Volcanism can produce flat lava plains or a wide variety of topographic features called volcanoes. The type of volcanic feature produced during a volcanic eruption depends on the viscosity of the magma/lava involved. Viscosity depends on temperature, composition, presence/absence of solid material in the melt, and amount of gas dissolved in the magma. The most important factor influencing viscosity is the amount of silicate (SiO2) in the magma - higher SiO2 concentrations result in stickier magmas. High-viscosity magmas can contain more gas than lower-viscosity magmas and are therefore more explosive. Explosive eruptions also can occur when lower-viscosity magmas encounter water. Such eruptions are called phreatic eruptions and usually produce large circular craters called maars.

Planum Mars

Figure 5.14 Magma viscosity and eruption rate determine the type of volcanic feature produced during an eruption. (a) Low-viscosity magmas often form flood basalts, as seen in this view of Lunae Planum, Mars, centered near 15.0°N 293.8°E. (Viking mosaic image, NASA/JPL.) (b) Slightly more viscous flows form low-relief constructs called shield volcanoes. Mauna Loa, Hawaii, is an example of a shield volcano. (Image by author.) (c) Cinder cones form when the gas content of the magma increases. The explosive release of this gas fragments the magma during the eruption, with these fragments cooling into cinders which form a conical structure. Sunset Crater in Arizona last erupted in AD 1064. (Image by author.) (d) Mt. St. Helens in Washington displays the classic steep-sloped profile of highly viscous composite volcanoes. (Image by author.) (e) This Space Shuttle radar image shows the Yellowstone caldera region. The heat associated with this region is revealed today through the area's active geysers and hot springs. Large eruptions occurred 2.0 Ma, 1.3 Ma, and 0.6 Ma ago, ejecting over 3700 km3 of material. (NASA/Shuttle/SIR-C/X-SAR.)

The type of volcanic feature produced during an eruption depends on viscosity and eruption rate. If two magmas have the same viscosity, the event with the higher eruption rate will generally produce a flatter structure than the event with the lower eruption rate.

Flat plains of lava flows are typically composed of basaltic rock (consisting of the minerals plagioclase, pyroxene, and olivine) and are called flood basalts (Figure 5.14a). Flood basalts are produced from low-viscosity magmas with high eruption rates. These eruptions have little to no associated explosive activity, resulting in very little topographic relief. The source of these massive eruptions is usually a vent or fracture. Flood basalts are a common form of volcanism, identified on all of the terrestrial planets as well as Earth's Moon.

Low-viscosity magmas produced by events with lower eruption rates result in topographically low structures called shield volcanoes (Figure 5.14b). These mountains have slopes typically <5° and are composed of basalt, although late-stage eruptions can be slightly more silicic due to differentiation within the volcano' s magma chamber. Eruptions emanate from the central crater (caldera) or from fissures along the volcano's flanks. Shield volcanoes can be extremely massive - the volume of Mars' Olympus Mons shield volcano is about 3 X 106 km3 (Smith etal., 2001a), or about the same volume of basalt as is found in the entire Hawaiian-Emperor Sea-mount chain. Shield volcano eruptions produce basaltic lava flows, which can be rough (aa) or smooth (pahoehoe) (Figure 5.15). Maps of surface roughness derived from MOLA analysis suggest that both pahoehoe and aa flows occur on Mars.

Fluid lavas producing flood basalts and shield volcanoes often move from one location to another through either lava channels on the surface or underground in lava tubes (Figure 5.16). Once the lava reaches a flat plains region, it will spread out along the sides into a large flow. The amount of spreading depends on the properties

Ascraeus Mons Jpl Viking

Figure 5.15 Low-viscosity lavas produce two types of lava flows. (a) Rougher flows are called aa flows, as seen in this image of the Bonito Lava Flow at Sunset Crater, Arizona. (b) Smoother flows, like these on Kilauea Volcano in Hawaii, are called pahoehoe flows. Pahoehoe flows are characterized by a ropy texture. (Images by author.)

Figure 5.15 Low-viscosity lavas produce two types of lava flows. (a) Rougher flows are called aa flows, as seen in this image of the Bonito Lava Flow at Sunset Crater, Arizona. (b) Smoother flows, like these on Kilauea Volcano in Hawaii, are called pahoehoe flows. Pahoehoe flows are characterized by a ropy texture. (Images by author.)

Lava Tube Mars
Figure 5.16 Low-viscosity lavas often form lava channels and lava tubes. These lava channels occur on the flank of Pavonis Mons. This is a perspective view of the region taken by MEx's HRSC camera. (Image SEMELC9ATME, ESA/DLR/FU Berlin [G. Neukum].)

of the lava, especially the viscosity and the amount of solid material contained in the flow. Lava flows are characterized by their aspect ratio (A):

w where t is the central thickness of the flow and w is the flow's width. The value of A depends on slope, g, and viscosity. As slope decreases, A decreases since the lava flow spreads out, decreasing t and increasing w. Conversely, A increases as viscosity increases since the increased stickiness of the lava will reduce spreading. Comparing the aspect ratios of different lava flows provides important information about topographic variations and the properties of the originating magmas.

Eruptions become more explosive as the silica content increases. The release of gas trapped in the magma by the SiO2 causes lava fountains. Lava fountain activity erupts small clumps of magma into the air, which then cool into cinders as they fall back to the surface. The accumulation of cinders creates a cone whose flanks have slopes close to the angle of repose (about 30° for cinders). These cinder cones (Figure 5.14c) are primarily basaltic in composition with cinder formation from a central caldera and lava flows often exuded from their base. Cinder cones commonly form on the flanks and later in dormant calderas of shield volcanoes as the magma chamber composition evolves.

Composite or stratovolcanoes are steep-sided volcanic constructs composed of alternating layers of ash and lava flows (Figure 5.14d). They occur in association with subduction zones on Earth, but have not been definitively identified on other planets. Stratovolcanoes contain very viscous and volatile-rich magmas, resulting in extremely explosive eruptions from the central caldera. Sticky lava domes often form in the caldera, plugging the conduit used by the magma to reach the surface. Pressure builds up under this dome until the gases can break through the surface, producing a sudden release of gas and ash. Hot clouds of ash can travel down the volcano flanks at speeds of hundreds of kilometers per second. These pyroclastic flows are the cause of much death and destruction associated with stratovolcano eruptions. However, stratovolcanoes can also experience quieter extrusions of lava flows, giving rise to the stratified structure. After an eruption, a lava dome begins to rebuild in the caldera, allowing the process to repeat itself.

The most explosive eruptions are ash flows called ignimbrites. The flow consists of hot gas and ash particles which cover large areas. These eruptions result from extremely viscous magmas with acidic compositions. On Earth they only occur in continental regions, usually leaving behind a large caldera which can be mistaken for a simple valley (Figure 5.14e).

Mars displays a range of volcanic features ranging in age from Noachian to late Amazonian, with recent volcanic activity concentrated in the Tharsis and Elysium regions (Hodges and Moore, 1994). Flood basalts are common as lava plains surrounding the big volcanoes in Tharsis and Elysium and in the ridged plains. Large expanses of intercrater plains seen between craters in the Noachian-aged southern highlands are probably ancient flood basalts. Flood basalts appear to have occurred throughout martian history (McEwen et al., 1999), although their areal extent has decreased over time in conjunction with the overall decline in volcanic activity.

Mars displays a number of very large but extremely low-relief (average slopes <3°) volcanoes called paterae (Latin for dish or saucer) (Figure 5.17a). Paterae are the

Figure 5.17 Various types of volcanic structures occur on Mars. (a) Tyrrhena Patera is a very low relief, highly eroded, and heavily cratered volcano centered near 21.5°S 106.5°E. (Viking mosaic image, NASA/JP.) (b) Olympus Mons is a huge shield volcano, 21.3 km high and -800 km in diameter. (NASA/MSSS.) (c) The Elysium Mons province contains three volcanoes, Hecates Tholus, Elysium Mons, and Albor Tholus. (NASA/MSSS.)

Figure 5.17 Various types of volcanic structures occur on Mars. (a) Tyrrhena Patera is a very low relief, highly eroded, and heavily cratered volcano centered near 21.5°S 106.5°E. (Viking mosaic image, NASA/JP.) (b) Olympus Mons is a huge shield volcano, 21.3 km high and -800 km in diameter. (NASA/MSSS.) (c) The Elysium Mons province contains three volcanoes, Hecates Tholus, Elysium Mons, and Albor Tholus. (NASA/MSSS.)

oldest volcanic structures on Mars and are primarily concentrated around the Hellas impact basin. Detailed analysis of Hadriaca, Tyrrhena, and Apollinaris Paterae indicate the main edifices formed by explosive volcanism during the Late Noachian and Early Hesperian periods, with subsequent effusive eruptions extending until the Early Amazonian (Crown and Greeley, 1993; Robinson et al., 1993; Berman et al., 2005). Paterae flanks are highly dissected by wind and/or fluvial activity. The ease with which these flanks have been eroded suggests they are composed of finegrained pyroclastic deposits, either in the form of pyroclastic flows or air-fall ash deposits. However, paterae compositions are consistent with low-viscosity basalt. Mafic (iron-rich) pyroclastic deposits can be produced either by water-magma interactions or by rapid ascent of deep-sourced magmas, and both mechanisms have been proposed for the martian paterae (Crown and Greeley, 1993; Gregg and Williams, 1996).

The low-albedo region of Syrtis Major is a low-relief (slopes <1°) volcano of basaltic composition (Bandfield et al., 2000; Hoefen et al., 2003) rising up to an elevation of ~2.3km (Hiesinger and Head, 2004). Schaber (1982) argued that Syrtis Major is a shield volcano with two calderas (Nili and Meroe Paterae), but Hodges and Moore (1994) and Hiesinger and Head (2004) argue that its low relief makes it more akin to the highland paterae. The Early Hesperian age of Syrtis Major places it in the same time period during which the highland paterae were forming, suggesting that during this period eruptions were too temporally limited, lava viscosities were too low, or the rate of eruption was too high to build larger edifices (Hodges and Moore, 1994). The volume of lava comprising Syrtis Major is estimated to be ~1.6 to 3.2 X 105km, comparable to estimated volumes of Amphitrites Patera (Hiesinger and Head, 2004).

Mars' big shield volcanoes were first revealed by Mariner 9 as circular depressions atop high mountains which poked through the global dust storm. As the dust settled, these features were recognized as the calderas topping the four massive shield volcanoes in the Tharsis region: Olympus Mons (Figure 5.17b), Ascraeus Mons, Pavonis Mons, and Arsia Mons. Olympus Mons rises to a height of 21.3 km, while the elevations of the other three shields range between 14.1 and 18.2 km (Smith et al., 2001a). Elysium Mons, the largest shield volcano in the Elysium province (Figure 5.17c), rises to an elevation of 14.1 km.

Smaller volcanic edifices with slightly higher slopes (up to 12°) are called domes or tholii (singular is tholus). These features appear to be small shield volcanoes based on their large calderas and other geomorphic features (Plescia, 1994, 2000).

The large and small shields are concentrated in the Elysium and Tharsis regions of Mars. The Elysium province contains three volcanoes: Albor Tholus, Hecates Tholus, and Elysium Mons (Figure 5.17c). Albor Tholus, 160 km in diameter, is a small shield probably composed of basalt. Crater analysis suggests a Noachian to

Hesperian age for this volcano (Hodges and Moore, 1994). Hecates Tholus is about 200 km in diameter, has a relatively small caldera, and displays a large number of channels on its flanks (Gulick and Baker, 1990; Mouginis-Mark and Christensen, 2005). The volcano is primarily Hesperian in age (Hodges and Moore, 1994), although HRSC crater data have been interpreted to indicate Amazonian activity on the northwestern flank (Neukum et al., 2004). The western flank of Hecates Tholus displays lower crater density and has been proposed to be a younger pyroclastic deposit from either a summit (Mouginis-Mark et al., 1982) or flank (Hauber et al., 2005) eruption.

Elysium Mons is the largest of the Elysium volcanoes, with a diameter of 400 km and a height of 14 km. It has a single summit caldera from which emanate short lava flows (<70 km long). Lava flows originating from flank vents are much longer, up to 250 km long, and display large variations in aspect ratio (Mouginis-Mark and Yoshioka, 1998). The flows likely originate as low-viscosity lavas, but viscosity increases exponentially with distance as degassing occurs and temperature drops (Glaze et al., 2003). Crater analysis suggests that Elysium Mons is the youngest of the Elysium Province volcanoes with a Late Hesperian to Early Amazonian age (Hodges and Moore, 1994).

The Tharsis region has the largest concentration of structures on Mars with 12 large volcanoes, many smaller features, and extensive lava flows (Figure 5.18). The Tharsis Bulge occupies about 25% of the surface and rises about 6 km above the mean planetary radius (Anderson et al., 2001; Phillips et al., 2001; Smith et al., 2001a). Gravity data suggest that this bulge has been largely constructed from voluminous lava flows, although some support by a mantle plume is also suggested (Smith et al., 2001a; Phillips et al., 2001; Kiefer, 2003). Crater and tectonic data indicate that much of martian volcanism was localized in the Tharsis region as early as the end of the Middle Noachian period (Solomon et al., 2005).

The Tharsis Bulge is capped by the three shield volcanoes Ascraeus, Pavonis, and Arsia Montes. Ascraeus, Pavonis, and Arsia are aligned along a northeast-to-southwest rift zone about 700 km apart. Long, narrow lava flows, indicative of high eruption rates, extend from these volcanoes. The three volcanoes are believed to have originated during the Noachian period with slow accumulation of fluid lavas from the summit calderas and surrounding concentric fissures. Multiple calderas often seen on the martian shields suggest several episodes of filling and withdrawal from the magma chamber. After the shields reached their maximum heights, the eruptions shifted to the northeast-southwest rift zone, producing younger flank eruptions. The low aspect ratios of the flow and presence of lava channels indicate very fluid lavas. Smaller shield volcanoes are seen on the flanks and caldera of Ascraeus Mons, suggesting the presence of large dike complexes within the larger shields (Wilson and Head, 2002). Flood basalts formed throughout the Tharsis

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