The reserve is based predominantly on two geological formations, the Lubombo Mountains, being made up of rhyolites, and basalt valleys to the west.
The geology of the area is associated with the sedimentation of the supercontinent Gondwanaland which existed when all the southern continents were in one mass, with southern Africa as the centroid. The sequence known as the Karoo supergroup began about 300 million years ago when sediments started to form in a basin (Wilson 1980).
Approximately 200 million years ago, these sediments were capped by volcanic rocks which extruded onto the land surface (the Karoo volcanics). The first to appear were basalts which make up the western lowlying margin of the reserve. These unusual basalts, low in olivine and silica and therefore basic, are terrestrial. They formed as huge sheets of lava extruded by means of fissures over many kilometres north to south. The theory is that at some point, continental material became included within the magma and extruded explosively at high pressures in the form of welded volcanic ash (tuff). This ash originated from the line of volcano stretching north to south from eastern Zimbabwe to northern Natal and now forms the upstanding ridges and high ground of the reserve.
The general description of the Lubombo volcanics was given by du Toit in 1930, but the most recent work has been by Cleverly in the late 1970's (Cleverly 1979). In Cleverly's terminology (1979) he gives the name Twin Ridge Beds to the volcanic geology of the Lubombo. The main Lubombo Escarpment is made up of the Jozini Rhyolite Formation, followed to the east near the Mozambique border by the Umbuluzi Rhyolite Formation and the Oribi Rhyolites (Ndzindza). The basalts formed as a supurating lava subaerially extruded, which oozed across the surface from elongated vents.
The rhyolites, being explosive rocks, are possibly formed by hot rushing clouds of molten rock and gas. They may also include the flows of high temperature magma. The rhyolites are extruded periodically and give rise to various flow banded features which may be representative of several dozen explosive incidents. Each incident begins with the initial explosion and magma flow producing a fine crystalline rock at ground level. The surface of the massive magma flow that follows the surface is in contact with the air and cools more rapidly. It solidifies but then becomes rebroken by the flow beneath it, producing "lava concrete" (autobreccia). A number of outcrops of this can be seen in the reserve.
In addition, the rhyolites also include various air fall debris such as lapilli (small stones), silica raindrops and others siliceous ejectamenta including obsidian (black volcanic glass). Occassionally, the frothy surface of the flows includes gas bubbles which later solidify as geodes. The whole sequence testifies to a period of great geological violence heralding the break up of the supercontinent Gondwanaland and the establishment of Africa as a separate continent. Remarkably, small sedimentary basins of muds and sands were formed in between rhyolitic flows and, where these are preserved, fossils occur in the sediments.
In terms of historical geology, the Karoo volcanics are significant to have occurred as the eastern margin of Africa was being formed. India, Antarctica, and Australasia were being pushed away along a tearing fracture from what today is the eastern edge of the continent. The Lubombo mountains provide insight into the process of continental formation due to the fact that they have been stable since this break up (190 million years). There is evidence of subduction along the plate margin after formation, and rhyolites have been noted at great depth from coastal boreholes in a vertical attitude, off northern Natal.
The different chemical composition of the basic basalts, acid rhyolites, and intermediate dacites are the controlling feature in the dramatic terrain of the reserve. The basalts, being low in silica, are chemically weak and therefore have been susceptible to deep weathering in the Late Tertiary Period. The rhyolites, being rich in silica, are chemically very resilient and therefore have eroded to a much lesser degree. Once continental formation had taken place, the interbedded layers of basalt, dacite, and rhyolite were downwarped to the east forming a monocline. The easily weathered basalts were then etched and eroded out from the sequence, leaving the ridges of rhyolite as the ribs of the strata.
On the western side of the reserve the lowlying relatively flat topography of Mbuluzi Nature Reserve and Simunye Nature Reserve is underlain by basalts. A minor flow of rhyolite produces the ridges adjacent to southern Mbuluzi Nature Reserve along the line of Lusoti hill. A switch from dacite to basalt, when weathered and removed, exists in the south. A further switch back to the massive flows of rhyolite produces the precipitous slopes to the east of the Siphiso and the high ground of Ndzindza.
In terms of the outcrops of rock, the basalts rarely are visible at the surface. They may be seen in a somewhat degraded form along stream beds, particularly the Siphiso near the restcamp, where it has a drab greenish grey colour with white spherules. The rhyolites outcrop is more obvious with ridges covered with angular rhyolite cobbles, varying in colour from maroon brown to reddish grey, with obvious orange crystals of felspar (Urie 1967, Wilson 1980).
Cleverly, R.W., The Volcanic Geology of the Lebombo Monocline in Swaziland. Trans. Geol. Soc. S. Afr. 82(343-348), 1979.
Uric, J.G., Explanatory Notes to Accompany Sheet 2631BD(14)
Geological Survey and Mines Department, Swaziland, 1967.
Wilson, A.C., Explanatory Notes to Accompany Sheets 2631BB(8) and 2632AA, Geological Survey and Mines Department, Swaziland, 1980.