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[47] 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 (Ascreus 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 [48] 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 d 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.





Olympus Mons. (a) This volcano, the largest on Mars, measures over 600 km across at the base, and is about 27 km high. It is surrounded by a well-defined scarp that is up to 6 km high. Flows drape over the scarp and extend onto tile surrounding plains. [n many places the scarp is associated with small block faults, indicating that faulting may, have played a part in its development. Parts of the plains surrounding tile volcano are characterized by ridged and grooved terrain that is faulted in places. The origin of this terrain is not known. (b) The stereogram permits a greater appreciation of the structure of Olympus Mons, especially the caldera and the scarp. [(a) 211-5360, (b) Left 211-5345. Right 211-5360: 18° N, 133° W]


Summit Caldera of Olympus Mons. This mosaic consists of several frames that show features on the surface as small as 18 meters across. The circular caldera on the left is almost 3 km deep and 25 km across and has wall slopes of about 32°. It probably formed as a result of recurrent collapse following drainage of magma out of the central conduit of the volcano during flank eruptions. The floor of the deepest caldera is featureless at this resolution, but the floor materials of other parts of the caldera complex are marked by fault patterns and ridges similar to mare ridges on the Moon. Fluting of the caldera walls suggests landslide activity. [211-5601: 18°N 133°W]

Terraces on Upper Slopes of Olympus Mons. The origin of the lava terraces is not known. In some respects they are analogous to terraced features seen on pahoehoe flows on Mount Etna Sicily , where they formed as a result of embankments developing at the fronts of lava flows and the accumulation of lava lakes behind the embankments. Some of the small craters appear to be rimless volcanic pits. [46B12; 17° N 132° W ]


Lava Flow Drapes over Olympus Mons Scarp. (a) Lava channels and partially collapsed lava tubes are visible along the crests of ridge-like flows. The surface features on these flows are similar to those developed on basaltic flows on Earth. Clearly the scarp in this area is older than the flows indicating that at least the youngest flows on the mountain occurred d after scarp formation. In this region the Olympus Mons flows make up the plains surface at the foot of the scarp. However, in other areas Olympus Mons flows have been overlain by the smooth-surfaced material of the plains. (b) The stereographic pair graphically portrays the ruggedness of the scarp. [(a) 47B25 (b) Left 46B34, Right 45B45; 21°N 130° W]



Arsia Mons. The summit is at about the same elevation as that of Olympus Mons rising 16 km above the Tharsis Ridge itself about 11 km high. The caldera is less complex than that of Olympus Mons being a single, large circular structure about 140 km in diameter. Surrounding the caldera are concentric graben; the main northeast-southwest trending fracture zone underlying the volcano is indicated by numerous collapse pits seen here on the upper side of the caldera. This mosaic shows an enormous flow-like feature that extends from the volcano flanks onto the adjacent plains and which consists of hummocky terrain with faint concentric features. The flow terminates in fine scale ridges parallel to the flow's front edge. The origin of this feature is not clear but it may be a major landslide that developed high on the flanks of the volcano at a time when the volcano slopes were unstable. The concentric ridges in the distal parts appear to run through all the topographic features without substantially modifying them. and may be pressure ridges that developed in the underlying terrain at the foot of the unit. [211-5317; 9°S, 123 °W]

[53] Possible Landslide Deposit on Arsia Mons. Hummocky terrain makes up most of this flow, and grades into the finely ridged, concentric flow front. These features may be pressure ridges at the front of the flow or, in some places deceleration ridges formed as the flow came to a standstill. Small lava flow fronts are visible on the smooth plains in front of the main flow. [49B89; 3°N 117°W]


Extensive Lava Flows from Arsia Mons. The flows that erupted from Arsia Mons extend some 1500 km away from the summit and bury the older cratered terrain of the southern hemisphere. Flow fronts are visible within the large crater Pickering (120-km-diameter) where they have been diverted around high ground associated with the central peak of the crater. Flows of this type associated with the big volcanoes may have lengths in excess of 1000 km and may resemble the large flows found in Mare Imbrium on the Moon. The discovery of these flows on the outer flanks of the major volcanoes on Mars has shown that the basal diameter of many of these volcanoes is considerably larger than was suspected from Mariner 9 data. [56A12; 34°S 133°W]


Arsia Mons Summit. Part of the caldera is visible at the upper left of' the picture. The summit of the volcano is cut by lines of pits marking the fracture zone running through the volcano. Most of the lava at the middle and bottom right of the picture appears to have originated from the fracture zone, and postdates the summit cone of the volcano. A well- defined channel/tribe system is visible toward the lower right of the picture pits at the head of this channel system represent the vent area. [52AO4; 12°S, 120°W]

Summit of Alba Patera. This volcano is only a few kilometers above the surrounding plain which coupled with its large diameter of some 1700 km gives it a much lower profile than the Tharsis volcanoes. The rim of an old caldera near the summit partly buried by younger lava flows is visible at the bottom left; at the bottom right a younger caldera is at the top of the youngest summit cone. Lava flows are well preserved and flows can be seen extending from near the lower right of the picture toward the upper left. [7B94; 41°N, 109°W]

[56] Lava-Covered Upper Flanks of Alba Patera. Different kinds of flows are visible. Large, relatively feat-topped flows with well-defined flow fronts occur in the middle of the frame. At the lower left are long flow-ridges, some of which extend for several hundred kilometers. The flat-topped flows are generally considered to have been fed by lava tubes. One flow has a sinuous channel-tube running along the crest of the ridge. Superposed impact craters on Alba Patera are more numerous than on Olympus Mons and Arsia Mons suggesting an older age for many of these flows. [7B24; 48°N, 115°W]


Ridge-Like Lava Flows on Alba Patera. This part of the flanks of Alba Patera has ridge-like lava flows with complicated dendritic patterns developed on them. Some of these channels may be directly associated with the formation of the lava flows, but some may have resulted from fluvial modification of the volcano flanks. Cutting the lava flows in this area is a well-defined graben, within which are numerous collapse pits. [7B53; 46°N, 119°W]

Biblis Patera. This volcano, situated between Arsia Mons and Olympus Mons, is much smaller than those so far described. Flow features on the flanks of the volcano are truncated by the surrounding plains, indicating partial burial by later deposits. The exposed part of the volcano has a basal diameter of about 100 km. Its original size may have been larger, although, from the small size of the caldera, it is unlikely-eve n considering the buried base-that it was ever as large as the giant Tharsis volcanoes. The summit caldera is surrounded by almost circular faults, which seems characteristic of Martian volcanoes. [44B50; 3°N, 124°W]


Ulysses Patera. This volcano lies just to the east of Biblis Patera in the northwest part of 'Tharsis. It is similar in size to Biblis Patera, is surrounded by younger flows, and has two superposed craters, probably of impact origin. These craters are older than the surrounding plains, and they have intersected the caldera walls and pushed material into the floor of the caldera. [49B85; 3°N, 121°W]

Tharsis Tholus. This 17O-km diameter volcano differs in form from the volcanoes previously illustrated. The caldera has a wide bench around one side. This bench may represent an early lava lake level before further collapse occurred in the middle of the caldera. Scarps intersecting the caldera appear to be normal faults rather than graben. The base of the volcano is covered by younger materials so its original size cannot be determined. [225A13; 13°N, 92°W]


Tyrrhena Patera. The flanks of this ancient, southern hemisphere volcano have been strongly modified and embayed. At the summit is an irregular depression that is continuous with a valley, extending down the outer flanks. Concentric graben surround the summit. The volcano is so degraded that there are no well-defined primary volcanic depositional features to provide clues regarding the nature of the erupted materials. However, the low profile of the volcano, and the way in which outliers of the volcano form mesa-like bodies, suggest ash flow deposits rather than lavas. [211-5730; 20°S, 252° W]


Hadriaca Patera. This volcano's caldera is much better defined than that of Tyrrhena Patera, but its flanks are strongly degraded by radial valleys. The volcano is younger than many of the surrounding craters, but still much older than the Tharsis volcanoes, as indicated by the numbers of superposed impact craters. [97A42; 30°S, 270°W]


Volcano-Like Features in the Chryse Basin. In the upper left of this picture, a light circular feature with a central pit and a very low profile straddles a sinuous line, which may be the trace of a dike or fracture. The feature is less than 10 km across, much smaller than any of the other volcanoes described. [4A36; 18°N, 35°W]