Features Due to Tectonic Activity

Many of the surface features on Venus can be attributed to tectonic activity— that is, to deformational motions within the crust. These include mountain belts, plains deformation belts, rifts, coronae, and tesserae.

Belts and Rifts

Found in the terrae, Venus's mountain belts are in some ways similar to ones on Earth such as the Himalayas of Asia and the Andes of South America. Among the best examples are those that encircle Lakshmi Planum, which include Freyja, Akna, and Danu Montes.

The tallest mountain range on Venus is Maxwell Montes, which is particularly

Akna Montes, a mountain belt on Venus bordering Lakshmi Planum in Ishtar Terra, in a radar image obtained by the Magellan spacecraft. North is up. NASA/Goddard Space Flight Center broad and comparable in size to the Himalayas, rising to about 11 km (7 miles) above the planet's mean radius. First observed as a bright feature in Earth-based radar observations of the planet made in the 1960s, the region initially was dubbed Maxwell after the British physicist James Clerk Maxwell, whose formulation of the laws that relate electricity and magnetism is the basis of radar. Subsequent Earth-based and spacecraft radar observations revealed its mountainous nature.

A major feature of Maxwell Montes is Cleopatra, a circular depression near its eastern margin that has a diameter of slightly more than 100 km (60 miles)

and a depth of more than 2.5 km (1.6 miles). Suspected after its discovery of being a volcanic caldera, Cleopatra was later generally recognized to be an impact crater.

In radar images Maxwell Montes is one of the brightest features on Venus. This high radar reflectivity, responsible for the region's early discovery, is due in part to its very rugged nature. It apparently is also due to the highest elevations on Venus being coated with some as-yet-unidentified material, perhaps an iron-containing mineral such as pyrite or magnetite, that is unusually reflective at radar wavelengths.

Venus's mountain belts typically consist of parallel ridges and troughs with spacings of 5-10 km (3-6 miles). They probably developed when broad bands of the lithosphere were compressed from the sides and became thickened, folding and thrusting surface materials upward. Their formation in some respects thus resembles the building of many mountain ranges on Earth. On the other hand, because of the lack of liquid water or ice on Venus, their appearance differs in major ways from their counterparts on Earth. Without the flow of rivers or glaciers to wear them down, Venusian mountain belts have acquired steep slopes as a result of folding and faulting. In some places the slopes have become so steep that they have collapsed under their own weight. The erosional forms common in mountainous regions on Earth are absent.

Although plains deformation belts are similar in some ways to mountain belts, they display less pronounced relief and are found primarily in low-lying areas of the planet, such as Lavinia Planitia and Atalanta Planitia. Like mountain belts, they show strong evidence for parallel folding and faulting and may form primarily by compression, deformation, and uplift of the lithosphere. Within a given lowland, it is common for deformation belts to lie roughly parallel to one another, spaced typically several hundred kilometres apart.

Rifts are among the most spectacular tectonic features on Venus. The best-developed rifts are found atop broad, raised areas such as Beta Regio, sometimes radiating outward from their centres like the spokes of a giant wheel. Beta and several other similar regions on Venus appear to be places where large areas of the lithosphere have been forced upward from below, splitting the surface to form great rift valleys. The rifts are composed of innumerable faults, and their floors typically lie 1-2 km (0.6-1.2 miles) below the surrounding terrain. In many ways the rifts on Venus are similar to great rifts elsewhere, such as the East African Rift on Earth or Valles Marineris on Mars; volcanic eruptions, for example, appear to have been associated with all these features. The Venusian rifts differ from Earth and Martian ones, however, in that little erosion has taken place within them owing to the lack of water.

CORONAE AND TESSERAE

Coronae (Latin: "garlands" or "crowns") are landforms that apparently owe their origin to the effects of hot, buoyant blobs of material, known from terrestrial geology as diapirs, that originate deep beneath the surface of Venus. Coronae evolve through several stages. As diapirs first rise through the planet's interior and approach the surface, they can lift the rocks above them, fracturing the surface in a radial pattern. This results in a distinctive starburst of faults and fractures, often lying atop a broad, gently sloping topographic rise. (Such features are sometimes called novae, a name given to them when their evolutionary relationship to coronae was less certain.)

Once a diapir has neared the surface and cooled, it loses its buoyancy. The initially raised crust then can sag under its own weight, developing concentric faults as it does so. The result is a circular-to-oval pattern of faults, fractures, and ridges. Volcanism can occur through all stages of corona formation. During the late stages it tends to obscure the radial faulting that is characteristic of the early stages.

Coronae are typically a few hundred kilometres in diameter. Although they may have a raised outer rim, many cor-onae sag noticeably in their interiors and also outside their rims. Hundreds of cor-onae are found on Venus, observed at all stages of development. The radially fractured domes of the early stages are

Oblique view of coronae in the Sedna Planitia lowlands of Venus, generated by computer from data collected by the Magellan spacecraft's radar imaging system. The topographic rise left of centre is a corona in an early evolutionary stage characterized by raised crust that is fractured in a radial pattern. The depression at the far right represents a corona in a later stage, in which the raised crust has sagged at the centre, with concentric fractures added to the radial ones. The image is highly exaggerated in its vertical direction—the more mature corona, for example, is about 100 km (62 miles) across but actually only about 1 km deep. NASA/JPL/Caltech comparatively uncommon, while the concentric scars characteristic of mature coronae are among the most numerous large tectonic features on the planet.

Tesserae (Latin: "mosaic tiles") are the most geologically complex regions seen on Venus. Gravity data suggest that the thickness of the crust is fairly uniform over much of the planet, with typical values of perhaps 20-50 km (1230 miles). Possible exceptions are the tessera highlands, where the crust may

Aine Corona and other volcanic features in a region on Venus to the south of Aphrodite Terra, shown in an image obtained from radar data gathered by the Magellan spacecraft. Aine Corona is the central large circular structure bounded by numerous arc-shaped concentric faults. It measures about 200 km (125 miles) across. Also visible are two flat-topped pancake domes, one to the north of the corona and a second inside its western border, and a complex fracture pattern in the upper right of the image. NASA/JPL

be significantly thicker. Several large elevated regions, such as Alpha Regio, are composed largely of tessera terrain. Such terrain appears extraordinarily rugged and highly deformed in radar images, and in some instances it displays several different trends of parallel ridges and troughs that cut across one another at a wide range of angles. The deformation in tessera terrain can be so complex that sometimes it is difficult to determine what kinds of stresses in the lithosphere were responsible for forming it. In fact, probably no single process can explain all tessera formation.

Tesserae typically appear very bright in radar images, which suggests an extremely rough and blocky surface at scales of metres. Some tesserae may be old terrain that has been subjected to more episodes of mountain building and faulting than have the materials around it, each one superimposed on its predecessor to produce the complex pattern observed.

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