Science and Technology Education Centre
(STEC@UKZN)

Drakensberg and Lebombo Groups

lavas flood the continents

Map Lebombo Drakensberg

The extensive lava field probably spread across much of Gondwana. A 1.5 kilometre thick accumulation of Jurassic age basalt flows can be seen along the Drakensberg Escarpment.

Dramatic outpourings of lava spread across much of Gondwana about 180 million years ago heralding the start of Gondwana breakup. Remnants of these once extensive lavas now form the Lesotho highlands and Lebombo mountains.

Uplift

1

Gondwana is uplifted and faulted along along the boundary between Africa and Antarctica. This causes rifting and the continents separate.

Rift widens

2

The rift widens and oceanic crust begins to form between the continents. Sediment is deposited in the newly formed Indian Ocean.

Mid oceanic ridge

3

Faulting ceases along the continental margins which are draped by sediment from rivers. New oceanic crust is continuously formed at the mid-ocean ridges.

Sills are horizontal and dykes are vertical intrusions of igneous rocks. Dolerite sills are common throughout inland KZN in sedimentary rocks of the Karoo supergroup.

Drakensberg Amphitheatre
The 'High Berg' is made up of basalt lavas from the Drakensberg group. Amphitheatre, Drakensberg (Photo: T. Reinhardt)

The magma (molten rock) made its way to the surface along a complex system of fractures. Crystallisation of magma within these fractures formed dolerite sills and dykes.

Dolerite dyke in basalt lava (move your mouse here)

The sills often form flat areas and weather to form a very dark red soil. A sill may also form resistant cliffs, such as at the Howick falls.

Howick Falls
Howick Falls

Photo source: https://en.wikipedia.org/wiki/Howick_Falls#/media/File:Howick-Falls-November-2009.jpg

In the Lebombo mountain the eruption of basalt was followed by another phase of volcanic activity involving the explosive eruption of rhyolites and huge quantities of volcanic ash. About  4800 m of volcanic material accumulated during this second phase of activity which now froms the Lebombo mountains. These volcanic events were followed by uplift and faulting that eventually separated Africa and Antarctica.

Lebombo Mountains
Lebombo mountain, south of Jozini
Lebombo Mountain Mkuze Falls
Lebombo mountain and Mkuzi falls, south of Jozini
General mineral composition of igneous rocks and their silica content

Basalt (igneous – volcanic)

Volcanic eruption

Basalt

The basalt was extruded at the surface as a volcanic eruption of lava. It is equivalent to dolerite (see chart above), except that it cooled quickly making the crystals very small (i.e. fine grained). The typical white to greenish blebs in the basalt were formed later when the minerals infilled preserved gas bubbles. Basalt is the most common rock on the Earth surface. It is dark coloured, fine-grained basic (low silica (45-52% and relatively high calcium, iron and magnesium) volcanic rock, composed mainly of the minerals calcium plagioclase and pyroxene, usually augite, with or without olivine. 

Dolerite (igneous – volcanic)

Volcanic eruption

Dolerite

Dolerite is a medium- to fine grained, dark crystalline rock which formed underground when lava feeding the volcanoes cooled in its feeder pipes – sills (horizontal) and dykes (vertical). Dolerite dykes and sills are very common, often seen intruding other rock layers. Because of its high iron content, dolerite weathers to a bright red soil. Dolerite is the medium grained equivalent of basalt and gabbro.

Rhyolite (igneous – volcanic)

Volcanic eruptions

GEM_KZNGeolrhyolite

Rhyolite lavas are fine grained, but differ from basalt in their lighter colour (normally light pinkish) due to a higher silica and lower magnesium and iron content than basalt lavas (which are dark). Some rhyolite lavas will have larger crystals of quartz and feldspar in a finer-grained groundmass.

When rhyolite lava erupts on the surface , smaller crystals often respond to the flow by aligning themselves into bands. This effect is called flow-banding.