Martian Volcanoes
Mars is only about one-half the size of Earth and yet has several
volcanoes that surpass the scale of the largest terrestrial volcanoes. The most
massive volcanoes are located on huge uplifts or domes in the Tharsis and Elysium regions
of Mars. The Tharsis dome is 4,000 kilometers (2,500 miles) across and rises to
astonishing 10 kilometers (6.2 miles) in height. Located on its northwest flank are three
large shield volcanoes: Ascraeus Mons, Pavonis Mons and Arsia Mons. Beyond the
dome's northwest edge is Olympus Mons, the largest of the
Tharsis volcanoes. Olympus Mons is classified as a shield volcano. It is 24
kilometers (15 miles) high, 550 kilometers (340 miles) in diameter and is rimmed by a 6
kilometers (4 miles) high scarp. A scarp is a A line of cliffs produced by faulting
or erosion; a relatively straight, clifflike face or slope of considerable linear extent,
breaking the general continuity of the land by separating surfaces lying at different
levels. It is one of the largest volcanoes in the Solar System. By comparison the
largest volcano on Earth is Mauna Loa which is 9 kilometers (6 miles) high and 120
kilometers (75 miles) across. Elysium Planitia is the second largest volcanic region on
Mars. Elysium Planitia is centered on a broad dome that is 1,700 by 2,400 kilometers
(1,060 by 1,490 miles) in size. It has smaller volcanoes than the Tharsis region,
but a more diverse volcanic history. The three volcanoes in this region include
Hecates Tholus, Elysium Mons and Albor Tholus.
The large shield volcanoes on Mars resemble Hawaiian shield volcanoes. They both have effusive eruptions which are relatively quiet and basalic in nature. Basaltic in the sense that it erupts dark-colored, igneous rocks composed of minerals that are relatively rich in iron and magnesium. Both have summit pits or calderas; a large, basin-shaped volcanic depression that is more or less circular in form. Most volcanic calderas are produced by collapse of the roof of a magma chamber due to removal of magma by voluminous eruptions or subterranean withdrawal of the magma, although some calderas may be formed by explosive removal of the upper part of a volcano and long lava flows. The biggest difference between Martian and Terrestrial volcanoes is size. The volcanoes in the Tharsis region are 10 to 100 times larger than those on Earth. They were built from large magma chambers deep within the Martian crust. Magma chambers are molten rock within the crust of a planet that is capable of intrusion into adjacent crustal rocks or extrusion onto the surface. Igneous rocks are derived from magma through solidification and related processes or through eruption of the magma at the surface. The Martian flows are also much longer. This is probably due to larger eruption rates and to lower gravity. One of the reasons volcanoes of such magnitude were able to form on Mars is because the hot volcanic regions in the mantle remained fixed relative to the surface for hundreds of millions of years. On Earth, the tectonic flow of the crust across the hot volcanic regions prevent large volcanoes from forming. The Hawaiian islands were created as the Pacific plate moved northwest. These volcanoes have a relatively short life time. As the plate moves new volcanoes form and the old ones become silent.
Not all Martian volcanoes are classified as shields with effusive eruption styles. North of the Tharsis region lies Alba Patera. This volcano is comparable to Olympus Mons in its horizontal extent but not in height. Its base diameter is 1,500 kilometers (930 miles) but is less than 7 kilometers (4.3 miles) high. Ceraunius Tholus is one of the smaller volcanoes. It is about the size of the Big Island of Hawaii. It exhibits explosive eruption characteristics and probably consists of ash deposits. Tyrrhena Patera and Hadriaca Patera both have deeply eroded features which indicate explosive ash eruptions. Mt. Saint Helens is an example of a terrestrial ash eruption.