Science Spotlight: Volcanism on Planet Venus

by Leandra Xochitl Marshall, published 25 July 2022
Venus. Photo Credit: NASA.
Venus. Photo Credit: NASA.
Venus is a terrestrial planet and the second planet from the Sun, referred to as Earth's "sister planet" due to its similar size, gravity, and bulk composition.

Surface Conditions

Toxic clouds obscuring the Venusian surface. Photo Credit: NASA.
Toxic clouds obscuring the Venusian surface. Photo Credit: NASA.
Surface conditions on Venus are extreme due to the planet's proximity to the sun, lack of water, and atmospheric composition. Venus originally possessed a small terrestrial ocean, but due to the immense heat the ocean evaporated, and the planet's hydrogen (H2) was lost to space. Venus is now dry and covered by reflective clouds of sulfuric acid that prevent its surface from being seen. The temperature is approximately 450 degrees Celsius, and the atmosphere is 90 standard atmospheres (atms) thick. Much of the planet's atmosphere is composed of carbon dioxide (CO2). Probes sent to Venus do not last very long under such conditions. The Venusian surface was somewhat of a mystery before it was first mapped by NASA's Project Magellan in 1990 and 1991.

The Magellan spacecraft sent short-wavelength gravity measurements and radar images back to Earth. A great deal of work on crustal and tectonic processes on Venus has been produced as a result, and there is still much to learn. The planet's surface is unlike Earth's because the absence of water has reduced erosion. About 80% of it is covered by volcanic plains. 70% of these plains contain wrinkle ridges, while 10% of the flows are lobate. Two continental highlands make up the rest of its surface. One highland lies in the planet's northern hemisphere, while the other is just south of the equator. The northern continent is called Ishtar Terra. The southern continent is called Aphrodite Terra.

Volcanism is the most important geologic phenomenon on Venus. Volcanic deposits cover most of the surface and occur in a variety of ways. Sulfur in the atmosphere may signify that eruptions have happened recently. Some forms of Venusian volcanism have terrestrial analogues, while others do not. Not all volcanic activity on Venus is well understood.

Terrestrial-Style Volcanism

Venus shares some similar planetary traits with Earth. The planet has a basaltic crust that is 25–30 km thick. Its mantle is approximately 1300 degrees Celsius. Mantle convection is dynamic, producing plumes that cause uplift, rifting, and volcanism. However, apart from its unusual global resurfacing events, Venus does not possess regular large-scale plate tectonics.

The surface composition of Venus is primarily basaltic. Satellite Aperture Radar (SAR) images show basaltic flows resembling those on Earth, as well as volcanic flows covering surface areas of a similar size to terrestrial flood-basalt provinces. However, these basaltic flows are more viscous than those found on Earth Some of the flows, festoons, are ridged lobate flows. These characteristics indicate an extraordinarily high viscosity. Like Earth, Venus has cinder cones, shield volcanoes, and calderas.

Shield Volcanoes on Venus

Venusian shield volcano and cinder cones. Photo Credit: NASA.
Venusian shield volcano and cinder cones. Photo Credit: NASA.
Shield volcanoes on Venus are several hundred kilometers in diameter and less than 2 km in elevation. They are low compared to shield volcanoes on Earth due to the density contrast between lava and rocks from the source and the surface. Density differences may be less on Venus than on Earth because the higher temperature gradient leads to shallower magma sources and volatile exsolution is restrained during magma ascent.

Calderas on Venus

Caldera on Venus. Photo Credit: NASA.
Caldera on Venus. Photo Credit: NASA.
Calderas are circular depressions and are characterized mainly by concentric patterns of fractures and a flat inner region. They are 40-80 km wide. Calderas form through the evolution of magmatic diapirs and pressure release melting in a diapir head, akin to the arachnoids and coronae discussed below. Moderate or poor volcanism is associated with calderas, and they can occur without the previous formation of composite or shield volcanoes. Despite the absence of water, there is evidence of explosive volcanism on Venus.
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