Tectonics is the study of the deformation of planetary crust and the origin of large-scale planetary features (Hamblin et al. 51). Understanding the tectonic activity of an body in the Solar System provides a framework that integrates the myriad geologic phenomena, occurring on the surface and operating inside that body, into a unified model. Fundamental details about the evolution and structure about the body are inferred and predicted. Tectonics provides a global view of a planet, a holistic model that ties together internal processes with the surface.
The surface of the Earth is divided into two major provinces, the light silicate continental crust and the denser, basaltic oceanic crust. The older continental crust covers about 40% of the surface, some of which is overlain by shallow seas. The rema inder is oceanic crust, relatively young and entirely under water (Hamblin et al. 13).
The continental crust has several domains. The first is the shield, vast eroded plains of intricately folded metamorphic rock with igneous intrusions. These surfaces are of extremely ancient origin, typically over a billion years old. The shields are regi ons of very low relief and practically featureless on a regional scale (Hamblin et al. 591). Some examples are the Canadian Shield in the northwest region of North America and much of Australia.
Similar to the shields are the stable platforms, essentially shield structures overlain by sedimentary rock. The sediments are typically limestones, shales, and sandstones deposited under water. Currently about 11% of the continental crust is under water (forming continental shelves and shallow seas), but this percentage has varied greatly in the past, according to the sedimentary evidence left in the stable platforms (Hamblin et al. 595). The platforms are often locally warped into broad basins and domes . The Great Plains of North America are a typical example. The Grand Canyon in Arizona is a region where erosion has cut down through the sedimentary layers to reveal the basement shield complex below (Hamblin et al. 592).
The third major continental regions are the active mountain belts, complexly folded and deformed structures of extremely high relief near the continental margins (Hamblin et al. 595). Formed by the convergence of two crustal plates, active ranges coincide with zones of active seismicity and andesitic (i.e. explosive) volcanism (Hamblin et al. 548). The Pacific Ring of Fire is the major example of this sort of feature, bounded by some of the highest peaks in the world and marked by some of the most violent eruptions and earthquakes in recent history (Hamblin et al, 529).
Oceanic crust is primarily basaltic (3.2 g/cm3) in composition, denser than the more silicate rocks of the continents (2.7g/cm3) but still not as dense as the peridotite of the Earth's mantle (Hamblin et al. 98). The basins are relatively undeformed, diff ering from the complex structure of the shields and stable platforms, and are also much younger, only several hundred million years old (Hamblin et al. 578).
Among the features of the oceanic crust are subduction zones, regions where the lithosphere descends back into the mantle at a convergence of two plates (Hamblin et al. 570). The result is a deep trench, which as noticed before correspond to margins of th e continents and intense volcanism and seismicity as well as active mountain belts on the continents.
Spreading centers, regions where new oceanic crust is created by upwelling mantle material at a divergent plate margin, are also features of note on the oceanic crust. The main result of this is the Oceanic Ridge, an enormous mountain range that threads t he ocean basins for thousands of miles. A broad, fractured swell, it covers 23% of the Earth's surface and is a purely extensional fault feature, unlike the compressional folded structure of the continental mountains (Hamblin et al. 566).
Hot spots, the surface interaction with rising deep-mantle plumes, occur on both crustal types. On the continents they take on the form of extruded basaltic plateaus and rifting (like Africa's Rift Valley). The oceanic equivalents are large shield volcano arcs and similar flood basalt plateaus (Hamblin et al. 540).
The surface of Venus is similarly divided into provinces. The highly deformed and complex highlands cover about 15% of the surface of the planet, whereas the low-relief lowlands account for 60%. A transitional form of surface, the uplands, comprise th e remainder of the planet, predominantly extensional in character, typified by extensional features such as rifts as well as volcanic activity (Christiansen et al, 202).
The highlands are platforms that rise as much as 5 km above the plains, with mountainous areas that extend upwards for as much as 11km. In addition to the folded mountains there are chaotically fractured areas called tesserae comprised of interconnected t roughs and ridges. The features of the highlands are both extensional and compressional in nature, indicating complex lateral movement of the crust. The two predominant examples are Ishtar Terra near the north pole and Aphrodite Terra near the equator (Ch ristiansen et al. 205).
The lowlands are quite dull in comparison, with local relief never varying by more than 500 meters. Long, sinuous ridge belts snake across the plains, essentially low-relief folds, and the near-total absence of impact craters suggest a relatively new surf ace. Again, the dominant features are compressional in nature, indicating some type of lateral crust movement. The largest of the lowland regions is Atalanta Planitia, a Gulf of Mexico-sized region near the north pole, adjacent to Ishtar Terra (Christians en et al. 202).
The uplands, however, are where the truly intriguing features are. Of intermediate height (2 km or so above the plains), the features here are primarily extensional in nature. Fracture belts are prevalent near the equatorial regions, with shallow grabens at the center (Christiansen et al. 229). Isolated domes and broad swells display fractures and rift valleys (Christiansen et al. 225).
Volcanic features are extremely prevalent, with a broad array of types. Coronae are concentric arrays of ridges and fractures surrounding a central plain, with an average diameter of 250 km (Christiansen et al. 238). Large shield volcanoes and massive ca lderas (100-1000 km across) are concentrated in the uplands, though they do also appear in the highland regions. Lava domes are flat-topped steep-sided domes with fractures cutting across them, about 500 meters high and from 10 to 100 km across (Christian sen et al. 239). Called "pancake" domes due to their shape, they may be comprised of more granitic magma, for basaltic lava is not viscous enough to maintain the form (similar to rhyolite domes on Earth), suggesting some true "continental&q;uot; crust may exist on Venus (Christiansen et al. 240).
Venus and Earth have relatively similar orbits (.7 AU and 1.0 AU respectively) and so condensed out of similar regions of the primordial solar nebula. Both planets have similar proportions of metals (iron, nickel), silicates (olivine, pyroxene) and fe ldspars (CA, K, Na). Volatiles and ices are not as abundant at this distance from the sun (Christiansen et al. 28). As a result of their similar compositions, Venus and Earth have similar densities, 5.24 g/cm3 and 5.52 g/cm3 respectively (Christian sen et al. 209).
Due to the similar size and masses of the two planets, they have very similar internal structure. The internal differentiation is a crucial aspect of a planet's geology, driving the tectonic processes (Christiansen et al. 30). Differing chemical affin ities and densities of the constituents of the planet caused the separation into layers (Christiansen et al. 468).
The heaviest components, the metals, sank to the center of the planet due to gravity's pull and higher densities. Thus an iron-nickel core formed, releasing gravitational potential energy in the form of heat that further melted the interior, inducing furt her differentiation (Christiansen et al. 31). The initial heat required to start this process originated from the accretionary heat of formation by meteorite impacts (Christiansen et al. 32).
The surrounding mantle, comprised primarily of olivine-rich rocks like peridotites, has enough radioactive materials within to be heated radiogenically by decay (Christiansen et al. 32). The resulting temperature gradient induces convection to dissipate h eat outwards. This convection is the source of deep-mantle plumes that directly affect surface features on both planets (Christiansen et al. 35). The convection cells may encompass the entire mantle or may be layered with both deep-mantle and upper-mantle convective regimes (Hamblin et al. 490).
The surface of the planet cools fastest after the accretionary, core formation, and radiogenic heat sources. The silicate and mafic (i.e. basaltic) constituents are concentrated in the outer layer due to their lower densities (Christiansen et al, 39). A s ilicate "scum" forms at the surface, analogous to the thin layer that forms on the surface of cooling milk. As this layer cools, it thickens and becomes the lithosphere, the lighter, less dense, brittle crust of the planet.
Both Earth and Venus are seen to have two major surface domains. The highlands are thicker regions of the crust with complex structure and high relief. The lowlands, regions of relatively smooth relief, cover far more surface area. We have no information as to composition of the highland regions, but at first glance the highland plateaus of Venus certainly seem analogous to the continents of Earth. The Venusian lowlands and Terran basins have similar morphology and composition.
However the differences are quite noticeable. Venusian highland and uplands contain far more diverse array of volcanic features. The lowlands of Venus display compressional features whereas the character of the oceanic basins on Earth are primarily extens ional. Also, Venus has upland regions which are not found on Earth.
All volcanoes on Venus are basaltic, formed from hot spot activity. Volcanic features are broadly distributed over the surface of Venus, compared to their concentration along plate margins on Earth. The Venusian distribution is far from random, however. V olcanic activity occurs more often in the upland areas as opposed to the highlands or lowlands (Christiansen et al. 244). The distribution of basaltic volcanism on Earth, however, is broadly distributed across both oceanic and continental crust.
The outermost physical layer of the Earth is the lithosphere, cool and rigid compared to the inner layers. The lithosphere is rather thin in comparison to other planets, 50 km thick under continental crust and only 8 km thick under the oceanic basins (Christiansen et al. 256). Combined with the low temperatures at the surface, the rock of the lithosphere is brittle and thus yields to stress by breaking. As a result, the lithosphere is broken into crustal plates, each acting as independent mechanical u nits, that are carried by the motions of the aesthenosphere below and interact. Lighter continental crust resists assimilation back into the mantle, but the denser oceanic crust is eventually recycled. It is these crustal plates that make plate tectonics a reality (Christiansen et al. 255).
The aesthenosphere is a structurally weak zone of the upper mantle where the temperatures and pressures are just right for partial melting of the constituent rocks (Hamblin et al. 24). The boundaries are determined by mechanical, not chemical differen ces, by analyzing speed of seismic waves, and so actually spans different chemical composition regimes (Hamblin et al. 519). The behavior of this zone is essentially plastic, allowing ductile flow of the material. The result is a decoupling of the lithosp here from the motions of the lower mantle (Christiansen et al. 255). Without the aesthenosphere, plate motions would grind to a halt, as there would be no lubricating layer to enable their movement across the surface.
The mesosphere is the region of the mantle below the aesthenosphere, where the bulk of the planet Earth is contained (82% by volume, 68% by mass), chiefly comprised of dense basaltic rocks such as peridotite. (Christiansen et al. 254). As noted earlie r, radiogenic heating from this region is the primary internal energy source of the planet, driving the convection cells that churn the aesthenosphere, and through it the tectonic plates.
The exact method of coupling between the surface tectonics and mantle convection is unknown at present. The motions of the plates and the convection of the mantle are thought to be pieces of a single, complex system, but theories to explain the mechanics of such a system are sketchy. The only clues available derive from surface observations and geophysical & geochemical studies of extruded igneous rock (Hamblin et al. 491).
One possible convective model involves full-scale convection throughout the entire mantle, with cells that encompass both aesthenosphere and mesosphere. The aesthenosphere would thus be directly coupled to the mesosphere and the motions of the plates woul d be directly dependent upon them. Spreading centers and subduction zones would then be located exclusively at upwellings and downwellings respectively of the convection cells (Hamblin et al. 491).
Another theory maintains that the aesthenosphere and mesosphere have separate convective systems. In this view, the plates ride upon the convection cells of the aesthenosphere and part of the upper mantle. Seismic activity has been used to detect lithosph eric slabs descending as much as 700 km into the mantle, so a significant portion of the upper mesosphere must be involved in this upper convective system. Below this regime is the deeper mantle convection, thought to be far slower and totally independent of the motions of the upper mesosphere. Material probably doesn't transfer across the boundary, but heat readily flows outward (Hamblin et al. 491).
In either case, phenomena known as mantle plumes can occur and are observed. Thought to originate from the deep mantle, these plumes take the form of cylindrical columns that are long-lived and independent of the plate motion above them. It is these plume s (hotspots) that cause the aforementioned flood basalts and shield volcanoes found on both continental and oceanic crusts (Hamblin et al. 492).
One possible origin for these plumes may be an "avalanche" of lithospheric slabs of oceanic crust. As slabs get subducted into the mantle, the accumulate at the 670 km seismic wave velocity discontinuity in the upper mesosphere, the boundary bet ween aesthenosphere and mesosphere as determined by seismic data. Eventually, enough slabs accumulate to break through the barrier and penetrate down to the core/mantle boundary. This avalanche may trigger an upward counterflow of plume material (Hamblin et al. 523).
All these mantle motions are essentially thermodynamic responses to the internal temperature gradient of the planet. Once the mantle convection and plumes have transported the radiogenic heat to the surface, it escapes mostly via sea-floor spreading and t he subsequent cooling & formation of new oceanic crust. A smaller percentage of the heat escapes from hotspots above mantle plumes (Christiansen et al. 247).
The first crust of the Earth was cooled basaltic "scum" that floated to the surface after the initial accretion and internal differentiation of the planet's interior. This early crust was most likely unstable until after the bombardment of t he Solar System by planetesimals (left over by the condensation of the primordial solar nebula) had ceased (about 4 b.y.a). Also, vigorous convection of the young hot mantle probably recycled crust as soon as it was formed (Christiansen et al. 307). Again , an excellent analogy is the skin that forms on heated milk, a thin layer of cooler and less dense impurities left over from the chemical differentiation below.
Convection cells and hot spots probably induced partial melting of some regions of the primordial crust. Silicate magmas were formed from the basaltic crust, which then due to lower density were able to resist reassmilation into the mantle. This is the su ggested scenario for the nuclei of the continents, but of course we are far from a total understanding (Christiansen et al. 310).
By erosion of these pre-continents, sediments were derived, which were then concentrated and exposed to further partial melting, giving rise to true granitic continental crust. Again, the lower density enabled these first continents to avoid destruction i n the mantle (Christiansen et al. 310).
New continental crust is formed today by the partial melting induced by descending plates at subduction zones. (Hamblin et al. 616). The overlying crust becomes heavily compressed and results in active mountain belts. These mountains eventually erode flat , forming the shields, and their sediment covers the shields to make stable platforms.
New oceanic crust is extruded at the spreading centers of the sea floor, presumably at upwellings of convection cells in the aesthenosphere. The mid-oceanic ridge is essentially the surface expression of this process. Oceanic crust has a short life (sever al hundred m.y.a.) of lateral movement before being subducted back into the mantle (Hamblin et al. 587). Trenches form at the subduction zones of these converging plate boundaries. The melting of the descending slabs provides enriched material to aid in o rogenesis, and may as noted earlier be responsible for mantle plumes.
The mantle of Venus is probably very similar to that of Earth's, with similar density and composition. As a result, the radiogenic heating is comparable, creating a temperature gradient high enough to require convective transport of heat outwards. Even le ss is known about the convective system of the Venusian mantle than that of Earth's, but there is reasonable certainty that convection is dominated by deep mantle plumes, based on observations of surface features (Christiansen et al. 245).
There is one major difference between the internal structures of Venus and Earth, however, directly related to the amount of water available. Venus may have begun its history with no water at all due to its closer proximity to the sun, reducing the volati les in that region of the solar nebula. Or the planet may have had water at some point, but lost it due to solar radiation. The hydrogen would have escaped into space while the oxygen reacted with the surface to form oxides. Either way, the available wate r on Venus is only a trace, confined to the upper atmosphere (Christiansen et al. 245).
Water plays a role in tectonic processes, as even a small relative amount (say 1%) in rock can lower the melting point by as much as 200 to 600 OK. Water can also increase the fluidity and mobility of mantle rock. As a result, temperatures in the upper ma ntle were never high enough to allow a partially melted aesthenosphere to form, cementing the lithosphere in place (Christiansen et al. 245).
Therefore, the lithosphere of Venus is directly coupled to the mantle, specifically the upwelling and downwelling of the mantle plumes. The high temperature and pressure at the Venusian surface (750 OK and 90 atm) causes the crust to be weak and ductile, so the overall picture is a plastic crust directly acted upon by mantle processes (Christiansen et al. 245). Plate tectonics, requiring brittle crust decoupled from the mantle, is not possible on Venus. The crust cannot form seperate, mechanically distinc t plates, and even if they did exist they would be unable to move freely without the aesthenosphere to float upon. The internal radiogenic heat driving the planet's convection is thus forced to escape via increased volcanism and surface swells and rifting (Christiansen et al. 247).
As noted earlier, the dominant characteristic of the highland regions are compressional and extensional features, specifically the folded mountain belts and chaotic tesserae. These can now be explained in terms of mantle plume downwelling (antiplumes). Th anks to viscous drag on the crust, as the antiplume builds (like a large water droplet on the underside of a flat surface) it drags the weak and ductile crust along with it, causing the lithosphere to pile high (into a folded mountain belt) directly above the site of the downward motion. After the antiplume has ceased, there is no longer any frictional drag to maintain the high relief , and the ductile, compressed crust relaxes laterally under the influence of gravity (Christiansen et al. 233). As the cru st relaxes from the compression, tesserae are formed by the extensional troughs cutting across the previous compressional ridges (Christiansen et al. 232).
Similarly to the highland regions, the lowlands also display compressional features. This is also thought to be related to downwelling in the mantle. Ridge belts that trundle through the plains are thought to be formed by these "cold spots." Th is hypothesis explains why we see no hot spot features like coronae or shield volcanoes in these regions of Venus, as well as why there are no extension features at all (Christiansen et al. 231).
As we have seen, the uplands are primarily large domes and rises cut by rift valleys and dotted with volcanic features. In sharp contrast to the lowlands, the features are all extensional in character. The current picture points to mantle plumes are the p rimary source of this activity, with different size plumes resulting in different types of phenomena (Christiansen et al. 242).
The large domes and rises are caused by large mantle plume heads, with associated features like rift valleys caused by the uplift and extensional cooling (Christansen et al. 225). Buoyant plume heads strain the surface outwards creating the initial rise; secondary, possibly multiple upwellings from the mantle plume head form the volcanoes, uplift, and other varied volcanic features (Senske et al. 1992). The volcanic activity is directly tied to the size of the upwelling plume, with the smaller plumes caus ing regular volcanoes, coronae, pancake domes, etc. and the larger ones responsible for domes and volcanic rises (Christiansen et al. pg. 242).
Coronae are especially interesting features, with no terrestrial analogue. They are essentially ring-shaped mountain ranges surrounding a low plain, encircled by concentric fractures and rifts. Rising mantle plumes first uplift the entire region, impartin g a circular shape as the mantle plume spreads radially outwards beneath the crust. As the plume cools and subsides, the central plateau sags and the outer sharp ring becomes defined but as the plume cools the central support for the plateau disappears, a nd so the inner portion of the dome relaxes gravitationally, forming the central depression, outer rim, and "moat" along the outside. Concentric fractures develop as the lithosphere is extended while bending. (Sandwell et al. 1992).
Coronae are seen in highland regions as well as the uplands. The highlands are a region of extreme downwelling and thus we would not expect upwelling-related features such as coronae to appear. One possibility is secondary and multiple plumes rising upwar ds from the downward flow, as described by Senske et al. (1992). The dynamics of the mantle plume interactions with the lithosphere are still not completely understood.
The surface of Venus is indeed molded by tectonic processes, driven by forces similar to those of Earth (hardly surprising, given the similar density, size, and composition of the two planets). However, the lack of water on Venus has inhibited th e formation of a plastic aesthenosphere to decouple the lithosphere from the mantle. Also, the high temperature and pressure at the surface cause the rock to be ductile in response to stress. As a result, a system of cool, brittle crustal plates moving th rough a plastic aesthenosphere in response to convection does not occur on Venus.
Instead, mantle-plume tectonics dominate, where large cylindrical units of buoyant rock rise upwards and directly affect the crust. The size of these plumes determines the type of feature that will occur on the surface in response. Secondary upwellings fr om the mantle plume may also affect the variety of coronae, arachnoids, ticks, pancake domes, and of course volcanoes of all sizes that are produced.
The downwelling that accompanies the rising plumes exerts a direct drag on the crust, producing lowlands with long compression ridges. As the downwelling grows even more extreme, the crust may be piled up and compressed, causing the crust to thicken and f orming a highland domain (Bindschadler et al. 1992). After the downwelling has ceased, gravitational relaxation causes extension of the crust, creating extensional features such as tesserae and fracture belts.
Most of the data on Venus used to derive these understandings of the planet comes from the Magellan mission to Venus by NASA during 1990-1993. The Journal of Geophysical Research published two exclusive volumes devoted to Magellan research, vol. 97 E8 and E10. The textbook Exploring the Planets by Eric Christiansen and W. Kenneth Hamblin (Prentice-Hall, 1995) nicely summarized the tectonic and volcanic discoveries found by the Magellan researchers, so references have been ma de with respect to that book instead of the individual contributors when possible. The other volume used in this paper was Earth's Dynamic Systems by W. Kenneth Hamblin and Eric Christiansen (Prentice-Hall, 1995). To distinguish between these two t exts, written by the same authors, published in the same year by the same company (!) Exploring the Planets was referred to as Christiansen et al. and Earth's Dynamic Systems was referred to as Hamblin et al.
Christiansen, Eric H., and Hamblin, W. Kenneth. 1995. Exploring the Planets. Englewood Cliffs: Prentice-Hall.
Hamblin, W. Kenneth, and Christiansen, Eric H. 1995. Earth's Dynamic Systems. Englewood Cliffs: Prentice-Hall.
Sandwell, D.T., and Schubert, G., Ridges, Trenches, and Outer Rises Around Coronae on Venus, J. Geophys Res., 97, E10, 1992.
Aubele, J. C., Crumpler, L. S., Head, J. W., Guest, J. E., Saunders, R. S., Venus Volcanism, J. Geophys Res., 97, E8, 1992.
Senske, D. A., Schaber, G. G., Stofan, E. R., Regional Topographic Rises on Venus, J. Geophys Res., 97, E8, 1992.
Bindschadler, D. L., Kaula, W. M., Schubert, G., Global Tectonics and Mantle Dynamics of Venus, J. Geophys Res., 97, E8, 1992.