Le Cercle des ufologues Index du Forum

Le Cercle des ufologues
Forum d'études du phénomène ufologique
ProfilProfil   Se connecter pour vérifier ses messages privésSe connecter pour vérifier ses messages privés   ConnexionConnexion   FAQFAQ   RechercherRechercher   MembresMembres   GroupesGroupes S’enregistrerS’enregistrer

San Diego University

Poster un nouveau sujet   Répondre au sujet    Le Cercle des ufologues Index du Forum -> Forum in English -> Astronomy -> Meteors and others spatial phenomena
Sujet précédent :: Sujet suivant  
Auteur Message

Hors ligne

Inscrit le: 01 Sep 2006
Messages: 1 361
Localisation: Nantes
J'accepte La charte: Oui j'ai lu et pris connaissance de la charte, j'e l’accepte et je m’engage à la respécter

MessagePosté le: 08/12/2007 14:26:47    Sujet du message: San Diego University Répondre en citant

For those who are interested by meteorites :

Meteorite impact cratering on Earth:
Geological and biological consequences
Christian Koeberl
Department of Geological Sciences
University of Vienna, Austria

Impact is a unique, short-time, high-energy geological process. The importance of impact cratering on terrestrial planets (Mercury, Venus, Mars), our Moon, and the satellites of the outer planets is obvious from the abundance of craters on their surfaces. On most bodies of the solar system that have a solid surface, impact cratering is the most important surface-modifying process even today. On Earth, active geological processes rapidly obliterate the cratering record. To date only about 170 impact structures have been recognized on the Earth’s surface. The come in various forms, shapes and sizes, from 300 km to less than 100 m in diameter, from Recent to 2 billion years in age. Mineralogical, petrographic, and geochemical criteria are used to identify the impact origin of such structures or related ejecta layers. The two most important criteria are the presence of shock metamorphic effects in mineral and rock inclusions in breccias and melt rocks, as well as the demonstration, by geochemical techniques, that these rocks contain a minor extraterrestrial component. In impact studies there is now a trend towards the use of interdisciplinary and multi-technique approaches to solve open questions. In this lecture we will take a look at impact craters on the Earth (and some other planets), and discuss how they formed and how they can be recognized. An aspect of impact cratering that may be underestimated is the influence of impacts on the geological and biological evolution of our own planet. Even the impact of relatively small asteroids or comets can have disastrous consequences for our civilization. There is a 1 in 10,000 chance that a large asteroid or comet 2 km in diameter (corresponding to a crater of about 25-50 km in diameter) may collide with the Earth during the next century, severely disrupting the ecosphere and annihilating a large percentage of the Earth's population. The biological evolution of our planets is punctuated by mass extinction events, of which the one 65 million years ago, which marks the Cretaceous-Tertiary boundary, is probably the best known one. Abundant impact debris marks this boundary, providing a clear link with a major impact event. The Chicxulub impact structure in Mexico, about 200 km in diameter, which resulted from the impact of an about 10-km-diameter asteroidal body, has been identified as the culprit. Understanding of impact structures, their formation processes, and their consequences should be of interest not only to earth and planetary scientists, but also to society in general.

And to have more informations, go there Arrow San Diego University
Have a nice week end Very Happy

RGB - Serenity is not freedom from the storm but peace within the storm (Native American Proverb)
Revenir en haut

MessagePosté le: 08/12/2007 14:26:47    Sujet du message: Publicité

PublicitéSupprimer les publicités ?
Revenir en haut
Montrer les messages depuis:   
Poster un nouveau sujet   Répondre au sujet    Le Cercle des ufologues Index du Forum -> Forum in English -> Astronomy -> Meteors and others spatial phenomena
Toutes les heures sont au format GMT + 2 Heures
Page 1 sur 1

Sauter vers:  

Portail | Index | Créer un forum | Forum gratuit d’entraide | Annuaire des forums gratuits | Signaler une violation | Conditions générales d'utilisation

Powered by phpBB © 2001, 2018 phpBB Group
Traduction par : phpBB-fr.com
MylMatriX theme for phpBB version 2+
Created by SGo
Based on the original subSilver Created by subBlue design