Olympus Mons
From Wikipedia:
This article is about the volcano on Mars
As a shield volcano, Olympus Mons resembles in its morphology the large volcanoes making up the Hawaiian Islands. The edifice is about 600km (370mi) wide.[7] Because the mountain is so large, with complex structure at its edges, allocating a height to the structure is difficult. It stands 21km (13mi) above the Mars global datum, and its local relief, from the foot of the cliffs which form its margin to the northwest to its peak, is nearly 22km (14mi)[3] (a little over twice the height of Mauna Kea as measured from its base on the ocean floor). The total elevation change from the plains of Amazonis Planitia, over 1,000km (620mi) to the northwest, to the summit approaches 26km (16mi).[2] The summit of the mountain has six nested calderas (collapse craters) forming an irregular depression 60km (37mi) × 80km (50mi) across[8] and up to 3.2km (2.0mi) deep.[9] The volcano's outer edge consists of an escarpment, or cliff, up to 8km (5.0mi) tall, a feature unique among the shield volcanoes of Mars. Olympus Mons covers an area approximately the size of Arizona, or about 295,254km2 (113,998sqmi).[10]
Being a shield volcano, Olympus Mons has a very low profile. The average slope on the volcano's flanks is only 5°.[9] Slopes are highest near the middle part of the flanks and grow shallower toward the base, giving the flanks a concave upward profile. The shape of Olympus Mons is distinctly unsymmetrical. Its flanks are shallower and extend out further from the summit in the northwestern direction than they do to the southeast. The volcano's shape and profile have been likened to a "circus tent" held up by a single pole that is shifted off center.[11]Because of the size of Olympus Mons and its shallow slopes, an observer standing on the Martian surface would be unable to view the entire profile of the volcano, even from a great distance. The curvature of the planet and the volcano itself would obscure such a synoptic view.[12]Similarly, an observer near the summit would be unaware of standing on a high mountain, as the slope of the volcano would extend beyond the horizon, a mere 3 kilometers away.[13]
However, the basal escarpment seen from the plains around the volcano would likely present an astonishing view. The sight of an immense wall of rock shooting up 5miles into the air is sure to impress any future explorers.
Geology
[19]In places along the volcano's base, lava flows can be seen spilling out into the surrounding plains, forming broad aprons, and burying the basal escarpment. (Note: Lava flows refer to both actively flowing lava and the solidified landforms they produce. The meaning here is the latter, since Mars has no active flows at the present time.) Crater counts from high resolution images taken by theMars Expressorbiter in 2004 indicate that lava flows on the northwestern flank of Olympus Mons range in age from 115 million years old (Mya) to only 2 Mya.[20]These ages are very recent in geological terms, suggesting that the mountain may still be volcanically active, though in a very quiescent and episodic fashion.[21] 
Calderason Olympus Mons summit. The youngest calderas form circular collapse craters. Older calderas appear as semicircular segments because they are transected by the younger calderas. The caldera complex at the peak of the volcano is made of at least six overlapping calderas and caldera segments (pictured).[22]Each caldera was formed by roof collapse following depletion and withdrawal of the subsurfacemagma chamberafter an eruption. Each caldera thus represents a separate pulse of volcanic activity on the mountain.[23]The largest and oldest caldera segment appears to have formed as a single, large lava lake.[24]The size of a caldera is a reflection of the size of the underlying magma chamber. Using geometric relationships of caldera dimensions from laboratory models, scientists have estimated that the magma chamber associated with the largest caldera on Olympus Mons lies at a depth of about 32km below the caldera floor.
.[25]Crater size-frequency distributions on the caldera floors indicate the calderas range in age from 350Mya to about 150Mya. All probably formed within 100 million years of each other.[26][27]Olympus Mons is asymmetricalstructurallyas well astopographically. The longer, more shallow northwestern flank displays extensional features, such as large slumps andnormal faults. In contrast, the volcano's steeper southeastern side has features indicating compression. They include step-like terraces in the volcano's mid-flank region (interpreted asthrust faults[28]) and a number ofwrinkle ridgeslocated at the basal escarpment. Why opposite sides of the mountain should show different styles of deformation is puzzling. The answer may lie in understanding how large shield volcanoes grow laterally and on how variations within the substrate of the volcano affect the final shape of the mountain. 
MOLA-generated oblique view of Olympus Mons, showing the asymmetry of the volcano. View is from northeast. The wider, gently sloping northern flank is to the right. The more narrow and steeply sloping southern flank (left) displays low, rounded terraces resembling rumpled carpet. These features are interpreted asthrust faults. The volcano's basal escarpment is prominent. The vertical exaggeration in this image is high (10x), so the relief is more striking than it would appear in reality. Large shield volcanoes grow not only by adding material to their flanks as erupted lava, but also by spreading laterally at their bases. As a volcano grows in size, thestressfield underneath the volcano changes from compressional to extensional. A subterranean rift may develop at the base of the volcano, causing the underlying crust to spread apart.[29]If the volcano rests on sediments containing mechanically weak layers (e.g., beds of water-saturated clay), detachment zones (decollements) may develop in the weak layers. The extensional stresses in the detachment zones can produce giant landslides and normal faults on the volcano's flanks, leading to the formation of a basal escarpment.[30]Further from the volcano, these detachment zones can express themselves as a succession of overlapping, gravity driven thrust faults. This mechanism has long been cited as an explanation of the Olympus Mons aureole deposits (discussed below).[31]Olympus Mons lies at the edge of the Tharsis bulge, a vast volcanic plateau that is very ancient. The formation of Tharsis was likely complete by the end of theNoachian Period. At the time Olympus Mons began to form inHesperiantimes, the volcano was located on a shallow slope that descended from the high in Tharsis into the northern lowland basins. Over time, these basins would have received large volumes of sediment eroded from Tharsis and the southern highlands. The sediments likely contained abundant Noachian-agedphyllosilicates(clays) formed during a early period on Mars when surface water was abundant.[32]The sediments would be thickest in the northwest where basin depth was greatest. As the volcano grew through lateral spreading, low-friction detachment zones preferentially developed in the thicker sediment layers to the northeast, creating the basal escarpment and widespread lobes of aureole material (Lycus Sulci). Spreading also occurred to the southeast; however, it was more constrained in that direction by the Tharsis rise, which presented a higher-friction zone at the volcano's base. Friction was higher in that direction because the sediments were thinner and probably consisted of coarser grained material resistant to sliding. The competent and rugged basement rocks of Tharsis acted as an additional source of friction. Thus, basal spreading of Mons Olympus was inhibited in the southeast direction, accounting for the structural and topographic asymmetry of the mountain. Numerical models of particle dynamics involving lateral differences in friction along the base of Olympus Mons have been shown to reproduce the volcano's present shape and asymmetry fairly well.[33]The detachment along the weak layers was likely aided by the presence of high-pressure water in the sediment pore spaces. This possibility has interesting astrobiological implications. If water-saturated zones still exist in sediments under the volcano, they would likely have been kept warm by a high geothermal gradient and residual heat from the volcano's magma chamber. Potential springs or seeps around the volcano would offer exciting possibilities for detecting microbial life.[34]
