Volcanic structures at Venus

Transcription

1 Volcanic structures at Venus TU Bergakademie Freiberg Abstract. This paper tries to figure out the context between magma vis cositys and the observed volcano structures. To create this paper, different papers from many authors were exploit. It was tried to make a general view which comprises the surface conditions, the supposed chemical com position, the structure of Venus and the magma composition. For a better understanding, the paper is divided into two parts. In the first part I try to figure out some typical volcanic structures. At second part I try to show the connection between volcanic structures and the viscosity of their forming magmas and lavas. Introduction. Venus earth little sister? Venus is in our solar system earth most similar. Her radius is approximately 95% of earth radius, her mass is approximately 80% earth mass. The chemical composition and also the density is almost equal. The most serious differences are: the high surface pressure (90bar), the high surface temperature (460 C) and the absence of plate tectonic.

2 2 Typical volcano structures at Venus The most part off the Venus surface is build off rolling hills with light ele vations. There are two huge mountain ranges, Ishtar Terra, at northern he misphere and Aphrodite Terra along the equator. Both are interpreted as an equivalent to early kontinental formations. (Bonin et al., 2001). The sur face of Venus probably is formed of basaltic rocks. Geochemical data, col lected by russian Venera and Vega missions, support this predication. Due to the flat, smooth surface and the relatively small number of impact cra ters, it is supposed that there was a fatal volcanic event wich was resurfa ceing the whole Venus. On the basis of impact crater counting, it is suppo sed, that event before 500 ma took place. (Bonin et al., 2001). Newer stu dies suppose that there was not one catastrophically event, but rather a long period of volcanic activities (Guest and Stofan, 1999; Hansen, 2005, 2007) Lava flows: Lava flows extend from some kilometers to several hundred kilometers. Their origin can be a volcano, a crack in the crust or a depression at sur face. Most of the sources aren t visible. Mylitta Fluctus a complex of 6 flow fields at Venus southern hemisphere is similar to the Columbia River flood basalt province at earth (Roberts, 1992). Each flood field ist created by numerous single floods. Most of the floods are several hundred kilome ters long and relatively flat (only few kilometers). It is assumed, that flood fields commonly have basaltic composition (Campbell, 1992).

3 Volcanic structures at Venus 3 Lava Channels: Lava channels can reach length from several hundred to thousand kilome ters. They form noticeable lines in the venusian plains. Simple channels normally aren t dentritical. They form long sinuous forms, called Canali and sinuous rilles. Canali are best preserved, in regions containing a flat relief. They have a high wide to depth ratio, which doesn t change over a long distance. Radar pictures show the presence of meanders, point bars and cut banks (Hamilton, 2001). Most of the sources and ends of the canali are covered by lava flows. The se flows are much younger than these, wich formed the canali (Parker, 1992). Kargel et al. (1994) compares the appearance from canalis with ter restrial rivers and fluviatile landforms. Small Volcanoes: Volcanoes at Venus are classified and subdivided on base of their morpho logy and size (Head et al., 1992). Volcanoes with a diameter less them 20 kilometers are classified as small. Normally they where find at flat plains, but they can also be developed at the flanks of larger volcanoes and in as sociation with Coronae and Arachnoids. Small volcanoes consist of shields, cones and sometimes also domes (Head et al., 1992). Small shields are circular or elongated. They have shallow slopes and aren t associated with flow deposits. Most of the small shields are identi fied by their soft contours and their dark radar colouring. Some shields also have diffuse contours with a central circular summit pit with a diame ter less then one kilometer. Along linear fracture belts at plains, clusters of small shield volcanoes are common.

4 4 Cones reach highs from 200 to m, usually they have steep slopes from 12 to 23. Individual flows aren t visual separable. Cones often can find as clusters at plains (Hamilton, 2001). Intermediate Volcanoes: Intermediate volcanoes have diameters between 20 to 100 kilometers. They consist of symmetrical shields, which were characterised by radial fractures and lava flows. Domes reach highs between 70 to 2000 m, they have diameters from less then 10 to 100 kilometers. They are usually surrounded by steep perime ters and have a relatively flat top. Usually they are pretty round formati ons. Their surface is darker then he surrounding material, this means that they have a rough surface (Pavri, 1992). Small craters are a common fea ture of the surfaces of all domes. Domes occur singly, in pairs, groups, or in overlapping clusters. Many are associated with Coronae, but the erupti ve mechanism is not clearly understood. Pavri (1992) supposed that they where formed by viscosed lava erupting uniformly from a central vent. Petford (2000) assumed that the lavas, forming pancake domes, had a rhyolitic (trondhjemitic) composition. Large Volcanoes: Large volcanoes reach dimensions from 100 to 600 kilometers. They whe re dominated by a positiv topography and radial lava flows. They occur at broad rises, tectonic junctions and higher elevations (Hamilton, 2001).

5 Volcanic structures at Venus 5 Calderas: Calderas are defined as circular to elongate depressions, which where not associated to well formed edifices. They show characteristically concentric patterns of surrounding fractures (Head et al., 1992). Coronae: Coronae are circular to elliptical structures with a diameter from 300 to 2600 kilometers. In opposite to impact craters, their center area is several hundred meters higher then the neighborhood. It is surrounded by up to 2 kilometer high ring walls. It is supposed that they are a result of Hot Spot equal structures at asthenosphere (Ernst & Desnoyers, 2003). Connection between volcanic structures and magma / lava viscosity To interpret the different volcanic structures at Venus it is necessary to in volve multiple factors like petrology and the viscosity of magma, the loca tion of the magma chamber and the width of extrusion ways and also the topography and the pressure and temperature at the venusian surface. Un fortunately there are only very few data from Venus. The most important are the radar map from the Magellan mission with a resolution from 100m / pixel, and XRF analyses as well as γ-ray spectroscopy by russian Venera missions. Due to the relatively light data, the presented models and explanations depend on similes from terrestrial appearances, laboratory ex periments and model calculations.

6 6 By cosmological reasons, it is certain, that the geochemical composition of Venus is pretty equal to Earth geochemical composition. The structure of Venus resembles earth structure. Venus has a Iron Nickel rich core, a mantle composed of magnesium-rich silicates and oxides and a basaltic crust (Stevenson, 2002). Due to the absent, better the very weak magnet field, it can be assumed that the core of Venus is cooled down and solified a long time ago (Stevenson, 2002). Fig. 1: Venus cutaway from core to crust. Modified from

7 Volcanic structures at Venus On the basis of the exterior aspect of the venusian surface at radar maps and sparsely geochemical data, it is to assume that the bulk of the surface is made of basaltic rocks. The data, accumulated by Venera 13 & 14 sup pose a composition analog tholeitic basalt. If it is assumed by terrestrial observations and add the high pressure and und temperature it is probably that the most lavas had low viscosity attributes. Lava channels, called canali, could formed by exotic carbonates and carbonate sulfate rich li quids (Kargel et al., 1994). Fig. 2: Venus Sinuous Channel, modified from Nasa JPL PIA

8 8 Older publications assume that at Venus no magma differentiation took place and so there shouldn t be a significant amounts of SiO² rich rocks. On the other Hand Petford (2000) showed, that the so called Pancake do mes could interpreted through forming by high viscosity melts. After his idea, the forming rocks could be venusian analogues to terrestrial trondh jemites and adakites. Fig. 3: Overlapping Pancake Domes, Venus Alpha Regio, modified from Nasa JPL PIA00215.

9 Volcanic structures at Venus 9 By his calculations, he realised that "melt viscosity appears to be more important than either density contrast or the local temperature field in controlling dyke ascend velocities and magma transport rates" (Petford, 2000) He estimated a dike width of less then 3 m and minimum ascend velocity of m/s. In his calculations he used a fixed melt viscosity of 105 Pa s. Petford (2000) find out that domes commonly associated with coronae. After his opinion coronae could be formed by larger upwelling of magma. Fig. 4: Artemis coronae, 2100 km diameter, modified from Nasa JPL PIA00101.

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