The Great Dyke

The Great Dyke (or Dike – American spelling) is a linear geological feature of layered intrusion of igneous, metal-bearing rock that runs north-south through the centre of Zimbabwe and is the most outstanding feature of the country’s map.

Appearance

The Dyke’s edges are sharply defined by two wide wetlands flanking each margin, which together with the linear nature of the Dyke make it a most noticeable feature when viewed from above. The Dyke spans approximately 550kms, and it varies along its width from about 3 to 12 kms making it one of the largest dyke-like layered volcanic intrusions on the planet and the longest linear mass of mafic rocks in the world. (“Mafic” describes a group of dark-coloured igneous rock that are high in magnesium and iron. The word ‘mafic’ is a combination of the words “magnesium” and “ferric” i.e. iron.)

Geological History of the Great Dyke

The Dyke is approximately 2.5 billion years old, making it a significant geological event in the Archaean Eon. (The Archaean Eon, lasting from roughly 4 billion to 2.5 billion years ago, is a period in Earth’s history which included the formation of Earth’s early crust, the emergence of the first continents, the development of the first life forms, and a vital shift in atmospheric composition.) 

The structure of the Dyke is a result of shifts in the earth’s crust and the outpouring of successive layers of molten lava over a number of centuries. ​It has been pushed up through the core of the oldest rocks on earth which form the continent of Africa (called the African craton).

In cross section, the Great Dyke looks somewhat triangular or keel-shaped or Y-shaped because of the action of lava having been squeezed up along deep faults or cracks in the earth’s crust.

History of Human Interest in the Great Dyke

• 1870 First documented in a sketch map by Carl Mauch
• 1897 The Dyke is clearly portrayed as an intrusive body on the Fletcher and Espin map of Matabeleland Province
• 1912  Named “Great Dyke” by Zealley. “The Selundi Hills and both valleys of the Tebekwe River are occupied by a portion of a large basic intrusion for which the name ‘Great Dyke’ is suggested” 
• 1914 Wagner suggested that the structure was not a dyke but an ‘elongated laccolite’.
• 1918 The Great Dyke was identified as a source of platinum – but mining of this mineral did not start until 1924.
• 1958-1960 Worst produced a fully comprehensive account of the Great Dyke was published. This has been followed by studies followed by various researchers on geochronology and mineralization.
• 1998 Worst attempted with very little success to rename the structure the ‘Great Graben’ saying, “The incorrect …term has been applied … since 1908. The term ‘dyke’ gives a completely incorrect and misleading geological connotation to this world-famous geological phenomenon”.  The term dyke has been retained.

The Great Dyke is a geological feature that is of great interest to geoscientists due to its enormous size, huge mineral resources and complex and enigmatic geological history. It is clearly visible from outer space.

Astronaut photograph ISS025-E-5538 was acquired on September 30, 2010, with a Nikon D2Xs digital camera using a 180 mm lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image was taken by the Expedition 25 crew.

Mineral Wealth

The economic importance of the Great Dyke cannot be overstated.

• Platinum group metals (platinum, palladium, rhodium, ruthenium, iridium, osmium) – the second-largest reserves of in the world after South Africa, much of it concentrated in this geological belt.

• Chrome ore (10 billion tonnes in 11 seams that are the thickest and richest on earth) mined throughout the Dyke, especially in the Darwendale, Lalapanzi and Mutorashanga areas. The three largest chrome mining companies are Maranatha, Ferrochrome, Zimalloys and Zimasco, although most mines are worked by the tribute system.

• Nickel – vast reserves over a 100 km stretch of the northern part of the Dyke.
• Asbestos – high-quality chrysotile mined from Ethel Mine
• Magnesite – workable deposits in southern part of the Dyke.
• Gemstones, including agate and chrysoprase
• Gold
• Copper,
• Cobalt,
• Iron,
• Vanadium,
• Silver,
• Tin

Mining

While the Great Dyke and its metal ores are products of geologic processes from the deep past, more recent mining events have also left their mark on the landscape.

Environmental and Social Impact

Mining along the Great Dyke provides significant employment and foreign currency earnings for Zimbabwe. However, extraction has environmental challenges:

• Open-pit and underground mining can cause habitat loss and soil contamination.
• Smelting operations release sulphur dioxide, contributing to air pollution.
• Waste rock and tailings may leach heavy metals into rivers.

Sustainable mining practices, environmental rehabilitation, and community engagement are crucial to ensuring the Great Dyke remains both an economic and ecological asset.

The Dyke and Dam Sites

Darwendale Dam (also known as Lake Manyame)

The Dyke is a barrier to ground water, making the edges marshy and the source of many springs and streams. Darwendale, Ngezi and Sebakwe Dams use natural sites along the Dyke. The Dyke passes through several significant cities and towns, such as Harare, Mhangura, Guruve, Mberengwa, Banket, Chegutu, Kadoma, Kwekwe, Gweru, Shurugwi, and Zvishavane.

Vegetation of the Great Dyke

Soils on the Great Dyke are strongly influenced by the underlying bedrock. Shallow, infertile soils over magnesium-iron bearing (mafic) rock dominate the northern and southern ends of the Dyke. The mafic rock in the central area produces reddish-brown granular clay soils which are best for agriculture.

The vegetation along the Great Dyke is generally open, ranging from grassland to shrubland, with riverine forests along watercourses. It is characterized by combinations of plants adapted to the unique chemical compositions of mineral-rich soils from the breakdown of primarily mafic rocks, especially serpentine (which makes up 70% of the Dyke area) which creates soils with chemical compositions that influence a vegetation that often contrasts sharply with the vegetation of surrounding areas. These soils, particularly those with high nickel concentrations, support a distinctive flora that includes numerous endemic species, some of which are listed below: 

• Euphorbia wildii: An endemic species first described by Professor Hiram Wild. 
• Aloe ortholopha: Another endemic, with a distinctive branching pattern. 
• Ozoroa longipetiolata: A tree species also found on the Great Dyke. 
• Pearsonia metallifera: A plant known for accumulating high levels of nickel. 

One very special aspect of the northern end of the dyke is two small stands of raphia palms (found no where else in Zimbabwe), protected in Tingwa and Miware Raphia Palm Botanical Reserves.

The northern part of the Great Dyke exhibits distinct botanical differences compared to the southern part, which is why Nyamaneche Sanctuary (in the north) and Sebakwe Great Dyke Botanical Reserve (near Kwekwe) are necessary protectors of the unique vegetation areas.

Conclusion

The Great Dyke of Zimbabwe is more than a geological curiosity. It is a 2.57-billion-year-old legacy of Earth’s formative processes and the foundation of Zimbabwe’s mining industry. Balancing economic exploitation with environmental stewardship will ensure this geological wonder continues to benefit science and society for many generations to come.