Vredefort crater - field night 1 report PDF

Preview

Summary

field night 1 report ...

Description

Vredefort Crater Simeon Kruger - 22377267 The Vredefort crater is located in the Free Sate Province of South Africa. The crater is believed to be formed by an asteroid that hit earth more than 2 billion years ago (Osinski, 2012). This means that the asteroid hit earth in the Paleoproterozoic era and is believed to be the second oldest crater on earth. The original crater was estimated to have a diameter of approximately 300 km’s, which means it is the largest crater on the earth (Reimold & Köber, 1999). The crater has since eroded away and is now encircled by a ring of hills with a 70km diameter. This is most commonly referred to as the Vredefort Dome. The Vredefort crater is named after a town called Vredefort, which is located near the centre. The Vredefort impactor Scientists first thought that a volcanic explosion caused the Vredefort crater. However, due to shatter cones found on the bed of the Vaal River, scientists now believe that it is an asteroid that caused the crater to be formed. The asteroid was estimated to be between 5-10 km’s in diameter (Osinski, 2012). This indicates that it was one of the largest asteroids to hit earth. Scientists believe that the asteroid was traveling at a speed of approximately 20 kilometres per second. U-Pb dating technique Different techniques were used to determine the age of the Vredefort crater. The most common technique used to date large terrestrial craters is the U-Pb technique. The U-Pb dating technique is a process where the igneous bodies are cooled. The process includes observing at what age certain minerals crystallize (Osinski, 2012). In order to achieve the most accurate result, minerals such as zircon and baddeleyite are used. Zircon and baddeleyite are used because it has trace amounts of uranium present but does not contain lead. In order to determine the age of the Vredefort crater, scientists analysed the crystallised zircon grains in the pseudotachylyte, which was found in the core rocks (Reimold & Köber, 1999).

Economic effects of Vredefort impact Witwatersrand is a neighbouring town of Vredefort and is mostly encircled by the original Vredefort crater. The core is encircled by an 18 km wide made up of Proterozoic sedimentary as well as volcanic rocks of the Witwatersrand and Ventersdorp supergroup (Grieve & Therriault, 2000). Witwatersrand is the world’s largest goldfield and have produced approximately 47,000 ton of gold since it’s discovery in 1886, which is also between 40-50% of gold ever mined. As a by-product of gold mining, there has been approximately 150,000 ton of uranium mined as well. Scientists believe that there are still 20,000 ton of gold reserve as well as 475,000 ton of uranium reserve (Grieve & Therriault, 2000). It is believed that the Vredefort impact played a crucial role in the process of gold remobilisation. Scientists believe that this is due to the alteration of hydrothermal activity. This activity would provide channels for fluid migration by remobilising the gold and uranium within the impact related structure. Conclusion The Vredefort crater is the largest crater on earth and is the second oldest crater. Due to the discovery of shatter cones, scientists concluded that an asteroid rather than a volcanic explosion caused the crater. It is believed that the impact of the crater caused alteration of hydrothermal activity causing the remobilisation of gold and uranium.

(Satellite image of the Vredefort Crater) References Osinski, G. (2012). Impact Cratering : Processes and Products (1st ed.). Chicester: Wiley. Reimold, W., Köber, C., Fletcher, P., Killick, A., & Wilson, J. (1999). Pseudotachylitic breccias from fault zones

in the Witwatersrand Basin, South Africa: evidence of autometasomatism and postbrecciation alteration processes. Mineralogy and Petrology, 66(1). doi:10.1007/BF01161721 Grieve, R., & Therriault, A. (2000). Vredefort, Sudbury, Chicxulub: Three of a Kind? Annual Review of Earth and Planetary Sciences, 28(1). doi:10.1146/annurev.earth.28.1.305 Di nop e d i a .( 20 14 ) .Vr e d e f or tc r a t e r .r e t r i e v e d f r om h t t p : / / di no pe d i a . wi ki a . c om/ wi ki / Vr e de f or t _c r a t e r( i ma g e )...