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

Highlands of tessera terrain rising from the plains region known as Leda Planitia in Venus's northern hemisphere, in an image produced from radar data collected by the Magellan spacecraft. Having an extraordinarily rugged appearance in radar images, the terrain displays several different patterns of ridges and troughs crisscrossing in various directions. Tesserae are the most geologically complex terrains known on Venus and may be the result of numerous consecutive episodes of mountain building. NASA/JPL

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,

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Canali, or lava channels, in Venus's Lo Shen Valles region, north of the equatorial elevated terrain Ovda Regio, shown in a radar image from the Magellan spacecraft. Collapsed source areas for some of the meandering lava flows are visible in the image. NASA/Goddard Space Flight Center 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. Located at the western end of the elevated region Eistla Regio, south of Ishtar Terra, it is about 2 km (1.2 miles) high and has a base 300 km (200 miles) in diameter. 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. Made up of many individual lava flows piled on

Sif Mons, in a low-angle computer-generated view based on radar data from the Magellan spacecraft. In this radar image, lava flows having rougher surfaces appear brighter than smoother flows and are therefore presumably more recent. NASA/JPL

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

Volcanic pancake domes in the elevated region Eistla Regio on Venus, in a radar image produced from Magellan spacecraft data. The two larger domes, each about 65 km (40 miles) across, have broad flat tops less than 1 km (0.6 miles) high. They apparently were formed from unusually thick lava that oozed to the surface and spread in all directions. NASA

Merged pancake domes on the eastern edge of the Alpha Regio highland area of Venus, in an oblique view generated by computer from radar data gathered by the Magellan spacecraft. The volcanic features, each about 25 km (15 miles) in diameter and about750 metres (0.5 mile) high, are thought to have been formed from the extrusion of extremely viscous lava onto the surface. The vertical scale of the image is exaggerated to bring out topological detail. NASA/ JPL/Caltech 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.

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