Origin, history, physical characteristics and tectonic features of Alba Mons - Mars

 Introduction

Alba Mons (also known as the Arcadia ring; formerly and still occasionally known as Alba Patera, a word that has since been restricted to the volcano's top caldera) is a volcano in Mars' northern Tharsis region. In terms of area, it is the largest volcano on Mars, with volcanic flow fields extending for at least a kilometer from its summit. Despite having a span equivalent to that of the United States, the volcano barely reaches an elevation of at its greatest point. This is roughly one-third the height of Olympus Mons, the world's tallest volcano. Alba Mons' flanks have extremely gentle slopes. The average slope along the volcano's northern (and steepest) face is 0.5 degrees, which is more than five times lower than the slopes on the other great Tharsis volcanoes. Alba Mons appears in wide profile to be a massive but scarcely elevated welt on the planet's surface. It's a one-of-a-kind volcanic edifice that doesn't exist either on Earth or on Mars.

Figure 1 Alba Mons as depicted by the Vikings. In orbital images, the relief of the volcano is hardly discernible. Tantalus Fossae is a large pattern of fractures on the volcano's eastern slope (right). Alba Fossae is a narrower fracture structure on the western flank. (Viking MDIM 2.1 color).

Alba Mons has a lot of distinguishing qualities in addition to its large size and low relief. Alba Fossae on the volcano's western flank and Tantalus Fossae on the volcano's eastern flank surround the center portion of the volcano with an incomplete ring of faults (graben) and fractures. In addition, the volcano contains unusually long, well-preserved lava flows that radiate outward from the volcano's center zone.

The massive lengths of certain individual flows (>) indicate that the lavas were extremely fluid (low viscosity) and large in volume. Many of the flows exhibit unusual morphologies, with long, sinuous ridges and discontinuous lava channels in the center. Water flow likely generated the branching pattern of shallow gullies and channels (valley networks) between the ridges (especially along the volcano's northern flank).

Alba Mons includes among the Tharsis region's oldest and most widely exposed volcanic deposits. Significant volcanic activity at Alba Mons terminated far sooner than at Olympus Mons and the Tharsis Montes volcanoes, according to geologic evidence. Alba Mons' volcanic deposits range in age from Hesperian to early Amazonian (approximately 3.6 to 3.2 billion years old).

Origin Name

Alba Patera was the volcano's official name for many years. Patera (paterae) is a shallow drinking dish or saucer in Latin. The phrase was coined to describe a group of ill-defined, scalloped-edged craters that seemed to be volcanic (or non-impact) in origin in early spacecraft photographs. The volcano was renamed Alba Mons (Alba Mountain) by the International Astronomical Union (IAU) in September 2007, with the word Alba Patera reserved for the volcano's two central depressions (calderas).

Alba comes from the Latin word for white and alludes to the clouds that may be seen from Earth-based telescopes regularly over the region. The Alba volcanic structure, also known as the Arcadia Ring, was first found by the Mariner 9 spacecraft in 1972. (about the partial ring of fractures around the volcano). In 1973, the International Astronomical Union (IAU) named the volcano Alba Patera. When the context is recognized, the volcano is commonly referred to as Alba.

Location and Size

The Arcadia quadrangle is centered on Alba Mons (MC-3). The nearby Diacria quadrangle contains a large portion of the volcano's western flank (MC-2). It covers roughly 2.5 million square kilometers and has a volume of about 2.5 million km3.

Figure 2 Alba Mons and its environs on a MOLA topographic map. The main structure is red to orange in hue, with a yellow-orange to green apron surrounding it. Because the volcano crosses the dichotomy line, the relief is highest to the north. Ceraunius Fossae, which sits beneath a portion of the volcano, stretches southward like a handle.

Although Alba Mons reaches a maximum elevation above Mars' datum, the difference in elevation between its summit and surrounding topography (relief) on the north side of the volcano (about) is much greater than on the south side (about) (about ). Alba spans the dichotomy line between the cratered uplands in the south and the lowlands in the north, which explains the asymmetry. The volcano's plains slope northward toward the Vastitas Borealis, which has an average surface elevation below datum (-). Alba Mons' southern half is built on a large north-south topographic ridge that correlates to Ceraunius Fossae's broken, Noachian-aged landscape (pictured left).

Physical Characteristics

The majority of Alba Mons' primary building is covered in a coating of dust about a foot thick. In high-resolution photographs of the top, the dust layer can be seen (pictured right). The wind has chiseled the dust into streamlined shapes in certain areas, and minor landslides have slashed it. Some isolated patches of dust, on the other hand, appear smooth and wind-free.

Figure 3 Views of Alba Mons' core building and summit dome from the south (top) and north (bottom) by MOLA (bottom). The vertical exaggeration is ten times.

Since the late 1980s, several experts have suspected that the beginning phase of Alba Mons eruptions featured a considerable amount of pyroclastics (and thus explosive activity). The proof came in the form of several valley networks on the volcano's northern sides that looked to have been formed by running water. This evidence, along with thermal inertia data indicating a surface dominated by fine-grained materials, indicates the presence of an easily erodible substance, such as volcanic ash. If the edifice was created mostly from pyroclastic-flow deposits, the volcano's extraordinarily low profile would be more simply explained (ignimbrites).

Recent data from the Mars Global Surveyor and the Mars Odyssey spacecraft have found no evidence that Alba Mons has ever had explosive eruptions. Another theory for the valley networks on the volcano's north side is that they were formed by the sapping or melting of ice-rich dust deposited during a recent, Amazonian-aged glacial period.

In conclusion, the current geologic investigation of Alba Mons reveals that the volcano was formed by lavas with basalt-like rheological qualities. If Alba Mons was the site of early explosive activity, the evidence (in the form of massive ash deposits) has been mostly hidden by younger basaltic lavas.

Figure 4 Alba Mons' central caldera complex. In comparison to other Tharsis volcanoes, the calderas are quite shallow. A tiny shield within the main caldera is crowned by a concentric circular structure (near center). The image spans roughly 200 kilometers (120 miles) (THEMIS daytime IR mosaic).

Tectonic Features

The massive labyrinth of fractures that surround Alba Mons is undoubtedly the volcano's most noticeable feature. Alba's tectonic characteristics, which include normal faults, graben, and tension fractures, are almost all extensional. Simple graben are the most typical extensional features on Alba Mons (and Mars in general). Graben are long, narrow troughs enclosing a down-faulted piece of crust and connected by two inward-facing normal faults. Alba boasts the most obvious display of basic graben on the planet.

Figure 5 Tantalus Fossae, on the eastern flank of Alba Mons, with a simple graben and horsts. Drainage into subsurface voids, probably caused by tension cracks, is suggested by a line of pit craters (THEMIS IR daytime mosaic).

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