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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 in addition to Maxwell Montes include Freyja, Akna, and Danu Montes. Maxwell Montes is particularly broad and comparable in size to the Himalayas.
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.
Plains deformation belts
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
Rifts (see rift valley) 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
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 coronae sag noticeably in their interiors and also outside their rims. Hundreds of coronae are found on Venus, observed at all stages of development. The radially fractured domes of the early stages are comparatively uncommon, while the concentric scars characteristic of mature coronae are among the most numerous large tectonic features on the planet.
Tesserae
Tesserae (Latin: “mosaic tiles”) are the most geologically complex regions seen on Venus. 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.
Volcanic features
Along with intense tectonic activity, Venus has undergone much volcanism. The largest volcanic outpourings are the huge lava fields that cover most of the rolling plains. These are similar in many respects to fields of overlapping lava flows seen on other planets, including Earth, but they are far more extensive. Individual flows are for the most part long and thin, which indicates that the erupting lavas were very fluid and hence were able to flow long distances over gentle slopes. Lavas on Earth and the Moon that flow this readily typically consist of basalts, and so it is probable that basalts are common on the plains of Venus as well.
Of the many types of lava-flow features seen on the Venusian plains, none are more remarkable than the long, sinuous canali. These meandering channels usually have remarkably constant widths, which can be as much as 3 km (2 miles). They commonly extend as far as 500 km (300 miles) across the surface; one is 6,800 km (4,200 miles) long. Canali probably were carved by very low-viscosity lavas that erupted at sustained high rates of discharge. In a few instances segments of canali appear to proceed uphill, which suggests that crustal deformation took place after the channels were carved and reversed the gentle downward surface slopes to upward ones. Other channel-like volcanic features on Venus include sinuous rilles that may be collapsed lava tubes, and large, complex compound valleys that apparently result from particularly massive outpourings of lava.
In many locations on Venus, volcanic eruptions have built edifices similar to the great volcanoes of Hawaii on Earth or those associated with the Tharsis region on Mars. Sif Mons is an example of such a volcano; there are more than 100 others distributed widely over the planet. Known as shield volcanoes, they reach heights of several kilometres above the surrounding plains and can be hundreds of kilometres across at their base. They are made up of many individual lava flows piled on one another in a radial pattern. They develop when a source of lava below the surface remains fixed and active at one location long enough to allow the volcanic materials it extrudes to accumulate above it in large quantities. Like those found on the rolling plains, the flows constituting the shield volcanoes are generally very long and thin and are probably composed of basalt.
When a subsurface source of lava is drained of its contents, the ground above it may collapse, forming a depression called a caldera. Many volcanic calderas are observed on Venus, both atop shield volcanoes and on the widespread lava plains. They are often roughly circular in shape and overall are similar to calderas observed on Earth and Mars. The summit region of Sif Mons, for example, exhibits a caldera-like feature 40–50 km (25–30 miles) in diameter.
Along with the extensive lava plains and the massive shield volcanoes are many smaller volcanic landforms. Enormous numbers of small volcanic cones are distributed throughout the plains. Particularly unusual in appearance are so-called pancake domes, which are typically a few tens of kilometres in diameter and about 1 km (0.6 mile) high and are remarkably circular in shape. Flat-topped and steep-sided, they appear to have formed when a mass of thick lava was extruded from a central vent and spread outward for a short distance in all directions before solidifying. The lavas that formed such domes clearly were much more viscous than most lavas on Venus. Their composition is unknown, but—given the knowledge of lavas on Earth—they are likely to be much richer in silica than the basalts thought to predominate elsewhere on the planet.
Volcanic edifices are not uniformly distributed on Venus. Although they are common everywhere, they are particularly concentrated in the Beta-Atla-Themis region, between longitudes 180° and 300° E. This concentration may be the consequence of a broad active upwelling of the Venusian mantle in this area, which has led to enhanced heat flow and formation of magma reservoirs.
The Venus Express spacecraft found evidence for active volcanoes on Venus. A sharp rise in the amount of sulfur dioxide in the atmosphere in 2006 could have been from volcanic eruptions. In 2008 a hot spot was observed coming into existence and then cooling down again in the Ganiki Chasma rift zone. Analysis of Magellan images in 2023 found the first direct evidence of active volcanism on the planet. Comparison of images of a volcanic vent taken eight months apart in 1991 showed changes that were most likely caused by an eruption.
