Welcome to my Geo-Tour
Ancient Rocks
Here I present pictures and offer explanations of the geology of various
areas I have visited or have read about and found of interest. I am fascinated
with ancient rocks, particularly those of Archaean age. Some of the rocks
exposed in Southern Africa are amongst the oldest in the world. Send me pictures of ancient rocks from your
country and I will gladly publish them on this site.
South Africa's Tswaing Meteorite Crater
See an aerial photo of the Tswaing Meteorite north of Pretoria
Tswaing Meteorite Crater
Click on any one of the links below to take you to the Geo-Tour from 4.4 to 1.4billion years of
South African and Australian rocks
The Earliest Piece of the Earth - 4.4billion year old zircons found in the
Western Australian desert.
Archean Granites of Johannesburg - 3.2billion years old
Malmani Dolomites - 2.2billion year old sedimentary rocks bearing early evidence
of life
Evidence of early aerobic life altering the early earths atmosphere - precipitation
of iron oxides
Fossil ripple marks in an ancient river bed - the Waterberg
Pilanesberg Alkaline Complex - 1.4billion years old
in search of those elusive 3 billion year old zircons ...
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The Earliest Piece on the Earth - 4.4billion year old zircons found in the Western Australian desert
The Western Australian Desert
Outcrop photographs of the Jack Hills, Western Australia showing the outcrops where a 4.4Ga zircon sample was collected from interlayered red quartzites and metaconglomerate. (SA Wilde, unpublished photographs). Read the whole story by clicking here.
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Archean Granites of Johannesburg - 3.2billion years old
The Norscot and Lonehill Koppies (a Koppie in the Afrikaans language means a hillock or small hill) form a number of Archaen granite tor clusters developed in the central and southwest parts of the Johannesburg granite dome which is 3.2billion years old.
The Lonehill Granite Tor
The Tors vary in composition and texture but generally consist of medium to coarse-grained homogeneous granodiorites and adamellites. At the Lonehill Koppie the tor consists of medium to fine grained grey granodiorite with specks of biotite (dark mica). The present shape of the hills is the product of differential weathering along the faults, fractures and joint blocks; the inner portions of the blocks became the present hill.
Archaean granite making up
the granite tors is composed of fine grained grey granodiorite with specks of
biotite mica
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Malmani Dolomites - 2.2billion year old sedimentary rocks bearing early evidence
of life
Fossil Ripple Marks from the Malmani Dolomites
The Malmani Dolomites are very extensive in large areas of
South Africa and represent the remains of an ancient inland sea in which cyanobacteria
were in the process of releasing oxygen into the atmosphere of the early earth
and in so doing removing carbon dioxide from the atmosphere and precipitating
it as carbonates. The Malmani Dolomites are around 2.2 billion years old.
The picture shows Fossil Ripple Marks from the Malmani Dolomites
at Toms Trading store on the R511 route to Hartebeespoort. The picture depicts
ripple marks and these occur above the stromatolite layers. It is to be
expected that ripples will be preserved with them, as stromatolites originate
in relatively shallow water.
The ripples are composed of interference ripples. Here, instead of
having parallel trains of ripple crests, two different motions in the water,
either caused by waves and/or currents, have generated more than one direction
of hydrodynamic influence. So one set of ripples first formed, and then a
second set was superimposed on the first, from a different current direction,
causing the second set to interfere with the earlier formed ones.
The ripple marks are symmetrical in form In both cases, the water was fairly
shallow, probably less than a few meters deep. Thank
you to Bruce Cairncross of the Rand Afrikaans
University for the explanation of the fossil ripple mark formation.
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Evidence of early aerobic life altering the early earths atmosphere - precipitation
of iron oxides
Mining operations at the Thabazimbi Iron Ore Mine
Due to the increasing oxygenation of the early atmosphere iron precipitated out of the earths early oceans which resulted in the accumulation of enormous deposits of iron oxides in many areas as found here at Thabazimbi, South Africa. To think that simple life forms could drastically alter the earth’s early atmosphere, which was essentially devoid of oxygen, to an atmosphere rich in oxygen illustrates the enormous influence simple life forms had over our planet during its early history.
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Fossil ripple marks in an ancient river bed - the Waterberg
Fossil ripple marks in sandstone of the Waterberg formation
About a 2-hour drive north of the city of Pretoria one comes across a range of ancient mountains known as the Waterberg. During a recent trip to this area I found a lot of evidence of ancient rivers and lakes in the form of fossil ripple marks. It thought it extraordinary that I was looking at a ripple mark created in a riverbed more than 1.8 billion years ago, at a time when the only life on earth was cyanobacteria.
Sedimentary Rocks of the Waterberg Mountains
The Waterberg Supergroup rocks
are a sequence of conglomerates, sandstones and shales. Ripples only form in
the finer grained rocks, i.e., the mudstones to medium-grained sandstones. The
photo shows what are called straight-crested to sinuous-crested ripples.
They
are also symmetrical ripples - the upstream and downstream side is a mirror
image of one another. This is indicative of ripples formed in relatively
shallow water by the action of waves moving on the water surface. The orbital
motion of the water, generated by the surface waves, moulds the sand on the bed
below into the ripples. The age of the Waterberg rocks is circa. 1 800 million
years (Proterozoic Age). Thanks to Prof. Bruce Cairncross for the explanation
of how these ripples formed.
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Pilanesberg Alkaline Complex - 1.4billion years old
The Pilanesberg Igneous Intrusion as seen from Lenong View
The Pilansberg Complex is an
almost circular igneous complex of 530 square kilometer in plan area and lies
at the junction of the acid phase and basic phases of the Bushveld Complex
north of Rustenburg. The complex consists of partly denuded alkaline volcanics. Intruded by ring structures
around a central core. The intrusive core and rings are dominantly nepheline
syenites, some of them have pronounced trachytoid textures called Foyaites.
The
sequence of emplacement of the complex involved: initial volcanic action, followed
by repeated ring-fracturing alternating with
the emplacement of the various intrusive members of the complex. Fracturing and
collapse resulted from the subsidence of magma in the underlying reservoir,
while intrusion correspondend with resurgence of the magma. This mechanism has
given rise to the formation of ring dykes and cone sheets. Later, a number of
dykes, of widely varying compositions, were emplaced radially with respect to
the complex and the complex itself was considerably distributed by faulting.
Based on radiometric dating it is believed that the complex is 1.4 billion
years old. Information courtesy of
Jos Lurie and taken from his book South African Geology, Eight Revised Addition.
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Tswaing Meteorite Crater
The Tswaing Meteorite Crater
Around 220 000 years ago a meteorite traveling at hypervelocity slammed into the Earth’s crust causing a violent explosion and creating an impact crater. The near circular crater has a 1.13 km diameter and is well preserved in the pinkish Nebo Granite which is the main granite type of the Bushveld Complex. The crater is situated north of the city of Pretoria.
