This book is a collection of 22 selected papers from the homonymous Conference held in September 2003 Milos, Greece. The aim of the conference was to serve as a forum for the presentation and constructive discussion of the state-of-the-art and emerging issues on the South Aegean Volcanic Arc.

In the first part of the book the tectonic- geodynamic setting and the present upper mantle structure of the Aegean area are discussed. It includes an interesting interpretation of data on the spatial distribution of intermediate focal depth earthquakes, fault plane solutions and deep velocity structures, to further investigate active tectonics related to the deep structure of the southern Aegean volcanic arc.

The second part deals with general volcanological, petrological and tectonic characteristics of the SAAVA presenting an extensive review of volcanological, chemical, isotope and tectonic data, using a large amount of new field and laboratory data. Interesting conclusions are presented regarding the present volcanic associations, the volcanic fields location and shape in respect to the large tectonic lineaments and the plate motions, the source of the SAAVA parental magmas.

Presented in the third part is an extensive review on the volcanic hazard assessment and the monitoring state of the SAAVA centers. Seismic and geodetic monitoring of the Santorini volcano and the recent (1995-1998) crisis of Nisyros volcano are presented and discussed.

The last part deals with hydrothermal deposits and processes in the SAAVA, as well as products and processes in adjacent areas with a particular interest and significance that link them to the SAAVA processes.
*Systematic re-evaluation on the geodynamic and tectonic setting of the Aegean active volcanic centers
*Thorough review with new data and ideas on the magma source region, the magma differentiation processes in both the deep and shallow levels, and the volcanological processes related both to the magma composition and storage depth as well as to the tectonic regime of the volcano growth area
*Up to date estimation of the volcanic hazard in the Aegean area, and a detailed presentation of the present state and the monitoring efforts of the South Aegean active centers

This volume covers the formation and biogeochemistry of a variety of important sediment types from their initial formation through their conversion (diagenesis) to sedimentary rocks. The volume deals with the chemical, mineralogical, and isotopic properties of sediments and sedimentary rocks and their use in interpreting the environment of formation and subsequent events in the history of sediments, and the nature of the ocean-atmosphere system through geological time.

Reprinted individual volume from the acclaimed Treatise on Geochemistry,
(10 Volume Set, ISBN 0-08-043751-6, published in 2003)
* Comprehensive and authoritative scope and focus
* Reviews from renowned scientists across a range of subjects, providing both overviews and new data, supplemented by extensive bibliographies
* Extensive illustrations and examples from the field

This book is a continuation of 'Acoustic and Elastic Wave Fields in Geophysics, Part I' published in 2000. The second volume is dedicated to propagation of linear plane, spherical and cylindrical acoustic waves in different media. Chapter 1 is devoted to principles of geometric acoustic in plane wave approximation. The eikonal and transport equations are derived. Ray tracing and wavefront construction techniques are explained. Chapter 2 deals with dynamic properties of wave fields. The behavior of pressure and displacements amplitudes in zero approximation is analysed in two ways: using Poynting vector and solving the transport equation. This chapter contains several examples related to shadow zones and caustics. In Chapter 3 using the results of analysis of high-frequency wave kinematics and dynamics some fundamental aspects of Kirchhoff migration are described. Chapters 4 and 5 are devoted to propagation of plane waves in media with flat boundaries in the case of normal and oblique incidence. Special attention is paid to the case when an incident angle exceeds the critical angles. Formation of normal modes in the waveguide is discussed. Chapter 6 deals with a spherical wave reflection and refraction. The steepest descent method is introduced to describe the behavior of reflected, transmitted, head and evanescent waves. In Chapter 7 propagation of stationary and transient waves in a waveguide formed by a flat layer with low velocity are investigated. Normal modes and waves related to the branch points of integrands under consideration are studied. Dispersive properties of normal modes are discussed. Chapter 8 describes wave propagation inside cylinder in acoustic media. Several appendices are added to help the reader understand different aspects of mathematics used in the book.

"Geosciences, Environment and Man" has three major objectives, which determine the division in three parts of this volume: I. To consider the main natural geological processes interfering with and therefore threatening the activities of man: earthquakes, volcanic eruptions, land movements, floods, wind and coastal risks; main prevention and mitigation measures against these natural hazards are presented. II. To examine the exploitation of earth's natural resources such as materials, ores and minerals, fossil fuels, water, radioactivity, and the resulting consequences on solid Earth balance and future. III. To assess the hold level reached by the activities of man on planet surface envelopes through agriculture, urbanization, industrialization, and communication; the local to global effects of human influence triggered by recent demographic growth on underground, soils, water and air characters are taken into account. Both deteriorating and beneficial aspects of Earth - the interactions of man are emphasized, as well as mitigation or restoration measures and perspectives.

This book examines different classical and modern aspects of geophysical data processing and inversion with emphasis on the processing of seismic records in applied seismology.

Chapter 1 introduces basic concepts including: probability theory (expectation operator and ensemble statistics), elementary principles of parameter estimation, Fourier and z-transform essentials, and issues of orthogonality. In Chapter 2, the linear treatment of time series is provided. Particular attention is paid to Wold decomposition theorem and time series models (AR, MA, and ARMA) and their connection to seismic data analysis problems. Chapter 3 introduces concepts of Information theory and contains a synopsis of those topics that are used throughout the book. Examples are entropy, conditional entropy, Burg's maximum entropy spectral estimator, and mutual information. Chapter 4 provides a description of inverse problems first from a deterministic point of view, then from a probabilistic one. Chapter 5 deals with methods to improve the signal-to-noise ratio of seismic records. Concepts from previous chapters are put in practice for designing prediction error filters for noise attenuation and high-resolution Radon operators. Chapter 6 deals with the topic of deconvolution and the inversion of acoustic impedance. The first part discusses band-limited extrapolation assuming a known wavelet and considers the issue of wavelet estimation. The second part deals with sparse deconvolution using various 'entropy' type norms. Finally, Chapter 7 introduces recent topics of interest to the authors.

The emphasis of this book is on applied seismology but researchers in the area of global seismology, and geophysical signal processing and inversion will find material that is relevant to the ubiquitous problem of estimating complex models from a limited number of noisy observations.
* Non-conventional approaches to data processing and inversion are presented
* Important problems in the area of seismic resolution enhancement are discussed
* Contains research material that could inspire graduate students and their supervisors to undertake new research directions in applied seismology and geophysical signal processing

The nature of global change in the Pacific Basin is poorly known compared to other parts of the world. Climate, Environment, and Society in the Pacific during the Last Millennium describes the climate changes that occurred in the Pacific during the last millennium and discusses how these changes controlled the broad evolution of human societies, typically filtered by the effects of changing sea level and storminess on food availability and interaction. Covering the entire period since AD 750 in the Pacific, this book describes the influences of climate change on environments and societies during the Medieval Warm Period and the Little Ice Age, focusing on the 100-year transition between these - a period of rapid change known as the AD 1300 Event.
* Discusses the societal effects of climate and sea-level change, as well as the evidence for externally-driven societal change
* Synthsizes how climate change has driven environmental change and societal change in the Pacific Basin
* Contains a comprehensive and up-to-date survey of the evidence for climate, environmental, and societal change, supported by a full list of references

Adsorption of Metals by Geomedia II serves as a needed resource for this topic which has received much attention during the past 25 years. The book provides an in-depth review of the field, followed by numerous chapters that document the current status of adsorption research for a variety of metals by geomedia ranging from individual minerals to sediments and soils. Adsorption mechanisms are detailed and precipitation is presented as a distinct sorption process. Virtually all factors affecting the extent of metal adsorption are examined, including the effects of selected anions, competition among metals, pH, metal concentration, loading, variable metal adsorption capacity, ionic strength, hydrogen exchange and stoichiometry, and solids concentration. A variety of adsorption models are briefly presented and some are used to extend laboratory studies to field sites.
This is a compilation of 25 peer reviewed papers from among the 60+ platform and poster presentations of the symposium "Adsorption of Metals to Geomedia II" at the American Chemical Society (ACS) Meeting, March 27-29, 2006 in Atlanta, Georgia, USA. This symposium is a follow-up to the original held in 1996.
* Learn the tools and techniques from leading academics and industry experts
* One stop practical resource and guide for those in the field
* Keep informed and up to date on all the latest advancements in technology

The history of Earth in the Solar System has been unraveled using natural radioactivity. The sources of this radioactivity are the original creation of the elements and the subsequent bombardment of objects, including Earth, in the Solar System by cosmic rays. Both radioactive and radiogenic nuclides are harnessed to arrive at ages of various events and processes on Earth.

This collection of chapters from the Treatise on Geochemistry displays the range of radioactive geochronometric studies that have been addressed by researchers in various fields of Earth science. These range from the age of Earth and the Solar System to the dating of the history of Earth that assists us in defining the major events in Earth history. In addition, the use of radioactive geochronometry in describing rates of Earth surface processes, including the climate history recorded in ocean sediments and the patterns of circulation of the fluid Earth, has extended the range of utility of radioactive isotopes as chronometric and tracer tools.
Comprehensive, interdisciplinary and authoritative content selected by leading subject expertsRobust illustrations, figures and tablesAffordably priced sampling ofcontent from the full "Treatise on Geochemistry""

Geochemistry of Earth Surface Systemsoffers an interdisciplinary reference for scientists, researchers and upper undergraduate and graduate levelgeochemistry students a sampling of articles on earth surface processes from The Treatise on Geochemistry that ismore affordable than the full Treatise. For professionals, this volume will provide anoverview of the field as a whole. For students, it will provide morein-depth introductory content thanis found inbroad-based geochemistry textbooks. Articleswere selected from chapters across all volumes of the full Treatise, and include: Volcanic Degassing, Hydrothermal Processes, The Contemporary Carbon Cycle, Global Occurrence of Major Elements in Rivers, Organic Matter in the Contemporary Ocean, The Biological Pump, and Evolution of Sedimentary Rocks.
Comprehensive, interdisciplinary and authoritative content selected by leading subject expertsRobust illustrations, figures and tablesAffordably priced sampling ofcontent from the full "Treatise on Geochemistry""

The book deals with the record of important Neoproterozoic to Early Palaeozoic events in southwestern Gondwana, that heralded the Cambrian explosion and the dawn of modern ecosystems. It contains a detailed account of the Neoproterozoic to Cambrian geological record in a poorly-known part of the world, which is at the same time key to understand fundamental processes at the Proterozoic-Cambrian transition. The emphasis is placed on litho-, bio-, chemostratigraphy and magmatism. The palaeoclimatic, tectonic, evolutionary radiation and extinction events and associated mineralizations will be identified and discussed. A synthesis of all data is provided at the end of the book, integrating the data from all cratons and fold belts in southwestern Gondwana. The events will be individualized, their impact discussed and correlations between different successions both within and outside Gondwana proposed.
The book is organized in three sections. Section one is an introduction to the neoproterozoic and Cambrian seen as a time of upheavals, extremes and innovations. Section two comprisesnineteen chapters dealing with theneoproterozoic-Cambrian events in southwestern Gondwana. Section three will provide a synthesis on every major topic, and a critical assessment of the global implications of the presented data.
* The book deals with the record of important Neoproterozoic to Early Palaeozoic events in southwestern Gondwana, that heralded the Cambrian explosion and the dawn of modern ecosystems.
It contains a detailed account of the Neoproterozoic to Cambrian geological record in a poorly-known part of the world, which is at the same time key to understand fundamental processes at the Proterozoic-Cambrian transition. The emphasis is placed on litho-, bio-, chemostratigraphy and magmatism."

Treatise on Geophysics: Mantle Dynamics, Volume 7 aims to provide both a classical and state-of-the-art introduction to the methods and science of mantle dynamics, as well as survey leading order problems (both solved and unsolved) and current understanding of how the mantle works. It is organized around two themes: (1) how is mantle convection studied; and (2) what do we understand about mantle dynamics to date. The first four chapters are thus concerned with pedagogical reviews of the physics of mantle convection; laboratory studies of the fluid dynamics of convection relevant to the mantle; theoretical analysis of mantle dynamics; and numerical analysis and methods of mantle convection. The subsequent chapters concentrate on leading issues of mantle convection itself, which include the energy budget of the mantle; the upper mantle and lithosphere in and near the spreading center (mid-ocean ridge) environment; the dynamics of subducting slabs; hot spots, melting anomalies, and mantle plumes; and finally, geochemical mantle dynamics and mixing.

Treaties on Geophysics: Core Dynamics, Volume 8, provides a comprehensive review of the current state of understanding of core dynamics. The book begins by analyzing a subject of long-standing and on-going controversy: the gross energetics of the core. It then explains the important elements of dynamo theory; actual fluid motions in the core; the basic physical principles involved in thermochemical convection in the core and the basic equations governing the convection; and turbulence and the small-scale dynamics of the core. This is followed by discussions of the state of knowledge on rotation-induced core flows; the use of first-principles numerical models of self-sustaining fluid dynamos; and the behavior of polarity reversals in numerical dynamo models. The remaining chapters cover the various roles the inner core plays in core dynamics and the geodynamo; experiments that have shaped knowledge about the flows in the core that produce the geodynamo and govern its evolution; and ways the mantle can affect core dynamics, and corresponding ways the core can affect the mantle.

Geodesy, which is the science of measuring the size and shape of the Earth, explores the theory, instrumentation and results from modern geodetic systems. The beginning sections of the volume cover the theory of the Earth's gravity field, the instrumentation for measuring the field, and its temporal variations. The measurements and results obtained from variations in the rotation of the Earth are covered in the sections on short and long period rotation hanges. Space based geodetic methods, including the global positioning system (GPS) and Interferometric synthetic aperture radar (SAR), are also examined in detail.

Self-contained volume starts with an overview of the subject then explores each topic with in depth detail
Extensive reference lists and cross references with other volumes to facilitate further research
Full-color figures and tables support the text and aid in understanding
Content suited for both the expert and non-expert

Treatise on Geophysics: Geomagnetism, Volume 5, provides an overview of the most important aspects of geomagnetism. The book begins by tracing the history of the study of geomagnetism. It then reviews global models of the Earth's magnetic field; the main sources of external magnetic field contributions; and the instruments and practices used to observe and measure the full range of features of the geomagnetic field. It discusses the origins of current knowledge of the secular variation of the Earth's magnetic field; crustal magnetism; geomagnetic excursions; the study of geophysical electromagnetic induction; the magnetization process; and the status of recent magnetic field data and their applications. The remaining chapters cover the geometry of the geomagnetic field and its temporal variability as recorded in volcanic and sedimentary rocks over the past few million years; the ocean crust as a recorder of geomagnetic field variations; and the theoretical basis for paleointensity experiments in igneous and sedimentary environments. The final chapter explains the concept of true polar wander (TPW), defined as shifts in the geographic location of Earth's daily rotation axis and/or by fluctuations in the spin rate (length of day anomalies).

Evolution of the Earth focuses on the formation of Earth. Topics include the differention of the core, mantle and crust; the formation of the ocean basins and continents; outgassing and volcanism; the initiation of plate tectonics, the origin and persistence of Earth's magnetic field; the growth of the inner core; changes in mantle convection through time; and the impact of life on the planet. The volume takes an interdisciplinary viewpoint that emphasizes the interplay of geophysics, other aspects of earth science and biological evolution. Some outstanding questions are identified and debated.
Self-contained volume starts with an overview of the subject then explores each topic with in depth detail
Extensive reference lists and cross references with other volumes to facilitate further research
Full-color figures and tables support the text and aid in understanding
Content suited for both the expert and non-expert

Treatise on Geophysics: Mineral Physics, Volume 2, provides a comprehensive review of the current state of understanding of mineral physics. Each chapter demonstrates the significant progress that has been made in the understanding of the physics and chemistry of minerals, and also highlights a number of issues which are still outstanding or that need further work to resolve current contradictions. The book first reviews the current status of our understanding of the nature of the deep Earth. These include the seismic properties of rocks and minerals; problems of the lower mantle and the core-mantle boundary; and the state of knowledge on mantle chemistry and the nature and evolution of the core. The discussions then turn to the theory underlying high-pressure, high-temperature physics, and the major experimental methods being developed to probe this parameter space. The remaining chapters explain the specific techniques for measuring elastic and acoustic properties, electronic and magnetic properties, and rheological properties; the nature and origin of anisotropy in the Earth; the properties of melt; and the magnetic and electrical properties of mantle phases.

Ground-penetrating radar (GPR) is a rapidly developing field that has seen tremendous progress over the past 15 years. The development of GPR spans aspects of geophysical science, technology, and a wide range of scientific and engineering applications. It is the breadth of applications that has made GPR such a valuable tool in the geophysical consulting and geotechnical engineering industries, has lead to its rapid development, and inspired new areas of research in academia. The topic of GPR has gone from not even being mentioned in geophysical texts ten years ago to being the focus of hundreds of research papers and special issues of journals dedicated to the topic. The explosion of primary literature devoted to GPR technology, theory and applications, has lead to a strong demand for an up-to-date synthesis and overview of this rapidly developing field. Because there are specifics in the utilization of GPR for different applications, a review of the current state of development of the applications along with the fundamental theory is required. This book will provide sufficient detail to allow both practitioners and newcomers to the area of GPR to use it as a handbook and primary research reference.
*Review of GPR theory and applications by leaders in the field
*Up-to-date information and references
*Effective handbook and primary research reference for both experienced practitioners and newcomers

This book serves as an up-to-date introduction, as well as overview to modern trace fossil research and covers nearly all of the essential aspects of modern ichnology. Divided into three section, Trace Fossils covers the historical background and concepts of ichnology, on-going research problems, and indications about the possible future growth of the discipline and potential connections to other fields. This work is intended for a broad audience of geological and biological scientists. Workers new to the field could get a sense of the main concepts of ichnology and a clear idea of how trace fossil research is conducted. Scientists in related disciplines could find potential uses for trace fossils in their fields. And, established workers could use the book to check on the progress of their particular brand of ichnology. By design, there is something here for novice and veteran, insider and outsider, and for the biologically-oriented workers and for the sedimentary geologists.
* Presents a review of the state of ichnology at the beginning of the 21st Century
* Summarizes the basic concepts and methods of modern trace fossil research
* Discusses crucial background information about the history of trace fossil research, the main concepts of ichnology, examples of current problems and future directions, and the potential connections to other disciplines within both biology and geology

Based on lectures given in the First Russian School-Seminar on electromagnetic soundings of the Earth held in Moscow on 15th November, 2003, this book acquaints scientists and technologists with the latest achievements in theory, techniques and practical applications of the methods of electromagnetic sounding. This three part text covers the methods considered for Earth electromagnetic sounding on a global, regional, and local scale; modern methods for solving forward and inverse problems of geoelectrics, particularily contemporary approaches to the EM data modeling and interpretation in the class of three-dimensional models; and the results of regional EM on-land and sea soundings
*Presents theoretical and methodological findings, as well as examples of applications of recently developed algorithms and software in solving practical problems
*Describes the practical importance of electromagnetic data through enabling discussions on a construction of a closed technological cycle, processing, analysis and three-dimensional interpretation
*Updates current findings in the field, especially with MT, magnetovariational and seismo-electriccal methods and the practice of 3D interpretaions

Principles of Sequence Stratigraphy provides an in-depth coverage and impartial assessment of all current ideas and models in the field of sequence stratigraphy. Even though widely popular among all groups interested in the analysis of sedimentary basin fills, both in academia and in the industry, sequence stratigraphy is yet a difficult undertaking due to an overwhelming jargon and the persistence of conflicting approaches as to how the sequence stratigraphic method should be applied to the rock record. This textbook examines the relationship between such conflicting approaches from the perspective of a unifying platform, demonstrating that sufficient common ground exists to eliminate terminology barriers and to facilitate communication between all practitioners of sequence stratigraphy.

This work is not only a review of past and current literature, but also includes key original viewpoints and is illustrated with new graphic material and numerous field examples of outcrop, core, well log and 3D seismic data in full colour. The book is addressed to anyone interested in understanding and applying the method of sequence stratigraphic analysis, from students to geologists, geophysicists and reservoir engineers. Sequence stratigraphy remains the most recent paradigm in the broad field of sedimentary geology, with tremendous potential to deciphering the record of global change and to improving the success of economic exploration.
* Offers theoretical guidelines of how the facies and time relationships are expected to be under specific circumstances such as subsidence patterns, sediment supply, topographic gradients, etc.
* Contains numerous high-quality and full-color diagrams, photographs and illustrations, virtually on every aid in comprehension of the subject
* Features a dedicated chapter on discussions and conclusions incorporating all previous chapters with references, basic principles and strategies
* Provides an extensive list of references for further reading, as well as an author and subject index for quick information access

The book deals with the study of three important volcanisms in the Campania Plain: Vesuvius, Campi Flegrei and Ignimbrites. The knowledge of the volcanic evolution of Vesuvius and Campi Flegrei has a particular relevance because of the hazards that these volcanoes pose to the about 1.5 million people living in the Neapolitan area.

The contributors to the volume bring new data (experiments on volatile solubility, fluid-melt inclusions, tectonic, geophysical, isotope, geochronology), which are helpful in the creation of new models for a better understanding of the behaviour of the volcanic systems. In particular a hydrothermal model is used to explain the ground movements (bradyseism) at Campi Flegrei. To develop such a model, the authors use an analogue for the evolving Campi Flegrei sub-volcanic system, the model of the porphyry mineralized systems. For Campanian Ignimbrite the authors highlight the impact crystal-liquid separation has on melt compositional evolution and particularly focus on trace element and Th isotope evidence for open-system processes in the magma body associated with the Campanian Ignimbrite.

The authors, for their interpretations, utilize thermodynamic and quantitative mass balance modelling of major and trace element data and semi-quantitative limits on Th and Sr isotopes to evaluate the role of crytal-melt separation, magma-fluid interaction, and assimilation of wallrock on the geochemical evolution of the Campamian Ignimbrite.

The oceans are vitally important to an understanding of how the Earth works as an integrated system because its chemical composition records transfer of elements through the Earth's geochemical reservoirs as well as defining how physical, biological and chemical processes combine to influence issues as diverse as climate change and the capacity of the oceans to remove toxic metals. Much modern marine geochemistry aims to link and integrate studies of the modern oceans with work using proxies to define how ocean chemistry and the ocean/atmospheric system has changed through time on a number of different timescales. Special focus in such work is the carbon cycle and its link to changes in greenhouse gases in the atmosphere. Volume 6 covers all the important topics needed for such an integrated approach, ranging from the contemporary ocean composition, transport processes in the ocean, paleoclimatology and paleo-oceanography from marine deposits, to the evolution of seawater composition.

Volume 3 examines the chemical composition of the Earth's crust, starting with the continental crust and the rocks exposed therein, moving on to oceanic crust (MORB and oceanic plateaus) and finishing with island arc crust. In addition to providing a descriptive geochemistry of the Earth's crust, the volume summarizes the processes responsible for crustal formation and modification, exchange between the crust and other Earth reservoirs (mantle, oceans and atmosphere), and examines the secular evolution of the crust.

Reprinted individual volume from the acclaimed Treatise on Geochemistry,
(10 Volume Set, ISBN 0-08-043751-6, published in 2003)
* Comprehensive and authoritative scope and focus
* Reviews from renowned scientists across a range of subjects, providing both overviews and new data, supplemented by extensive bibliographies
* Extensive illustrations and examples from the field

High-pressure mineral physics is a field that is strongly driven by the development of new technology. Fifty years ago, when experimentally achievable pressures were limited to just 25 GPa, little was know about the mineralogy of the Earth's lower mantle. Silicate perovskite, the likely dominant mineral of the deep Earth, was identified only when the high-pressure techniques broke the pressure barrier of 25 GPa in 1970s. However, as the maximum achievable pressure reached beyond one Megabar (100 GPa) and even to the pressure of Earth's core on minute samples, new discoveries increasingly were fostered by the development of new analytical techniques and improvements in sensitivity and precision of existing techniques.

The book consists of six sections which group the papers according to their main topics: a) Elastic and Anelastic Properties; b) Rheology; c) Melt and Glass Properties; d) Structural and Magnetic Properties; e) Diffraction and Spectroscopy; f) Pressure Calibration and Generation. As many papers cover multiple topics, readers may find papers of interest in different sections. All papers are prepared with emphasis on technical details suitable for a technical reference. Many on-line software resources are also listed in as detailed a manner as possible. However, the URL of the software sites may be subject to change without notice.
* State of the art in a very important branch of geophysics, namely the experimental determination of material behavior at the extreme conditions of planetary interiors
* Emphasis on technical details suitable for a technical reference
* Includes many on-line software resources

This second volume of Frontiers articles contains a broad array of fascinating topics. The first four
papers focus on the Sun, the origin of the solar system and the early history of the Earth. The fifth, on komatiites, is also relevant to the early Earth but also forms a nice transition to three articles that focus on subduction and mantle processes. The final three articles focus on changes in our surface environment including biogeochemical cycles, the evolution of hominids and, finally, the future threat posed by near Earth asteroids.