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"This volume contains a sizable suite of contributions dealing with regional impact records (Australia, Sweden), impact craters and impactites, early Archean impacts and geophysical characteristics of impact structures, shock metamorphic investigations, post-impact hydrothermalism, and structural geology and morphometry of impact structures - on Earth and Mars"--
On Earth, catastrophic impact of an asteroid or comet with truly global consequences has not happened during our written history, and the threat seems very small. Giant scars on our planet’s surface are relics of an impact history stretching back more than 2 billion years, and there is no assurance it cannot happen again. In Australia there are 36 structures ranging from tens of metres to tens of kilometres in diameter, and recognised to varying degrees of certainty as having been formed by giant meteorite impact. In clear and concise language this book begins with ancient beliefs and myths about craters and then explains how they are actually formed and provides details of their structure. Using the record in the rocks, the authors also assess the likelihood of future impacts and their possible effects.
This book presents a comprehensive overview of Australian impact structures and related mineralization, including a discussion of the significance of many of these structures for crustal evolution. The book focuses in particular on Archaean impact ejecta/fallout units in the Pilbara Craton of Western Australia, large exposed and buried impact structures, and on the geophysical evidence for possible to probable impact structures. Thanks to their long-term geological stability, Precambrian and younger terrains in the Australian continent contain 38 confirmed impact structures and 43 ring and dome structures, many of which constitute possible to probable asteroid impact structures. The impact structures have been the subject of more than half a century of studies and range from several tens of meter-large craters to buried structures larger than 100 km in diameter. Discoveries of impact fallout units in the Pilbara Craton have defined the Pilbara as one of the two best documented terrains where Archaean impact ejecta/fallout deposits are identified, the other terrain being the Kaapvaal Craton in southern Africa. A synthesis of evidence from both cratons indicates periods of large asteroid bombardments during ~3.47 – 2.48 billion years-ago, including peak bombardment about 3.25—3.22 billion years-ago. The latter period coincides with an abrupt transformation of an early Archaean granite-greenstone crust to mid to late Archaean semi-continental crustal regimes, underpinning the significance of heavy asteroid impact events for crustal evolution. Apart from proven impact structures, Australian terrains display a range of circular features, including morphological and drainage rings, circular lakes, volcanic craters, tectonic domes, oval granite bodies, mafic igneous plugs, salt diapirs, and magnetic, gravity and seismic anomalies, many of which are of a likely impact origin. Thermal and hydrothermal processes associated with impact cratering bear important consequences for the formation of mineral deposits, such as Ni at Sudbury, Pb-Zn at Siljan and Kentland. Impact structures may also provide sites for the accumulation of hydrocarbons, whereas in some instances fracturing associated with impact structures allows outward migration of oil and gas.
This book describes a search for geological evidence of meteorite impact structures in Britain. The statistics of impact structures indicate that Britain should have Phanerozoic impact structures up to tens of kilometres in diameter. A constant theme is the importance of atmospheric break-up of small asteroids and comets. These fragmenting bodies produce anomalously shallow craters with low rims and central peaks; three British structures of this type are identified.Analysis of fireball statistics implies that damaging fireball explosions occur over the British Isles on a time-scale of decades. On a time-scale of millennia, however, more damage is done by Atlantic impact tsunami.
When in 1981 Louis and Walter Alvarez, the father and son team, unearthed a tell-tale Iridium-rich sedimentary horizon at the 65 million years-old Cretaceous-Tertiary boundary at Gubbio, Italy, their find heralded a paradigm shift in the study of terrestrial evolution. Since the 1980s the discovery and study of asteroid impact ejecta in the oldest well-preserved terrains of Western Australia and South Africa, by Don Lowe, Gary Byerly, Bruce Simonson, Scott Hassler, the author and others, and the documentation of new exposed and buried impact structures in several continents, have led to a resurgence of the idea of the catastrophism theory of Cuvier, previously largely supplanted by the uniformitarian theory of Hutton and Lyell. Several mass extinction of species events are known to have occurred in temporal proximity to large asteroid impacts, global volcanic eruptions and continental splitting. Likely links are observed between asteroid clusters and the 580 Ma acritarch radiation, end-Devonian extinction, end-Triassic extinction and end-Jurassic extinction. New discoveries of ~3.5 – 3.2 Ga-old impact fallout units in South Africa have led Don Lowe and Gary Byerly to propose a protracted prolongation of the Late Heavy Bombardment (~3.95-3.85 Ga) in the Earth-Moon system. Given the difficulty in identifying asteroid impact ejecta units and buried impact structures, it is likely new discoveries of impact signatures are in store, which would further profoundly alter models of terrestrial evolution. .
Impact cratering is arguably the most ubiquitous geological process in the Solar System. It has played an important role in Earth’s history, shaping the geological landscape, affecting the evolution of life, and generating economic resources. However, it was only in the latter half of the 20th century that the importance of impact cratering as a geological process was recognized and only during the past couple of decades that the study of meteorite impact structures has moved into the mainstream. This book seeks to fill a critical gap in the literature by providing an overview text covering broad aspects of the impact cratering process and aimed at graduate students, professionals and researchers alike. It introduces readers to the threat and nature of impactors, the impact cratering process, the products, and the effects – both destructive and beneficial. A series of chapters on the various techniques used to study impact craters provide a foundation for anyone studying impact craters for the first time.
The technique of the mapping of planetary surfaces and the methods used for the identification of various planetary landforms improved much in the last 400 years. Until the 20th century, telescopic observers could interpret planetary landforms solely based on their appearance, while today various data sets acquired by space probes can be used for a more detailed analysis on the composition and origin of the surface features. Before the Greeks, the Earth and the Heavens were indisputably of different origin and nature. It was a major philosophical breakthrough - first appeared as an a priori theory, later based on observations - that the Heavens (planetary bodies) and the Earth share common features: gravity, composition and solar distance may be different, but the nature of the physical processes shaping the landforms are essentially the same. It has been a long way since we have arrived from the first telescopic description of lunar craters to the identification of various geological formations on Mars or on minor planets. Relief features of the Moon have first been observed by Galileo Galilee, via his telescope. During the next centuries, a multitude of Lunar landforms have been identified. Theories based on observations have been connected together by a scientific paradigm which explained their origin in a logical and seemingly undisputable manner. Telescopes showed a Lunar surface full of circular landforms, called craters, a landscape with no parallel on Earth. But the individual landforms had a morphological equivalent, volcanoes, which naturally led to the conclusion that craters had been created by volcanic processes. Maria ("seas") served as natural basins for water bodies. Observations clearly showed that water and air are hardly found on the Moon, the lack of clouds indicated the lack of precipitation. But the flat surface of the maria (obviously composed of marine sediments) and the meandering valleys suggested the presence of liquid water and a higher atmospheric pressure in the past - during the age of active volcanism and degassing. There were no observable active volcanic processes but some craters (though to be volcanoes) have been observed as being active: flashes of light - interpreted as eruptions - have been reported by several observers. The presence of pyroclasts thrown out from the volcanic vents of craters provided an independent evidence: meteor showers and individual meteorites falling from the sky - originating from Lunar craters. The logical and interconnected set of explanations based on observations proved to be completely false by the second half of the 20th century. The new paradigm interpreted the very same features in a new context. The case of Mars was different. There were no telescopes capable of observing relief forms (no shadows on Mars are visible from the Earth, because Mars always shows a nearly full Mars phase), so only albedo features could be seen and used for interpretation. The lack of visible relief features were interpreted as a lack of considerable topography: an unnoticed distortion in the observational data. The hue and contrast of dark and bright, orange, grey and white spots have changed seasonally, the polar areas clearly showed a polar cap made of ice and snow, but clouds have not been observed. Since Mars is farther away from the Sun than the Earth, it was evident that temperature values are lower there. Scientists concluded that Mars is an ancient, arid world. Then contemporary geology taught the theory according to which waters on the Earth are going to infiltrate underground in time, making the surface dry - observations showed that this had already happened on Mars. The last surface reservoirs of water were the polar caps. Some observers reported seeing a global network of linear features, but other have only seen very few of such albedo markings. These features were interpreted as "canals," made by a civilization for irrigation, carrying water from the poles to all around the flat plains of Mars. What was observable from the Earth were the broad stripes of irrigated vegetation (like those along the Nile), the canals themselves were too narrow to be visible from here. All theories converged - supposing that the features seen by some, but not seen by others, were real. There was no chance for verification until spacecrafts have been developed which were able to make local observations. Instead of canals, the first pictures returned revealed a surface full of craters - a landform not expected by anyone. A paradigm shift was needed to explain the features of the "new" Mars. On the Moon, features were observable, but the interpretation was wrong. On Mars, only blurred albedo markings could be observed, along with sharp lines of imagination, which again were interpreted falsely. In the case of Venus, there was no data on surface features. Only its bright cloud top could be observed from the Earth. But this fact along with the planet's orbital parameters provided enough information for a popular view on its surface conditions: a hot world (inferred from its proximity to the Sun) and also a rainy one (from its complete cloud cover). The conclusion: Venus is a global jungle possibly with dinosaurs, like the hot and wet world of the then-discovered Mesozoic era. Our current knowledge originated from these early attempts of interpreting surface conditions and geological origin of landforms from a very little set of available data. Today we have a huge set of images and other physical data which makes it possible to create models on the inner structure and thermal history of planetary bodies. Combined data sets lead to better supported models on the formation of surface features. Today we believe that most models give reliable explanation for the origin of planetary landforms. New, higher resolution images reveal new sets of meso- and microscale landforms, while images from previously not imaged dwarf planets, satellites, asteroids and cometary nuclei show landforms never seen before. In the future exoplanets are expected to provide brand new types of relief features no predictable by our Earth-and Solar System bound imagination. There are so many different landforms on planetary surfaces that it is nearly impossible for anybody to overview all of them who does not work exactly with that certain feature type. The Encyclopedia helps with presenting the landforms in searchable, alphabetical order. The book contains more than a simple list of various features: it provides context and connections between them and point to their origin. For example sand dunes were found on Venus, Mars and Titan, fluvial valleys and shorelines are present on Mars and Titan, impact craters have many different types - all are presented and explained here. Beyond the texts, references, schematic figures, images and planetary maps accompany the description of landforms, providing a wide background for detailed analyses even for geomorphologists working in planetary science. This book is to help the reader to discover the great variety of planetary landforms.
The Australian vegetation is the end result of a remarkable history of climate change, latitudinal change, continental isolation, soil evolution, interaction with an evolving fauna, fire and most recently human impact. This book presents a detailed synopsis of the critical events that led to the evolution of the unique Australian flora and the wide variety of vegetational types contained within it. The first part of the book details the past continental relationships of Australia, its palaeoclimate, fauna and the evolution of its landforms since the rise to dominance of the angiosperms at the beginning of the Cretaceous period. A detailed summary of the palaeobotanical record is then presented. The palynological record gives an overview of the vegetation and the distribution of important taxa within it, while the complementary macrofossil record is used to trace the evolution of critical taxa. This book will interest graduate students and researchers interested in the evolution of the flora of this fascinating continent.