the big one

Warning: This is a geo and history entry. Prepare yourselves to see/ read about me at my geo-histo-geekiest! Also, it’s a long entry so make yourselves comfortable and prepare for an enlightening read.

This weekend Trevor and I went to the Big Hole. I think I’ve mentioned it before, but the Big Hole is the name of the diamond mine (owned by DeBeers) that is located in Kimberely. In fact, the Big Hole is the entire reason that Kimberley exists.

Trevor and I.

In 1866 a young boy, Erasmus Jacobs, picked up a pretty stone while he was playing near the Orange River on his father’s farm. When a friend of the family, Schalk van Niekerk, was visiting the boy’s father the interesting looking stone caught his eye and he offered to buy it. Erasmus’ parents simply gave van Niekerk the stone believing that it was nothing more than a unique pebble. That interesting looking stone turned out to be a 21.25-carat diamond that was later named Eureka.

But it wasn’t until 1871, when two brothers – by the name of De Beers – found an 83.50-carat diamond on the slopes of their farm in Colesberg Kopje, that the rush for diamonds began. This “New Rush” brought thousands of different people to Kimberley. Kimberley quickly became the largest city in the area as waves of immigrants moved to the area from all over the African continent. Kimberley became the first city in South Africa with electricity and it also had other comforts such as a telegraph, a tram system and running water.

A schematic drawing of the Big Hole and some fun facts.

From mid-July 1871 to 1914 (the mine was closed when World War I began), over 50,000 miners dug out the Big Hole with picks and shovels. The big hole covers a surface area of 42 acres and is 463 meters wide. It was dug to a depth of 240 m, but then partially in-filled with debris reducing its depth to about 215m; since then groundwater has filled the hole to a depth of 40 m, leaving 175 m visible. After the mine filled in with debris and water, mining continued and the Kimberley Mine was mined to a depth of 1097 meters.

The Big Hole with the Kimberley skyline in the background.

One of the unique things about the New Rush was that when individuals arrived they could make/ buy individual claims of land on which they could look for diamonds. As a result, Kimberley was built up in an erratic manner around the central diggings. Today the streets of Kimberley still reflect the patterns of erratic mine work (and maintaining a sense of direction while navigating them is a tough feat). During the 1880s however, several individuals began amalgamating smaller mining stakes and companies into large companies. In 1888, Cecil Rhodes bought out his competition and created De Beers Consolidated Mines, which to this day sill retains a monopoly over the world’s diamonds. 

Kimberley certainly has a unique history. But I’m sure you are all wondering – why were so many diamonds found in Kimberley? Well, allow me put my geology cap on and tell you!

Before the discovery of the pipes at Kimberley, diamonds were known only from alluvial river gravels in India, Brazil and South Africa – the Vaal River. The mines of Kimberley showed for the first time that diamonds could also be found in a certain type of volcanic rock that formed steep, deep pipes. Diamonds formed in the mantle are picked up and brought to the surface in magmas that explode as volcanoes.  This rock type, new to geology at the time, was identified and named ‘kimberlite’ by Carvill Lewis, an American geologist.

An enthused me near a pile of kimberlites.

The kimberlite pipes of Kimberley are shaped like carrots underground. In a complete pipe, three distinct zones of slightly different rock can be recognized: an upper crater zone filled with layered volcaniclastic kimberlite (layered VK); a pipe zone where the VK is generally not layered (massive VK); and a deeper root zone filled with magmatic kimberlite (MK) that has not exploded. The latter is the intrusive part of the volcanic complex, while the VKs are the kimberlite magmas that exploded. Kimberlite is an igneous metamorphic rock composed mainly of mantle olivine and other rock and mineral fragments (called xenocrysts and/or xenoliths), all set in a very fine groundmass (also referred to as the matrix). Interestingly, there are no modern kimberlite volcanoes erupting anywhere on Earth today. The youngest kimberlite volcano (Mwadui in Tanzania) is 45 million years old.

Not all volcanoes are sources of diamonds. Diamond-bearing rock as found in kimberlite volcanoes is rare. Magmas that intrude directly from the mantle and intersect thick lithosphere are more likely to contain diamonds. Most volcanoes around the world however, are found in mid-ocean spreading ridges or at subduction zones where the lithosphere is too thin to host diamonds. Kimberlite magma is the result of deep mantle melting – possibly as deep as 300 km – that occurs under thick lithosphere. This unique package of thick lithosphere and deep mantle melting are the conditions necessary for the growth and preservation of diamond and these conditions are only found beneath the oldest continental crust, called Archaean cratons.

I really enjoyed going to the Big Hole. Trevor and I had a good time trying to throw rocks into the Big Hole from the viewing platform – we were told this was impossible and neither of us successfully got a rock in the hole. I invite all of you to come visit and give it a try! I also thoroughly scavenged the ground and took home four pieces of kimberlite (to anyone reading from the WM Geology Department – I promise to bring home some samples for our collection), which I’m sure will not be the only kimberlite rocks that I collect while here.