Lebombo’s Rhyolite
The Lebombo Mountains, also called Lubombo Mountains, is
a long, narrow mountain range in South Africa, Swaziland, and
Mozambique, southeastern Africa. It is about 500 miles (800 km)
long and consists of volcanic rocks. The name is derived from a
Zulu word, Ubombo, which means “big nose.” In South
Africa the mountains extend from south of the Mkuze River
(KwaZulu-Natal province) north into Kruger National Park (Limpopo
province). The Lebombo Mountains form the boundary between the
province of KwaZulu-Natal, South Africa and Swaziland, between
Swaziland and Mozambique, and between Mozambique and the South
African provinces of Mpumalanga and Limpopo, extending north of the
Olifants River. The average elevation of the range is about 1,970
feet (600 meters) above sea level; Mount Mananga, on the border
between Mpumalanga province and Swaziland, rises to about 2,500
feet (760 meters). A number of rivers, including the
eastward-flowing Mkuze, Olifants, Pongola, Ingwavuma (Ngwavuma),
and Usutu, cut their way through the range, and the latter two have
formed especially spectacular gorges. An immense storage dam has
been built in the Pongola gorge. The vegetation on the range is
mostly tropical forest and savanna, with ironwood and ebony on the
better-drained slopes. In the narrow ravines, tree growth is dense
and includes the large khoya tree, which resembles mahogany.
Volcanic rocks of the Karoo sequence form a remarkable if not
unique geological feature along the eastern margin of southern
Africa, where they are preserved in the Lebombo Monocline. The
monocline is first evident to the southwest of the town of
Empangeni in Zululand, South Africa, and from there extends
northwards to the Zimbabwe border, a distance of approximately 750
km. In Zimbabwe a similar sequence of Karoo volcanic rocks is found
in the Nuanetsi-Tuli Syncline, and along the Mateke-Sabi monocline
(Cox et al., 1965). The volcanic rocks described here comprise that
part of the Lebombo south of the Swaziland border and represent a
small, arbitrary portion (- 2 000 km 2) of the major volcanic belt.
In spite of frequent citation in the literature as a monoclinal
structure containing a thick, bimodal assemblage of basic and acid
rocks (LOMBARD, 1952; STRATTEN, 1970; COX, 1972) the southern
Lebombo and Lebombo province as a whole, has received little
detailed attention since the findings of Du TOIT (1929) were made
known.
Rhyolite can be considered as the extrusive equivalent to
the plutonic granite rock, and consequently, outcrops of rhyolite
may bear a resemblance to granite. Due to their high content of
silica and low iron and magnesium contents, rhyolite melts are
highly polymerized and form highly viscous lavas. They can also
occur as breccias or in volcanic plugs and dikes. Rhyolites that
cool too quickly to grow crystals form a natural glass or
vitrophyre, also called obsidian. Slower cooling forms microscopic
crystals in the lava and results in textures such as flow
foliations, spherulitic, nodular, and lithophysal structures. Some
rhyolite is highly vesicular pumice. Many eruptions of rhyolite are
highly explosive and the deposits may consist of fallout tephra or
of ignimbrites.
Rhyolite is a fine-grained light-colored acidic volcanic rock.
Rhyolite is chemically the equivalent of granite, and is thus
composed primarily of quartz and orthoclase feldspar with
subordinate amounts of plagioclase feldspar, biotite mica,
amphiboles, and pyroxenes. Rhyolite lava exhibits a typical banded
structure produced by its flow pattern. Rhyolite lavas occur in
continental and submarine volcanoes, especially island arcs, and in
igneous dikes. Rhyolite lavas are typically highly viscous and are
explosively ejected from volcanoes.
The Rhyolitic Lava Flows of the Lebombos
Along the Lebombo monocline acid and basic magmas extruded
alternatively as fissure eruptions, to a thickness of approximately
12 km during the time interval from the Triassic/Jurassic boundary
to the Cretaceous.
Textural evidence suggests that the rhyolites were emitted as lava
flows. The rate of cooling or the grade of crystallization,
respectively, produced a series of textural zones. The upper parts
of the lava flow-units are intricately flow folded. It is
postulated that the Lebombo rhyolites were generated in the upper
mantle.
The Lebombo belt represents a striking volcano-tectonic structure
near the south-eastern margin of the African continent. Over an
almost N-trending distance of about 700 km and a maximum width of
40 km, a thick pile of eastward dipping volcanics of late Karroo
age (Stormberg-formation) form a monocline. This monocline marks
the southward extension of the African Rift system. The igneous
activity took place over a period of 70 m.y. stretching from the
Upper Triassic to the end of Jurassic and continued with small
alkaline intrusions into the Cretaceous (VAIL, 1968).
In the cross-section of North Swaziland-Lourenzo Marques the
volcanic sequence of nearly 13 km thickness comprises tholeiitic
basalts at the base, which are overlain by predominantly rhyolitic
lavas, capped by the upper basalts and intercalated rhyolites of
the Little Lebombos.
A brief account is presented for the Lebombo volcanic succession
which crops out in Natal, South Africa. The volcanic belt is of
late Karoo age and is composed of a thick sequence of basaltic
lavas (Sabie River Formation) overlain by an equally voluminous
succession of acid-flows (Jozini Formation) erupted over a period
of about 70 m.y. Field relationships indicate that the Lebombo
basalt pile consists of simple and compound flow units. The
rhyolite succession consists of thick (80-284 m) flows units
characterised by features found in both ignimbrites and rhyolitic
lavas respectively. It is postulated that they were extruded as
high temperature, low volatile pyroclastic flows. The Bumbeni
volcanic complex which crops out near the southern termination of
the Lebombo Mountains disconformably overlies the Jozini Formation
and is characterized by a suite of rocks that includes rhyolite
lavas, air-fail and ash-flow tufts, syenite intrusions and basic
intermediate lavas. Dolerite dykes are ubiquitous throughout the
succession and an extremely dense concentration of basic intrusions
located along the western margin of the belt gives rise to the Rooi
Rand dyke swarm. Rare sill-forms are found associated with the
mafic volcanics. Acid intrusives are represented by simple and
composite quartz-porphyry intrusions and rhyolite dykes. The
structure of the Lebombo is that of a faulted monocline, tilted to
the east, developed prior to the fragmentation of eastern
Gondwanaland. The volcanic belt is located at the tectonic contact
between two major lh'ecambrian elements, the 3,000 m.y. Kaapvaal
craton to the west and the southerly extension of the 550 m.y.
Mozambique belt to the east. It is bounded to the south by the
1,000 m.y. old Natal-Namaqua mobile belt.
Sources:
Wikipedia
Britannica Online
Springerlink
Remember that it is illegal (and dangerous) to get out of
your vehicle at any place in the Park except at places especially
designated and clearly marked to do so.
To log your find you have to answer the following
questions and email them to me (do not post the answers in your
log):
1. The pathway to the lookout goes through good exposures of
rhyolite. Take a look at the rocks and let me know if you can find
any flow patterns in the rocks.
2. What is the color and grain size of the rocks and do you think
it cooled quickly or slowly?
3. Name at least two (2) kinds of birds you saw in the area and
quote the correct nr. from the painted wall at the lookout.
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