Geology

Plant and animal life can’t exist without all the abiotic factors in the environment. One of the abiotic factors is soil and I asked a local geologist to educate me a bit about the geology of our farm.

The soil is weathered granite and very low in nutrients. Dykes are diabase and about 2900 million years old. Minerals in the dykes are feldspar (white) and hornblende (dark grey). It formed as igneous rocks from magma that forced into cracks.

A piece of this rock broke off after a veldt fire and the unweathered Diabase can be seen on the ‘scars’

These round structures are weathered patterns known as spheroidal or onion skin weathering.

On more than one place we have granite outcrops. The light coloured granite is Kaap Valley Tonalite and contains minerals like feldspar (white), quartz (transparent) and hornblende (black). The age is about 3220-3230 million years. It lies under the Kaap Valley and was a magma that didn’t burst out to the surface. It moved underneath the older Barberton sediments and lifted and deformed them.

The light coloured granite can be seen here where a piece of the top surface has splintered off.

The light coloured granite can be seen here where a piece of the top surface has splintered off.

The granite when exposed, is often seen as smooth domed surfaces like the one the giraffe is standing on.

Many interesting rock shapes are found where some parts eroded faster over the millennia than others to form these holes and patterns.

The holes were probably occupied by softer minerals like calcite or feldspar, which have been weathered out.

The natural weathering process can be observed in the pictures below.  The hornblende and feldspar decompose rapidly to softer, more erodable clay minerals, releasing the quartz grains to form a coarse sandy soil with little nutrient value.

The upper photo clearly shows the breakdown of the granite around mineral boundaries, the first stage in the formation of a soil.

First stage in the formation of a soil

The small fractures seen in the two photos below, also allow water to penetrate and alter the granite.

Weathering of granite in process

Granite weathering

We broke one of the rocks with an interesting shape to see how it looks on the inside… most amazing, don’t you think?

This rock could be called a ferricrete – the hot climate draws water to the surface, which combines with iron oxide and calcite which then cements together fragments of rock in the soil. It is usually found as a layer beneath the surface of the soil.

Small quartz veins can be seen on the surface in some areas.  Fluids containing silica move through fractures in the rock and precipitate as quartz veins.

The prospectors in the old days would look for these veins as they sometimes contained gold. By crushing and panning the gold could be recovered.

I often find chert Stone Age arrow points on the weathered granite. The closest chert can be found in the mountains in Barberton about twenty kilometres away. Quite a distance to travel on foot!

Arrowhead made from banded chert. The banding may reflect seasonal changes.

Chert arrowhead

Chert is a chemical precipitate from silica rich ocean water.  The Archaean oceans contained large quantities of dissolved silica.  Organisms like diatoms and radiolarian that extract silica from seawater for use in their internal skeletons, did not exist in the Archaean.  The dark chert contains organic carbon.  Bacteria microfossils have been found in the Barberton cherts, and may represent some of the earliest life forms on the planet.  Millimetre scale spheroids are also sometimes found in the Barberton chert units.  These can be meteorite spherules formed from the dust of meteorite impacts.  The dust coalesces to form these small balls in the atmosphere, which then fall to earth and get preserved in the chert gel on the Archaean ocean floor.  They are evidence of the oldest known meteorite impacts.

The following picture was provided by Chris Rippon, my geologist friend, to show how the spheroids from Barberton looks like.

Thanks Chris for all the information!

More Information about General Geology terms:

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