Geology of Zimbabwe

With the array of beautiful stones available across Zimbabwe it seems almost inevitable an art movement should arise from these raw materials. In order for you to really understand the basis of this art it’s necessary to understand the geology of Zimbabwe and the Great Dyke.

The Zimbabwe Craton

A craton is the ancient “core” of a continent, a piece of the continental crust so thick that it cannot be destroyed by the movement of tectonic plates. Africa contains five such cratons mostly made up of igneous rocks such granite, which is why so many huge granite outcrops can be seen across Zimbabwe. These rocks of the Zimbabwe craton are estimated to be 3.46 billion years old having been stable for the last 2.5 billion years.

Geology of Zimbabwe
Granite outcrop of the Zimbabwe craton.

As a result of this thick craton central Zimbabwe forms a high plateau between 1000 and 1600 meters altitude. This provides a subtropical highland climate which receives abundant sunlight without the searing heat associated with lowland areas at these latitudes such as in the Zambezi and Limpopo valleys in the north and south of the country respectively.

Incidentally the massive heat and pressure found at the base of cratons provides the conditions in which diamonds are formed. These are later brought to the surface by volcanic eruptions, explaining why there is such a wealth of diamonds found in Africa.

The Great Dyke 

The formation of the Great Dyke, source of the majority of sculpting stone, occurred just after the stabilisation of the Zimbabwe craton some 2.5 billion years ago. Considering the earth is known to be around 4.5 billion years old, it really is amazing to think your sculpture was formed so long ago!

The Great Dyke is a linear formation of ultra-mafic igneous rocks stretching roughly north to south some 340 miles across the country, passing just west of Harare. In simple terms these ultra-mafic rocks formed from magma which rose from beneath the craton intruding into massive faults within the Zimbabwean granite. This formed a ridge up to 460 meters high and between two and seven miles wide.

Geology Zimbabwe
Satellite image of the Great Dyke (NASA, 2002).

Over time these igneous rocks were altered through a metamorphic process known as serpentinisation. During this process the rock is subject to high pressure and low temperature in addition to large amounts of water being absorbed. This destroys the mineral structure of the rock, forming serpentine and other new minerals including iron.

Mtoroshanga Pass, Great Dyke Zimbabwe
The Mtoroshanga Pass in the Great Dyke (BT Wursten, 2005).

The result of these incomprehensibly long processes is stone perfect for sculpting. Soft enough to sculpt by hand while still hard enough to last a lifetime. Without large crystals these stones can be polished by hand to be incredibly smooth, taking on a brilliant shine and revealing the colours within after wax is applied.