Figure 8 - uploaded by James Head
Content may be subject to copyright.
Tectonic deformation in the mountains of Venus. In this view of a huge dome in the Freya Montes region of Ishtar Terra, numerous tectonic features are testimony to the intense deformation accompanying the creation of this and adjacent tessera terrain. Ringing the dome to the east and west are broad folds caused by shortening and contraction. On top of the dome are seen a set of intersecting extensional structures (graben) indicating that the dome underwent stretching and collapse. Width of the image is about 75 km. NASA Magellan radar image.
Source publication
During the latter part of the last century, a profound change took place in our perception of the Earth. First, this change was holistic: Plate tectonic theory provided a unifying theme that seems to explain disparate observations about the Earth and how it works, and lets us see the Earth as a planet. Secondly, actually seeing the Earth from the M...
Context in source publication
Context 1
... heat very efficiently. This, together with their small diameters, results in their lithospheres becoming a relatively large percentage of their radii early in history. It is then extremely hard to start the subduction that apparently resulted in plate tectonics on Earth. Breaking a thick rigid layer and pushing it into the interior on a small planet is not easy. But what about Venus, the most Earth-like of the planets in terms of its size, density, and position in the Solar System? Does Venus have plate tectonics? Exploration of Venus was motivated by just such questions and following numerous missions by the Soviet Union and the US, the Magellan mission obtained global high-resolution radar images in the 1990s. These spectacular images (Figure 8) revealed mountain ranges, rift zones, and an extremely young surface geologically (Figure 3), general properties that were very similar to the Earth and its plate tectonic system. But most surprisingly, there was no supporting evidence for ...
Similar publications
We investigate the conditions that will promote explosive volcanic activity on Venus. Conduit processes were simulated using a steady-state, isothermal, homogeneous flow model in tandem with a degassing model. The response of exit pressure, exit velocity, and degree of volatile exsolution was explored over a range of volatile concentrations (H2O an...
We examine the morphology and chemistry of the Vikrahraun basaltic eruption emplaced at Askja Volcano, Iceland, from Oct. 26–Dec. 17, 1961. The eruption had three eruptive events, initiating with aʻa and followed by alternating aʻa and pahoehoe lava flow emplacement. We determine that while the eruption is chemically homogenous (Fe/Mg = 1.9–2.2, 47...
We have used carefully controlled laboratory simulations to develop a model which relates lava flow morphology to effusion rate and rheology. Through comparisons with measured and estimated eruption rates on Earth, this approach allows us to constrain eruptive styles and compositions of extraterrestrial lava flows. By applying this model to lava fl...
The submarine eruption of Tagoro volcano in the cicinity of El Hierro island in 2011 renewd an exceptional exceptional opportunity to study in situ and to a short timescale the process of recolonization and recovery of an ecosystem recently destroyed by the massive emissions of CO 2 and H 2 S with the accompanying features of great increase in wate...
Paterae, defined by the International Astronomical Union as ``irregular crater[s], or complex one[s] with scalloped edges,'' are some of the most prominent topographic features on Io. Paterae on Io are unique, yet in some aspects they resemble calderas known and studied on Earth, Mars, and Venus. They have steep walls, flat floors, and arcuate marg...
Citations
... In this respect, impact cratering has been an ongoing and recurring geological process throughout the history of the planets (including Earth), that operates at many scales and has substantial geological, environmental, biological consequences, and can even force large geodynamic events (Head, 2001). As we argue in this communication, impact processes may trigger an unstoppable chain of events in cascade leading ultimately to the development a full scale metallogenic scenario, in this case, the Mid-Cretaceous Chilean Iron belt. ...
A cascade of tectonic and magmatic events that took place in the Pacific and northern Chile during Mid-Cretaceous led to formation of one of the World's largest Kiruna-type iron belts with reserves of ~2000 Mt (60% Fe). Geological evidence indicates that a major change occurred in Mid-Cretaceous time, when superplume emplacement and plate reorganization processes took place in the Pacific. Although this scenario is well documented, no proposals have been put forward on the actual event that may have triggered the subsequent cascade effect. In this regard, a large meteoritic impact in the Pacific may have initiated massive volcanism leading to formation of the Ontong-Java Plateau. Such an impact may have been the geologic equivalent to the falling domino principle: once the first piece is knocked over, the rest fall quickly. Thus, once the meteorite impacted the Mid-Pacific, subsequent and sequential plume emplacement, massive volcanism, plate reorganization, increased plate stress along the Pacific margin, fault zone formation and emplacement of the Chilean Iron Belt would have taken place within a relatively short time span in Mid-Cretaceous time.
... If so what are these processes and how did their ''Wilson cycles'' work? Are there any broad themes that emerge in the geological evolution of Venus (e.g., Solomon and Head, 1982), such as those seen on the Earth, Moon, Mars, and Mercury (e.g., Head and Solomon, 1981;Head, 2001)? If so, what are these themes and how did they change with time? ...
The surface area of Venus (∼460×106km2) is ∼90% of that of the Earth. Using Magellan radar image and altimetry data, supplemented by Venera-15/16 radar images, we compiled a global geologic map of Venus at a scale of 1:10M. We outline the history of geological mapping of the Earth and planets to illustrate the importance of utilizing the dual stratigraphic classification approach to geological mapping. Using this established approach, we identify 13 distinctive units on the surface of Venus and a series of structures and related features. We present the history and evolution of the definition and characterization of these units, explore and assess alternate methods and approaches that have been suggested, and trace the sequence of mapping from small areas to regional and global scales. We outline the specific defining nature and characteristics of these units, map their distribution, and assess their stratigraphic relationships. On the basis of these data, we then compare local and regional stratigraphic columns and compile a global stratigraphic column, defining rock-stratigraphic units, time-stratigraphic units, and geological time units. We use superposed craters, stratigraphic relationships and impact crater parabola degradation to assess the geologic time represented by the global stratigraphic column. Using the characteristics of these units, we interpret the geological processes that were responsible for their formation. On the basis of unit superposition and stratigraphic relationships, we interpret the sequence of events and processes recorded in the global stratigraphic column. The earliest part of the history of Venus (Pre-Fortunian) predates the observed surface geological features and units, although remnants may exist in the form of deformed rocks and minerals. We find that the observable geological history of Venus can be subdivided into three distinctive phases. The earlier phase (Fortunian Period, its lower stratigraphic boundary cannot be determined with the available data sets) involved intense deformation and building of regions of thicker crust (tessera). This was followed by the Guineverian Period. Distributed deformed plains, mountain belts, and regional interconnected groove belts characterize the first part and the vast majority of coronae began to form during this time. The second part of the Guineverian Period involved global emplacement of vast and mildly deformed plains of volcanic origin. A period of global wrinkle ridge formation largely followed the emplacement of these plains. The third phase (Atlian Period) involved the formation of prominent rift zones and fields of lava flows unmodified by wrinkle ridges that are often associated with large shield volcanoes and, in places, with earlier-formed coronae. Atlian volcanism may continue to the present. About 70% of the exposed surface of Venus was resurfaced during the Guineverian Period and only about 16% during the Atlian Period. Estimates of model absolute ages suggest that the Atlian Period was about twice as long as the Guineverian and, thus, characterized by significantly reduced rates of volcanism and tectonism. The three major phases of activity documented in the global stratigraphy and geological map, and their interpreted temporal relations, provide a basis for assessing the geodynamical processes operating earlier in Venus history that led to the preserved record.
... If so what are these processes and how did their ''Wilson cycles'' work? Are there any broad themes that emerge in the geological evolution of Venus (e.g., Solomon and Head, 1982), such as those seen on the Earth, Moon, Mars, and Mercury (e.g., Head and Solomon, 1981;Head, 2001)? If so, what are these themes and how did they change with time? ...
The recent wave of data on exoplanets lends support to METI ventures (Messaging to Extra-Terrestrial Intelligence), insofar as the more exoplanets we find, the more likely it is that “exominds” await our messages. Yet, despite these astronomical advances, there are presently no well-confirmed tests against which to check the design of interstellar messages. In the meantime, the best we can do is distance ourselves from terracentric assumptions. There is no reason, for example, to assume that all inferential abilities are language-like. With that in mind, I argue that logical reasoning does not have to be couched in symbolic notation. In diagrammatic reasoning, inferences are underwritten, not by rules, but by transformations of self-same qualitative signs. I use the Existential Graphs of C. S. Peirce to show this. Since diagrams are less dependent on convention and might even be generalized to cover non-visual senses, I argue that METI researchers should add some form of diagrammatic representations to their repertoire. Doing so can shed light, not just on alien minds, but on the deepest structures of reasoning itself.
We utilize a theoretical analysis of the generation, ascent, intrusion and eruption of basaltic magma on the Moon to develop new insights into magma source depths, supply processes, transport and emplacement mechanisms via dike intrusions, and effusive and explosive eruptions. We make predictions about the intrusion and eruption processes and compare these with the range of observed styles of mare volcanism, and related features and deposits. Density contrasts between the bulk mantle and regions with a greater abundance of heat sources will cause larger heated regions to rise as buoyant melt-rich diapirs that generate partial melts that can undergo collection into magma source regions; diapirs rise to the base of the anorthositic crustal density trap (when the crust is thicker than the elastic lithosphere) or, later in history, to the base of the lithospheric rheological trap (when the thickening lithosphere exceeds the thickness of the crust). Residual diapiric buoyancy, and continued production and arrival of diapiric material, enhances melt volume and overpressurizes the source regions, producing sufficient stress to cause brittle deformation of the elastic part of the overlying lithosphere; a magma-filled crack initiates and propagates toward the surface as a convex upward, blade-shaped dike. The volume of magma released in a single event is likely to lie in the range 102 km3 to 103 km3, corresponding to dikes with widths of 40-100 m and both vertical and horizontal extents of 60-100 km, favoring eruption on the lunar nearside. Shallower magma sources produce dikes that are continuous from the source region to the surface, but deeper sources will propagate dikes that detach from the source region and ascend as discrete penny-shaped structures. As the Moon cools with time, the lithosphere thickens, source regions become less abundant, and rheological traps become increasingly deep; the state of stress in the lithosphere becomes increasingly contractional, inhibiting dike emplacement and surface eruptions.
