Aki Roberge
April 21, 1998
Second Year Seminar Paper
Johns Hopkins University
The Planets After Formation
Volcanism:
Another way for a planetary body to transport internal heat to its surface is technically called advection. Here, heat is lost from the interior by the melting of rock at depth and direct transport of this lava to the surface, where it cools and becomes solid. This process is more commonly referred to as volcanism. There are three general ways in which advection may proceed.
Hot-spot volcanism:
Here, a hot mantle upwelling thins and weakens the lithosphere in a localized spot. This allows magma (molten mantle material) to erupt to the surface at that spot. This type of volcanism accounts for about 10% of the Earth's heat flow and formed the Hawaiian islands.
Sea-floor spreading:
Magmas rise from the middle mantle and erupt as basalts (volcanic rock rich in calcium and magnesium) at mid-ocean ridges, the constructive boundaries of continental plates.
Volcanism at subduction zones:
When oceanic lithosphere slides under a continental plate, the cold material heats up, due to friction and the heat of the mantle. This melts the cold oceanic lithosphere, creating pockets of magma between the subducting plate and the over-riding one. These magmas may then be forced upwards, creating volcano chains like the Japanese islands. The friction between the plates also causes earthquakes.
A) Volcanic Features (on Earth and on other worlds)
1) Simple lava flows:
In these flows, the molten rock has low viscosity and gas content. There are two common forms of basaltic lava flows on Earth. Aa (pronounced "ah-ah") lava is rough and chunky and is formed when solidified blocks on top of the flow get broken and mixed up. Below is shown an Aa lava flow in the Hawaiian islands.
The other kind is pahoehoe (pronounced "pa-hoy-hoy"). It is smooth and sometimes has a glassy surface if it cools rapidly. Below is a picture of a pahoehoe flow, again in the Hawaiian islands.
2) Fluidized eruptions:
In this type of eruption, clouds of pulverized, red-hot rock fragments are suspended in hot gas and flow like a flash flood.
Another term for this type of eruption is nuee ardente, French for "glowing cloud". At Mt. Pelee, on Martinique in 1902, a
nuee ardente raced across St. Pierre at 150 meters per second, killing 29,000 inhabitants.
These eruptions are extremely deadly, due to the speed of their advance.
3) Shield volcano:
This is a gently sloping mountain built by fluid lava flowing from a central vent; these volcanoes may grow very large.
The largest on Earth is Mauna Loa, Hawaii. It rises 9 km from its ocean base and is really the largest single mountain on Earth.
(Mt. Everest is only 8 km above sea level.) However, Mauna Loa is dwarfed by the largest shield volcano in the Solar System,
Olympus Mons on Mars. Its base is large enough to cover Missouri and it rises 24 km from its base.
Below is an image of Olympus Mons, looking directly down onto its peak.
4) Caldera:
When a source of lava flows drains out, the overlying rock layers can sag and collapse, forming a crater called a caldera.
One is visible in the picture of Olympus Mons above, right at the center of the peak.
5) Lava tubes/channels:
Lava tubes are underground flow cavities that often collapse after the lava drains out, like calderas.
Lava channels are surface flows that are typically tens of meters across and several kilometers long. One from the Hawaiian islands is shown below.
6) Cinder cones:
These are conical hills typically 50 to 300 meters high. They are composed of volcanic ashes and get built up around volcanic vents that eject gas, cinders, ash, and boulders. Some Hawaiian cinder cones are shown below, with Mauna Kea in the background.
7) Mare:
These are lava-covered plains that are common on the Moon, visible even to the naked eye as dark patches.
They typically occur in the bottoms of old impact craters, because the impact made fractures in the underlying rock through which
lava could rise. Since the lunar gravity is 1/6 that of the Earth, the flat, smooth mare surfaces imply that the lavas were very
fluid and thin. On Mars, mare-like plains occur near the largest volcanoes. Mare Orientale on the Moon is shown below.
8) Rilles:
These are long valleys that can be formed in four ways, the collapse of lava tubes, the build-up of levees along the sides of lava channels, the drainage of a lava channel, or by grabben formation. In the last mechanism, an area is stretched, forming a trench. This can occur as two plates move apart at a fault line. Hadley Rille, on the Moon, is shown below (with astronaut.)
So one may now see the need for exactness when studying planetary surfaces from a distance.
If you see a squigly line, it could have been formed by liquid water flow, lava flow, or by faulting.
I will now discuss the most volcanically active world in the Solar System separately.
B) Io
Io is covered with lava flows, but unlike Earth, most of the flows are composed of molten sulfur, not molten silicates. There are lots of volcanic calderas, but no large shield volcanoes. Due to the satellite's low gravity and lack of appreciable atmosphere, huge fountain-like eruptions rise unimpeded to great heights. One is visible on the edge of the moon in the image above, taken by the Galileo spacecraft, and enlarged in the upper inset. Another is shown in the lower inset, looking almost straight down in it. (You may click on the image above to see it in more detail; click on your browser's Back button to return to this page.) These eruptions are more like geysers than terrestrial eruptions.
This intense volcanism is driven by tidal heating. Io's global heat flow is 2 Watts/square meter, while Earth's is only 0.06 Watts/square meter. An active geothermal area on the Earth, like Iceland, emits 1.7 Watts/square meter. Thus, the entire surface of Io is like a volcanic hot spot on Earth. But it has been suggested that the tidal energy dissapated by the volcanism may vary with time, and that we are observing a particularly active period. Many questions remain about Io's volcanism.