across water or wet ground).
This is not a volcanic feature.
This small mound is called a "mensa", which is a relatively flat-topped hill (called a "mesa" on Earth) with cliff-like edges. The crater on the top of the mensa is a small impact crater and not the result of volcanism. Surrounding the mensa is a lobate debris apron, thought to be the remnants of ancient glaciers. Ground-penetrating radar data from the Mars Reconnaissance Orbiter has revealed that these aprons contain ice inside of them today!
Shield Volcanoes
Characterised by shallow slopes and broad areas, shield volcanoes are bulge-like volcanoes that share a profile similar to that of a knight's shield. Mars’ Olympus Mons is an example of a shield volcano. The large volcanoes of Hawaii, such as Mauna Kea and Mauna Loa, are examples of shield volcanoes on Earth.
Practice identifying shield volcanoes
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Mid-latitude crater fill in a complex impact crater
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Volcanic vent inside a simple crater
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Olympus Mons, the largest volcano in the solar system, in daytime infrared
This is a complex impact crater.
The central peak (or "uplift") in this circular feature on Mars, as well as the terraces inside the rim, reveal that this is a complex impact crater and not a volcano. The crater floor is covered with fill material though to have been ice-rich in Mars’ past, and main contain remnants of ice today.
This is a volcanic vent inside an impact crater.
The gash across the bottom portion of this simple impact crater is a volcanic vent, which has erupted in Mars’ past and filled the floor of the crater with lava that has since cooled to form rock. The crater itself also lies within a larger lava flow formed by nearby volcanic activity.
Yes, this is a shield volcano!
This is Olympus Mons, the largest volcano in the solar system. While the volcano is nearly 25 km tall, its diameter is nearly the size of the state of Arizona (624 km). The crater-shaped depressions near its peak at the centre of the image are the caldera complex of the volcano.
Lava Tubes
Lava tubes form when the outside of a lava flow cools and hardens while the molten interior continues to flow. This results in a hollow, cylindrical shell of rock called a lava tube. In satellite imagery, lava tubes can look like convex worms (if well preserved), a linear series of pits (if the tubes have partially collapsed), or like concave worms (if the top of the tube has eroded away completely).
Practice identifying a lava tubes
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Mid-latitude crater fill in a complex impact crater
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Volcanic vent inside a simple crater
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Olympus Mons, the largest volcano in the solar system, in daytime infrared
Yes, this is a lava tube!
This depression (called a "skylight") is the collapsed roof of a lava tube. Part of the tube is in deep shadow, while the illuminated part of the floor of the tube is littered with boulders from the roof collapse. It is distinct from an impact crater as it lacks a raised rim or ejecta blanket, and the floor is too deep to be an impact crater.
These are simple impact craters.
While skylights on lava tubes can occur in chains along the length of the tube, the raised rims and bowl shape of these circular features indicates that they are simple impact craters.
This is a pingo.
Pingos are periglacial features formed by underground water freezing, forming ice. Since ice has a greater volume (takes up more space) than liquid water, the ground containing this newly formed ice expands to form the mounds of pingos. The tops of these mounds often collapse, forming a circular crater-like feature. They are very similar in shape to volcanic cinder cones or rootless cones—so similar in fact that it is very hard to tell them apart in satellite images of Mars! Geologists have to look at the terrain surrounding the mounds, searching for evidence of volcanic vs. glacial/periglacial activity, to be able to distinguish pingos from volcanic cones.
Vents
A volcanic vent is a location on the surface of a moon or planet that ejects gas, lava, and ash. Volcanoes form through vents in their centres.
Practice identifying vents
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Mid-latitude crater fill in a complex impact crater
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Volcanic vent inside a simple crater
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Olympus Mons, the largest volcano in the solar system, in daytime infrared
This is a valley.
The image here shows a portion of a valley on Mars whose floor is covered by fill material thought to have once been ice-rich, and may still contain ice today. There are no flows on the plains around the valley, indicating that it is likely not a volcanic vent.
This is part of Mars’ north polar cap.
Each of the lines in this image represents a separate layer of water ice or dust in the northern polar cap of Mars. The layers here are exposed along a scarp—a steep wall along the edge of the polar cap.
Yes, this is a volcanic vent!
The crack running across the image here is a volcanic vent. Can you see the light, rough-looking material that extends to the north (top of the image) from the vent? This is an ancient lava flow that has hardened to form rock. There are many simple craters in this image as well, but near the centre of the image is a dome-shaped feature with a "crater" on its top—this is a volcanic cone.