Tswaing

Key facts

Country

South Africa

Diameter

1.13 km

Age

about 220,000 years

Location

25.41° S, 28.08° E

Tswaing Crater lies about 40 km north-west of Pretoria in South Africa. It is one of the best-preserved young impact craters on Earth and one of the most accessible, with a museum and walking trails. The name Tswaing means "place of salt" in Setswana, a reminder of the salty lake on its floor.

The impact. About 220,000 years ago a meteorite, probably a stony body a few tens of metres across, struck the granite of the region. It formed a bowl about 1.13 km wide and originally perhaps 200 m deep. The crater has since been partly filled with sediment, and the floor now lies about 100 m below the rim. The rim rises about 60 m above the surrounding land.

Salt and soda. A shallow lake on the crater floor is fed by groundwater and rain. Evaporation concentrates the dissolved minerals, making the water very salty and alkaline, rich in sodium carbonate (soda) and sodium chloride (salt). Local people collected salt from the lake for centuries. In the early twentieth century a soda works operated in the crater, extracting soda for industry, and the remains of the works can still be seen.

A long debate. For many years Tswaing was thought to be volcanic, perhaps a collapsed volcanic vent. The question was settled by a scientific drilling project in 1988, which drilled about 200 m into the crater floor. Beneath the lake sediments, the core reached broken granite and impact breccia containing shocked quartz and fragments of melted rock. Chemical traces in the breccia suggested the impactor was a chondrite, the most common type of stony meteorite.

A climate record. The thick lake sediments recovered by drilling turned out to be a valuable archive. They hold a record of changing rainfall and climate in southern Africa over about 200,000 years, preserved in layers of mud, pollen and minerals.

Visiting. Tswaing is a protected heritage site with a small museum and a trail of about 7 km that leads from the visitor centre over the rim and down to the lake. The crater is a pleasant place to walk, with bushveld vegetation and many birds, especially around the lake. Visitors should stay on marked paths and not remove rocks. Guided walks are offered, which are a good way to learn about the crater's geology and its human history.

Why it matters. Tswaing is one of the few young craters made by a stony impactor rather than an iron one. Stony meteorites are more likely to break up in the atmosphere, so a crater of this size from a stony body shows that it must have been large and strong enough to survive its fall largely intact.

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