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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.

Venusian Volcanism

Ternary diagram demonstrating the formation relationship between Venusian volcanic features. Photo Credit: Krassilnikov 2006.
Ternary diagram demonstrating the formation relationship between Venusian volcanic features. Photo Credit: Krassilnikov 2006.
There are four types of volcanic features unique to Venus: coronae, novae, “pancake” domes, and arachnoids. These features are classified by their geomorphology and formation mechanisms.

Coronae

Corona with “pancake” dome. Photo Credit: NASA, Pearson Education.
Corona with “pancake” dome. Photo Credit: NASA, Pearson Education.
Coronae (singular corona) have diameters of 100-1000 km. They are circular with ridges and fractures, have an interior that is topographically inflated or deflated, have a surrounding moat, and inner volcanic or tectonic structures. Corona shapes can vary. Radial fracturing, concentric fracturing, and compressional tectonic structures are common features of coronae. Coronae result from doming and fracturing due to the interaction of mantle diapirs with the gravitational relaxation of the lithosphere.

Novae

Nova. Photo Credit: NASA, Wiki Commons.
Nova. Photo Credit: NASA, Wiki Commons.
Novae (singular nova) are 100-300 km in diameter and radially fractured. Their fractures form starburst consisting of multiple blocks. A nova commonly features uplifted topography or inflation.. Novae are the result of doming and fracturing of the surface due to interaction of mantle diapirs with the lithosphere. Radial fracturing results from dike emplacement.

Pancake domes

Pancake domes. Photo Credit: NASA, Pearson Education.
Pancake domes. Photo Credit: NASA, Pearson Education.
A pancake dome is an unusual “pancake” shaped lava dome only found on Venus. These features often form in groups or clusters. They are generally associated with other forms of Venusian volcanism, such as coronae and novae.

Arachnoids

Arachnoid. Photo Credit: NASA.
Arachnoid. Photo Credit: NASA.
Arachnoids are characterized by circular patterns of ridges and radial fracture patterns or ridges extending outward. They are 50-175 km diameter depressions with small amounts of associated volcanism. They are formed by the evolution of mantle diapirs. High tangential stress during the formation of arachnoids leads to radial fractures. Some arachnoids resemble coronae.

Global Resurfacing Events

The geologic evolution of Venus is still uncertain due a lack of samples and age constraints. Radar images obtained by the Magellan mission revealed a low number of craters. The impact crater record gives a mean production age of 300–600 million years ago (Ma) and a uniform distribution over the entire surface of the planet. No more than 10% of the craters have been altered by volcanism. These observations suggest a catastrophic volcanic event removed all evidence of older surface material and previous cratering. In other words, Venus underwent a resurfacing event—a gradual resurfacing or global resurfacing event ending at 300–600 Ma. The last global resurfacing took place in less than 100 Ma and was followed by decreased volcanic activity. Two potential resurfacing mechanisms have been suggested to explain the above observations: resurfacing by plate tectonics and resurfacing by plume activity.

Plate tectonics, the creation of lithosphere at spreading ridges and subduction at trenches, could account for resurfacing. Because the last resurfacing event took 100 Ma to complete, the average surface age during this time must have been 50 Ma. This age corresponds to the average age of the oceanic lithosphere on Earth. The lack of evidence for present-day plate tectonics on Venus does not mean that plate tectonics could not have existed before the event, or that plate tectonics could not have accounted for it. However, it is not clear whether plate motions could stop within the 100-Ma time scale required by the crater record.

The other suggested mechanism of resurfacing is plume generation. Because less than 20% of the planet has been resurfaced over the last 500 Ma, the global production rate must have been at least 25 times higher than the present-day rate for the duration of the event. Such a rate could be achieved by increasing the number of plumes, increasing the potential temperature by 200 degrees Celsius, reducing the lid thickness, or reducing the solidus temperature. However, it is not clear that any of these processes could produce a resurfacing event over 100 Ma. Plume activity is not sufficient to explain the style of resurfacing required by the crater record. Therefore, the true mechanism for periodic resurfacing events on Venus is not yet evident and remains up for debate. Venus might exhibit similar behavior in the future, but there is no guarantee that it will occur within our human lifetime.

Sources / references

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