Kalkkop Crater EarthCache

OVERVIEW
Before setting off to do this earth cache you should realise that this is a very remote site – as reflected in the terrain rating. You’ll have to drive on at least 70km of dirt roads, some of which wind and twist their way down to little twee-spoor-paadjies. But… if you enjoy this sort of thing you will not be disappointed. It is a beautiful, off-the-beaten-track piece of the Karoo that will afford you the perfect opportunity to reflect on our place in the universe and, if you’re as lucky as us, meet some of the warmest and most generous people you’re likely to encounter, along the way and stay much longer than anticipated...

KALKKOP HISTORY
Kalkkop is a lonely, low limestone hill amongst the sandstones and mudstones of the Karoo, approximately 50km due south of Graaff-Reinet (photo above). Only in the 1940’s aerial photographs revealed that it is a perfectly circular white deposit, approximately 640m in diameter, so it was thought to be a crater of volcanic origin. In 1947 a mining company drilled a borehole on Kalkkop that revealed it contains remarkably pure limestone down to a depth of about 90 metres. Below that lie fragmented, but typical Karoo rocks.

That the Kalkkop is an impact crater was not suspected – even many of the craters on the Moon were initially thought to be of igneous origin. Then space exploration changed everything. In 1969 man landed on the Moon and brought back rocks. Soon it was realised that the craters there are not formed by volcanism but rather by impacts. Gradually geologists became aware of the ubiquity of impact craters in the Solar System and techniques were developed to identify impact structures on Earth.

In 1992 another borehole was sunk in the centre of the Kalkkop crater to a depth of 152m. Drilling was suspended for a while and later extended to 380m. The drill core revealed that below the limestone lie breccias, rocks made up of angular fragments of pre-existing rocks, a unique characteristic of impacted rock, thus confirming it as an impact crater.

KALKKOP’S FORMATION
Two hundred and fifty thousand years ago a meteorite, estimated to have had a diameter of about 20m, hit the unsuspecting Karoo plains and formed a bowl-shaped crater. Rainwater dissolved CaCO₃ from the surrounding Karoo rocks and accumulated in the bottom of the crater to form a shallow, brackish lake. During warm, drier spells the water would then evaporate, leaving behind their calcareous mineral load. In this way layer upon layer of white limestone was gradually deposited, until the crater was filled. Subsequently the Karoo became drier and the crater with its limestone fill and surrounding areas began to erode away. Because the limestone weathers slower than the surrounding Karoo rocks it resulted in the low limestone hill we see today (see schematic representation below).

Analysis of the 1992 drill core included a world first - a technique to confirm its meteoritic impact origin was used for the very first time on the Kalkkop crater’s rocks. Abundances and isotopic ratios of osmium and rhenium were measured, both for the breccias and for the Karoo sandstones and mudstones surrounding the area. The breccias below the limestone showed osmium isotope ratios closer to that expected for meteoroids than for typical Karoo rocks, indicating the presence of an extra-terrestrial component in the breccia. The meteorite itself would have exploded and vaporised on impact, but a small amount of meteoritic vapour would have been incorporated into the fragmented target rock, which formed the breccia. This was the first time that this, now well-established, “osmium isotope systematics” technique had been used to confirm the impact origin of a crater structure anywhere on Earth.

METEOROIDS, ASTEROIDS AND IMPACT CRATERS
A meteoroid is simply a small rocky or metallic body travelling through space that ranges in size from small grains to 1 meter-wide objects. Traditionally anything larger than this is called an asteroid. The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter and are thought to be remnants of the formative period of the Solar System. This belt is estimated to contain almost 2 million asteroids larger than 1km in diameter, a further ~30 million larger than 100m and many millions of smaller ones. Most meteoroids are fragments from asteroids or comets, while others are collision impact debris ejected from very large bodies such as planets or moons.

When a meteoroid enters the Earth's atmosphere at a speed typically >20 km/s, aerodynamic heating produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. This phenomenon is called a meteor, or colloquially a "shooting star" or "falling star". Larger incoming objects, such as asteroids or comets, often explode in the atmosphere and burn out before they can reach Earth’s surface. If a meteoroid, comet or asteroid or a piece thereof withstands ablation from its atmospheric entry and impacts with the ground, then it is called a meteorite.

If the meteorite is large enough the collision with the ground will create an impact crater. South Africa has only four confirmed impact craters that are listed in the “Earth Impact Database” (“and mapped here”). The Vredefort crater (or Vredefort dome) is probably the best known and has World Heritage Site status. It is also the oldest and largest crater in South Africa - thought to have been created by a hefty asteroid almost 10km across! In contrast, few people know about the Morokweng crater that is invisible to the naked eye, as it is buried underneath the Kalahari sands. The Kalkkop and Tswaing craters are the two babies in the South African crater family, both in terms of age and size.

Below are Google Earth photos of the three visible South African craters; Kalkkop, Tswaing & Vredefort. Note the different scale of the Vredefort photo (image from 90km above Earth while the other two are from only 3km).

Because of the relative difference in their numbers, there is an inverse correlation between the size of an object and the frequency with which such an object will hit Earth, i.e. the larger the object the more unlikely it is to hit Earth. Small objects frequently collide with Earth. Stony asteroids with a diameter of around 4m impact Earth approximately once per year. Asteroids with a diameter of 7m enter Earth's atmosphere with as much kinetic energy as Little Boy (the atomic bomb dropped on Hiroshima, ~16 kilotons of TNT) about every 5 years. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized. Objects with a diameter of roughly 50m strike Earth approximately once every thousand years, producing explosions comparable to the one known to have detonated roughly 8.5km above Tunguska, Siberia in 1908.

The most recent, significant meteor event occurred when a near-Earth asteroid entered Earth's atmosphere over Chelyabinsk, Russia on 15 February 2013, with a speed of ~19km/sec (68,000 km/h), almost 60 times the speed of sound. The light from the meteor was brighter than the Sun and eyewitnesses felt intense heat from the fireball (see photo below). Due to its enormous velocity and shallow atmospheric entry angle, the object exploded in an air burst over Chelyabinsk Oblast, at a height of around 30km. The explosion generated a bright flash, producing a hot cloud of dust and gas and many surviving small fragmentary meteorites, as well as a powerful shock wave. With an estimated initial mass of about 12,500 metric tonnes and measuring about 20 metres in diameter, it is the largest known natural object to have entered Earth's atmosphere since the 1908 Tunguska event that destroyed a wide, remote, forested area of Siberia. The Chelyabinsk meteor is also the only meteor confirmed to have resulted in a large number of injuries.