Tswaing Crater, S. Africa

In March of 2023, I explored the Tswaing Crater, 40 km north-northwest of Pretoria, South Africa, near the major settlement of Winterveld, and within a 1.946 ha conservation area managed by Ditsong Museums, (pictured below).

Public domain map of South Africa
Google Earth map of the location of Tswaing Crater 2023, relative to Pretoria, South Africa.

The Tswaing Crater was formerly known as the Pretoria Salt Pan. For many years it was considered to be a volcanic cone with a sunken floor containing a soda lake.

Google Earth map of the Tswaing Crater 2023
A trail map of the Tswaing Crater that was provided by the Park Ranger.

The crater consists of a ring of rock rising some 60 meters above the surrounding land surface, and it was presumed that the hills were formed by volcanic rock pushed out of the central neck, with the subsequent collapse of the center into the magma chamber.

A schematic east-west cross-section through the bowl-shaped Tswaing Crater, based on evidence from surface mapping and a borehole drilled in 1989, (image taken from “50 Must-See Geological Sites in South Africa”, Whitfield 2015)

However, the rocks forming this ring are granite, a deep-seated intrusive rock approximately three million years old, and not volcanic rock that has erupted at the surface. This crater is unique in South Africa: the central floor lies about 60 meters below the level of the surrounding land surface, and underlying the soda lake area variety of fine-grained muds which contain fragments of rock, some containing magnetite, (an iron oxide).

A view of the Tswaing Crater from the south-eastern rim, (looking northwest from the Shoemaker Viewpoint), in South Africa 2023.

Pictured above, the near-circular Tswaing Crater has a 1.13 km, (from rim to rim) diameter. From this “Shoemaker Viewpoint” the picture is taken 60 m higher than the surrounding landscape and 100 m above the crater floor. The rim is well preserved and consists of a up-tilted granite from the 2.050 million old “Bushveld Complex” or the Nebo Granite.

The schematic representation of the sequence of events during the formation of an impact crater, (image taken from “Tswaing Meteorite Crater” Reimold, Brandt, De Jong & Hancox 1999)

It is now concluded that this bowl-shaped geological feature, (The Tswaing Crater), is caused by the impact of a 50 m chondrite, (stony meteorite), 220,000 years ago, which exploded and vaporized on impact (imaged above). Rocks and gravel were thrown outwards and deposited around the hole, forming the crater rim. Originally the crater floor was about 150 m lower than the present level, however, the past 220,000 years the crater has been gradually filled in by deposits of sand and gravel, coming from the rim due to weathering. The water of the lake is derived from groundwater and rainfall.

The Bushveld Complex covering wide parts of the Northern Province and Mpumalanga Province, (image taken from “Tswaing Meteorite Crater” Reimold, Brandt, De Jong & Hancox 1999)

The Tswaing Crater and the surrounding area consists predominantly of the pinkish Nebo Granite, which is the main granite type of the Bushveld Complex. Large parts of the surrounding countryside are covered by coarse sands and grits derived from recent weathering of the underlying Nebo Granite.

A Nebo Granite sample found on the rim of the Tswaing Crater 2023

The Bushveld Complex is very important in its own right. It is a magmatic structure stretching from Pretoria in the southwest to Phalaborwa in the northwest, and from Middelburg in the south to Potgietersrus in the north. It contains a fill of several kilometers of magmatic rocks which form a sequence of near-horizontally layered rock types. The Bushveld Complex hosts some of the world’s most important ore reserves, including much of the world’s chromium, vanadium, platinum, silver, and tin.

Geological map of the Tswaing meteorite crater; (image taken from “Tswaing Meteorite Crater” Reimold, Brandt, De Jong & Hancox 1999)

Granite is found on the higher areas of the inner and outer rims of the crater. In hand specimens, the fresh granite appears as a coarse grained, pink rock. The largest grains which are pink or light brown consist of the mineral K-feldspar and are interlocked with large quartz grains of milky white of light-grey appearance. Minor amounts of the minerals hornblende and biotite appear as dark specks, (pictured below).

Granite found on the road midway down the Tswaing Crater 2023.

Highly altered granite is also common in the crater and is particularly abundant in parts of the crater rim. It is recognized by its typical whitish to greyish parts of the crater rim. The weathering of rock of the crater rim is the result of surface waters penetrating the granite along fractures and joint planes. The fractures, which are very common in the granite rim, allow easy access for the surface water, (pictured below). At the top of the rim the granite dips outwards at varying degrees, indicating that the rim granite had been tilted upwards. The upper, steepened granite and the Karoo sediments, (younger 220 million year old sedimentary-rocks that are gritty in texture), overlying the granite are also broken by a number of small, nearly vertical, radial faults, which have caused the numerous depressions seen along the rim crest trail.

Highly weathered rim rock, (altered granite) at the Tswaing Crater 2023

At the crater’s rim, the granite weathers into large, rounded boulders or into smaller, angular, crumbly fragments of feldspar and quartz, (pictured below).

Crumbly fragments of granite on the road near the mid-crater wall within the Tswaing Crater 2023.
Section of the crater rim along the access road from the southeast into the Tswaing Crater. (a) upper crater wall showing ejecta breccia overlying Karoo grits, which overlies steeply outward-dipping granite. (b) mid-crater wall exhibiting an anticlinal fold structure and (c) lower crater wall with generally shallow, inward-dipping granite. [Image taken from “Tswaing Meteorite Crater” Reimold, Brandt, De Jong & Hancox 1999]

The mid-section of the rim consists of buckled or folded granite. This fold has been locally fractured and displaced along some small-scale faults. This folded structure, also known as an anticline, is clearly exhibited along the road entering the crater. Folds, like this one, result from initial outward motion: the granite is first driven outwards in a horizontal direction and then deflected upwards towards the surface, (pictured below).

Anticline fold structure in the granite within the mid-crater wall seen on the road of the Tswaing Crater 2023.

Intrusive magmatic rocks are present in many places along the crater rim and are particularly abundant in the northern sector of the rim, with only a few occurrences elsewhere. These igneous rocks include lamprophyre, trachyte, phonolite, and carbonatite. They occur in the form of thin dikes or sills that are fractured and are displaced by small faulting during the cratering event. The intrusive rocks are approximately 1.3 million years old, which are much older than the cratering event, (pictured below).

A lamprophyre sample from a dike, found along the northside of the Tswaing Crater.

Gradually descending under a canopy of lush vegetation, the access road reaches the crater floor and turns towards a narrow strip of land that stretches into the crater lake.

The access road from the rim to the bottom of the Tswaing Crater in 2023

Of particular interest here are the research boreholes, near the center of the lake. It was information that was obtained from this 1988/1989 borehole that led to the conclusion that the Tswaing Crater is an impact, and not of volcanic, origin, (pictured below).

Schematic diagram illustrating the sequence of materials identified in the Tswaing drill core of 1988/1989, (image taken from “Tswaing Meteorite Crater” Reimold, Brandt, De Jong & Hancox 1999).

Pictured above, the drill core obtained from this borehole showed about 90 meters of crater-fill sediments overlying tens of meters of loose suevitic material and fragmented granitic rock. The fragmented rock in the drill core passes gradually downwards into less affected and, finally, non-fragmented rock. In the lower portion of the core, below the crater-fill breccia, only blocks separated by fractures were observed. Many diagnostics of shocked metamorphic-effects were observed, (which are uniquely associated with meteorite impact craters, in material recovered between the 90 and 150 meters depth.

The Tswaing Crater lake in South Africa 2023

Pictured above, the crater lake is saline, as the water entering the crater, (both as run-off from the slopes and from the boreholes (groundwater)), picks up various minerals from the rocks and carries them to the lake where they are concentrated by evaporation. It is the organic material, (fine weathered material and the minerals in the water), that give the lake its dark color and strange odor.

A look at some of the plants within the Tswaing Crater in South Africa 2023.

There is a very large diversity of trees, shrubs, grasses and other floral species, (some 480 plant species), that have been identified so far in the Tswaing Crater. The upper parts of the crater rim are covered by broadleaved woodland, (mainly Combretum trees), and the lower parts by thorny trees, (mainly Acacia woodland). These variations are related to differences in the slope angles at various levels of the crater rim and the types of soils of the crater interior.

Looking up at the southern rim of the Tswaing Crater, (notice the large boulders of granite of the Shoemaker Viewpoint).

Tswaing Crater is one of the world’s best preserved and best studied meteorite impact craters. It is quite clear that the Tswaing Crater is an important geological heritage-site. It has not only been proposed as a National Monument, but has also been selected to become a site museum.

I’m standing at the Shoemaker Viewpoint at the rim of Tswaing Crater in South Africa in 2023.
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