A volcano is a point on a moon or planet's surface where the hot material from the interior of the body erupts onto the surface. Molten rock underground (magma) breaks thru to the surface where it becomes ash, lava or both. Volcanoes are closely associated with Earth quakes. Volcanoes may erupt, and then as the magma chamber below drains, the top of the volcano collapses making a huge crater called a caldera.
Volcanos have been discovered on planets such as Venus, Earth, and Mars. Several moons also show signs of volcanism. (IO has silica-sulfur eruptions, whereas other moons have volcanoes were the magma is water ice.)
Types of Volcano
Volcanic eruptions are classified by how explosive they are. Low silica magma are less viscous and flow rapidly. High silica lava flows slowly. If gases are dissolved in the magma, they may boil free explosively as the pressure is released resulting explosive eruption throwing up ash.
- shield volcanoes are caused when low silica lava pores out of a central crater, building up broad cones with shallow slopes.
- rift volcano are caused when a long crack in the ground opens and lava pours out over a broad region. In Earth's history huge eruptions of this type have buried vast areas under lava deposits hundreds of meters deep, & are called flood basalts.
- Stratovolcanos are caused when a mixture of lava and ash are erupted. They have a broad shallow base, but they become steeper as they approach the central cone.
- Cinder cones are small volcanoes made up of ash. They are fragile and easily erode away.
- Some eruptions are explosive, which may blow apart some or all of an existing mountain. Such explosive eruptions are called pyroclastic (and are very dangerous). A pyroclastic flow is a cloud of red hot dust mixed with poisonous gases which flow down hill at a couple hundred km / hour. Pyroclastic flows are very dangerous.
- Venus has a form of volcano not found elsewhere in the solar system called 'pancake volcanoes'. These are circular, flat, with cliffs all around their perimeter.
Some craters on Mars may look like a volcano but are not.
- A 'rootless' volcano is a cinder cone like formation, which is very small. It is formed when lava flows over wet, sedimentary rock. The water boils, blasting up thru the lava field, leaving a field of small cones which look like volcanoes. However there is no path down to the magma chamber so they are said to be rootless. These have been seen on Mars.
- Sometimes a small meteor crater on a hill may have the appearance of caldera.
Locations of Volcanoes
On Earth, volcanoes are found either at the edges of Tectonic plates, or above mantle 'hot spots' where for some reason, hot mantle material cuts thru the crust. (The number of hotspots on Earth is hotly debated.) Mars is believed to only have a short history of plate tectonics, so it is likely that the Martian volcanic regions are based on various hot spots.
Creation of Ores
Vulcanism is a major way that commercially exploitable ores form. Planets without volcanoes are more likely to undifferentiated 'junk' rocks, whereas those with volcanoes will have more variety in rock types including ores.
In the magma chamber, magma rich with elements from the mantle begins to cool. The minerals with high melting temperatures freeze out first, creating minerals rich in such elements. At the top of the magma chamber easily melted minerals rich in elements like Fluorine, Chlorine, and Silicon become richer. Molten metal can be found. Sometimes a crack suddenly opens up and magma quickly squeezes thru it, forming mineral intrusions. Veins of Copper, Silver, and Gold have formed this way. Many metals for sulphates and are deposited by lava deposits.
Hot water can dissolve valuable minerals. When the water moves away from the geothermal heat source they pull the minerals with them, which then form deposits when the water evaporates. (See Hydrothermal Vents below.)
Diamonds can form in magma pipes which freeze underground.
In South Africa in the Transvaal Basin, there is the Bushveld Igneous Complex. This is thought to be a massive intrusion of mantle material thru the crust, and contains many of the richest ores on the planet. Platinum group metals (PGM), Iron, Tin, Chromium, Titanium and Vanadium are found there. Uranium mines have been considered. 80% of the Earth's Platinum come from this area. Some theorists think that such intrusions may be more common on Mars than on Earth.
Volcanos on Mars
The Elysium quadrangle contains an area of high ground called the Elysium Planitia, formed by Shield volcanoes. Volcanoes there erupted explosively (pyroclasticly) depositing large areas of ash. Crater counts (few craters means younger terrain), suggest that these volcanoes erupted in the last 50,000 to 200,000 years, which is very young geologically. There is certainly geothermal power in this area. The fact that there we have found recent volcanism in one part of the planet increases the odds that we can find geothermal power in other regions of the planet.
Syrtis Major quadrangle contains the Syrtis Major Planet which is a vast Hesperian aged shield volcano 1,200 km in diameter and 2 km high. It has 2 calderas: Nili Patera and Meroes Patera. It is of interest because it seems to have dacite and granite (which are common on Earth, but relatively rare on Mars).
In Arabia quadrangle is Arabia Terra, a large upland region. There are several irregular craters which form a Martian igneous province. These show similarities to Terrestrial supervolcanoes.
In the Southern Hemisphere, especially around the Hellas Basin are several volcanoes. These are extremely old and show much erosion.
Volcanic plains are common all over Mars. Much more of Mars is made up of igneous formations than Earth.
Volcanic Terrain and Settlement Locations
The lava tubes of an extinct volcano may provide an ideal settlement location. (They represent a cheap way to place a habitat fairly deep underground where the temperature will be more stable and radiation shielding is provided by the rock above.
Volcanos are associated with hydrothermal vents. In these structures water forced through hot magma exits from cracks and may leave behind concentrated ore that have been leached from the rock that the hot water moves through. Thus the water can make some areas mineral poor, while concentrating the elements elsewhere.
Terraforming and Volcanoes
It has been suggested that atom bombs may be able to crack open the crust, to reactivate volcanoes on Mars. This would add more gas to thicken the atmosphere.
By digging pits deep into the the crust of Mars, heat and gases could escape, forming a weak artificial 'volcano'.
By creating numerous hollow tungsten tear drop shaped objects, filled with radioactive material (and neutron reflectors), these could grow red hot and melt their way into the crust over suspected magma pools. More nuclear teardrops would be added as the earlier ones sank out of sight. They would melt a path deep into the crust allowing gases, water vapour, and perhaps, magma to rise up thru the crust forming an human triggered volcano. Alternately a similar plan could melt nitrate deposits underground, to outgas trapped Nitrogen.
// Link below goes to a study of recent vulcanism on Mars. This is consistent with Mars-quakes picked up in this region by the Insight lander.