Volcanism On Mars - Differences in Volcanic Styles Between Earth and Mars

Differences in Volcanic Styles Between Earth and Mars

The most common form of volcanism on the Earth is basaltic. Basalts are extrusive igneous rocks derived from the partial melting of the upper mantle. They are rich in iron and magnesium (mafic) minerals and commonly dark gray in color. The principal type of volcanism on Mars is almost certainly basaltic too. On Earth, basaltic magmas commonly erupt as highly fluid flows, which either emerge directly from vents or form by the coalescence of molten clots at the base of fire fountains (Hawaiian eruption). These styles are also common on Mars, but the lower gravity and atmospheric pressure on Mars allow nucleation of gas bubbles (see above) to occur more readily and at greater depths than on Earth. As a consequence, Martian basaltic volcanoes are also capable of erupting large quantities of ash in Plinian-style eruptions. In a Plinian eruption, hot ash is incorporated into the atmosphere, forming a huge convective column (cloud). If insufficient atmosphere is incorporated, the column may collapse to form pyroclastic flows. Plinian eruptions are rare in basaltic volcanoes on Earth where such eruptions are most commonly associated with silica-rich andesitic or rhyolitic magmas (e.g., Mount St. Helens).

Because the lower gravity of Mars generates less buoyancy forces on magma rising through the crust, the magma chambers that feed volcanoes on Mars are thought to be deeper and much larger than those on Earth. If a magma body on Mars is to reach close enough to the surface to erupt before solidifying, it must be big. Consequently, eruptions on Mars are less frequent than on Earth, but are of enormous scale and eruptive rate when they do occur. Somewhat paradoxically, the lower gravity of Mars also allows for longer and more widespread lava flows. Lava eruptions on Mars may be unimaginably huge. A vast lava flow the size of the state of Oregon has recently been described in western Elysium Planitia. The flow is believed to have been emplaced turbulently over the span of several weeks and thought to be one of the youngest lava flows on Mars.

The tectonic settings of volcanoes on Earth and Mars are very different. Most active volcanoes on Earth occur in long, linear chains along plate boundaries, either in zones where the lithosphere is spreading apart (divergent boundaries) or being subducted back into the mantle (convergent boundaries). Because Mars currently lacks plate tectonics, volcanoes there do not show the same global pattern as on Earth. Martian volcanoes are more analogous to terrestrial mid-plate volcanoes, such as those in the Hawaiian Islands, which are thought to have formed over a stationary mantle plume. (See hot spot.)

The largest and most conspicuous volcanoes on Mars occur in Tharsis and Elysium regions. These volcanoes are strikingly similar to shield volcanoes on Earth. Both have shallow-sloping flanks and summit calderas. The main difference between Martian shield volcanoes and those on Earth is in size: Martian shield volcanoes are truly colossal. For example, the tallest volcano on Mars, Olympus Mons, is 550 km across and 21 km high. It is nearly 100 times greater in volume than Mauna Loa in Hawaii, the largest shield volcano on Earth. Geologists think one of the reasons that volcanoes on Mars are able to grow so large is because Mars lacks plate tectonics. The Martian lithosphere does not slide over the upper mantle (asthenosphere) as on Earth, so lava from a stationary hot spot is able to accumulate at one location on the surface for a billion years or longer.

On October 17, 2012, the Curiosity rover on the planet Mars at "Rocknest" performed the first X-ray diffraction analysis of Martian soil. The results from the rover's CheMin analyzer revealed the presence of several minerals, including feldspar, pyroxenes and olivine, and suggested that the Martian soil in the sample was similar to the "weathered basaltic soils" of Hawaiian volcanoes.