VOLCANIC ACTIVITY on Earth can be divided into two basic types: eruptions that occur repeatedly from the same conduit and slowly build roughly circular mountains, and eruptions from any widely spaced vents, usually fissures, that create extensive lava plains. Both types are found on Mars. Volcanic rocks are of particular interest to the geologist because they originate deep within the planet and provide a means of assessing the conditions and processes that operate there. Although we are unable to examine the rocks on Mars directly, the volcanic features give an indication of rock composition. For example, silica rich lavas tend to have higher viscosities and yield strengths than silica poor lavas and so form differently shaped flows; volatile rich, viscous lavas tend to produce abundant ash during eruptions, so ash deposits rather than lava flows are the predominant landform. The volcanoes are also interesting in that their shapes and sizes provide information on thermal conditions in the interior of the planet. The volcano height gives a means of estimating the depth of melting, and the degree of sagging of the crust under the weight of the volcano permits the viscosities of the crustal materials and hence the temperature profile to be calculated.
Martian volcanoes are most common in the region of Tharsis, where three large volcanoes (Ascraeus Mons, Pavonis Mons, and Arsia Mons) form a northeast southwest line. Another large volcano, Olympus Mons, is located about 1500 km northwest of the line. All four are enormous by terrestrial standards. Olympus Mons is more than 600 km across and towers approximately 27 km above the mean surface level. Alba Patera, just to the north of Tharsis, although only a few kilometers high, is 1700 km in diameter. The Hawaiian volcanoes, which are among the largest on Earth, are generally less than 120 km in diameter and 9 km above the ocean floor. Surrounding the massive Martian volcanoes are extensive lava plains and many smaller volcanoes such as Biblis Patera and Tharsis Tholus. Volcanoes occur in regions of the planet other than Tharsis, but tend to be smaller and older.
Each of the three Tharsis shield volcanoes has a caldera complex at its summit, apparently formed by repeated collapses following eruptions. On the flank of each edifice is a faint radial texture formed by numerous long, thin flows, some with central channels. The general morphology of the flows is similar to those on the flanks of the Hawaiian shield volcanoes and suggests fluid flow. Various concentric features such as terraces, breaks in slope, and lines of rimless depressions are superposed on the radial texture. On the northeast and southwest sides of each volcano, numerous pits in the shield coalesce to form alcoves that evidently were sources of enormous volumes of lava. Flows spread from these alcoves over the adjacent plains, covering the lower flanks of the volcanoes and extending several hundred kilometers from the source. Thus, eruptions from the Tharsis volcanoes formed both the volcanic edifices and the surrounding plains.
The main edifice of Olympus Mons resembles the Tharsis shields except that it is surrounded by a cliff that, at some points, reaches 6 km in height. In several places, lava has flowed over the cliff and across the surrounding plains, extending the volcanic edifice beyond the scarp. All around Olympus Mons, blocks of strongly ridged terrain extend as far as 1000 km from the scarp and constitute the so called aureole. The origin of the aureole is unclear, but suggestions are that it is the remnant of a pre Olympus volcano, that it consists of eroded ash flow tuffs, or vast thrust sheets.
Alba Patera, just to the north of Tharsis, differs from the volcanoes already described. Although it is more than 1700 km across, it is only about 2 km high. Many flow features are visible on its flanks. These features are often as many as 10 times larger than their terrestrial counterparts, but otherwise show great similarity. The nature of Alba Patera's flow features again suggests fluid lavas.
Relatively featureless plains cover much of the planet's surface. The origins of most of the plains are not known. Although some may be largely aeolian and fluvial, evidence indicates that most are volcanic. The plains around the larger volcanoes have numerous flow features and are almost certainly volcanic. Other plains have ridges and rille like features that resemble those on the Moon and so are suspected of being volcanic like the lunar maria. Where visible in section, the plains are layered, perhaps indicating interbedded materials of different origins.
Terraces on Upper Slopes of Olympus Mons
Lava Flow Drapes over Olympus Mons Scarp
Possible Landslide Deposit on Arsia Mons
Extensive Lava Flows from Arsia Mons
Lava-Covered Upper Flanks of Alba Patera
Ridge-like lava Flows on Alba Patera
Volcano-Like Features in the Chryse Basin