One day, about 2023 million years ago, the earth was about to change. An asteroid (a really big meteorite) with a diameter of 10 km was racing towards the earth. (This is about the size of five Table Mountains put together.) It was travelling at a speed much faster than a bullet from a gun. The sky lit up when the huge rock burned as it entered the earth’s atmosphere. Then it hit the earth with an explosion that was a thousand times bigger than the biggest bomb known to man. The asteroid penetrated the crust of the earth’s surface and disappeared underneath the ground. Nothing in a 300 km radius would have survived the explosion. (This means that everything was devastated from Pretoria al the way to Bloemfontein.) Clouds of dust and rocks rose up from the earth and blocked the rays of the sun totally from reaching the earth’s surface - as if it was in the middle of a very dark night. It got very cold for a long time - some people say up to a few years. When the dust clouds disappeared, life returned to what it was previously very, very slowly.
Today, all that can be seen of the impact site is a piece of barren rock sticking out of the ground. If you look on the horizon, you’ll see the rim of mountains caused by the impact. The co-ordinates will take you to the location with the best view.
Geology map from www.parys.info
To log this cache you need to:
- Post a picture of yourself and your visiting group with your GPS. It would be nice if the rock dome is in the picture. Note: Since the area has now been fenced off, a picture with the hill in the background at the place where you were closest to ground zero will also be great.
- State the distance of your home town from the Vredefort Earthcache (which is at the impact site) in your log.
- Work out the Seismic Effects of the blast at your home town and include it in your log.
- Work out the Air Blast Details at your home town and include it in your log.
- Do you think you would have sustained any injuries in the blast if you were at your home town at the time of the impact?
- Note: There aren't a physical cache container - the treasure is in what you learn.
Educator's Guide to Impact Craters
The Vredefort site is a complex crater (or this site). If a similar meteorite was on its way to the earth now, it would measure 10 on the Torino Impact Hazard Scale.
Effects of the impact
The effects of an impact like this one are truly mindboggling. If you were to live in Pretoria at the time of the Vredefort Impact, you would have experienced the following effects:
- Time for maximum radiation: 7.18 seconds after impact
- Visible fireball radius: 119 km
- The fireball appears 135 times larger than the sun
- Duration of Irradiation: 26 minutes and 20 seconds
- Effects of Thermal Radiation: Clothing ignites and much of the body suffers third degree burns.
- The major seismic shaking will arrive at approximately 40 seconds after the impact.
Richter Scale Magnitude: 9.8
- The ejecta will arrive approximately 3 minutes 26 seconds after the impact.
- Average Ejecta Thickness: 18.4 m.
- Mean Fragment Diameter: 11.9 cm.
- The air blast will arrive at approximately 10 minutes after the impact.
- Max wind velocity: 4860 km/h
- Sound Intensity: 129 dB (Dangerously Loud)
- Damage Description: Multistory wall-bearing buildings will collapse, highway truss bridges will collapse and up to 90 percent of trees blown down; remainder stripped of branches and leaves.
These amazing impact effects was obtained from this site by Robert Marcus, H. Jay Melosh, and Gareth Collins. See what the effects would have been at your location by entering the following parameters:
- Distance from Impact: Your distance from Vredefort
- Projectile Diameter: 10000.00 m = 32800.00 ft = 6.21 miles
- Projectile Density: 3000 kg/m3
- Impact Velocity: 17.00 km/s = 10.56 miles/s
- Impact Angle: 45 degrees
- Target Density: 2500 kg/m3
- Target Type: Sedimentary Rock
Other interesting links:
Deep Impact - The Vredefort Dome
More pictures and article references of the Vredefort dome at this site.
Near Earth Objects - Presentation to the All Party Parliamentary Astronomy and Space Environment Group by Professor Mark Bailey
More impact craters in Africa at this site.
Lots more pictures of impact craters.
Lots more technical detail about impact craters in general at this site.
Lots of articles covering meteorite impacts at this site.
Background image sourced from this site.