Nelspruit’s Granite
Domes
Driving on the N4 highway from Gauteng eastward, the high
plateau grasslands of Mpumalanga stretch for hundreds of kilometres
until you go down to an area which we know as the Lowveld.
In geology terms, the Lowveld of Mpumalanga consists mostly out
of intrusive igneous rocks, of which the Nelspruit granite
batholith, the oldest magnetite-series granite so far known at 3105
Ma-old, is the most important.
Driving more eastward to Nelspruit, the provincial capital, you
are met with many granite outcrops from different sizes. This is
typical of the landscape in this area and is known as granite
domes.
A dome, in structural geology, is a deformational feature
consisting of symmetrically-dipping anticlines; their general
outline on a geologic map is circular or oval. The strata in a dome
are up warped in the center; if the top of a dome is eroded off,
the result will be a series of concentric strata that grow
progressively older from the outside-in, with the oldest rocks
exposed at the center. Many geologic domes are too large to be
appreciated from the surface, and are apparent only in maps. Some
batholiths are also known as domes.
Formation
Granite forms plutons and batholiths of igneous rock several
kilometers below the surface as magma slowly cools and
crystallizes. The granite is under great overhead pressure.
These granite formations are then exposed to the surface through
the process of erosion accelerated by continental uplift acting
over many tens of millions to hundreds of millions of years. This
process remove several tens of kilometers of overlying rock in many
areas, exposing the once deeply buried batholiths. The pressure on
the granite is reduced and it then expands and forms fractures or
sheet joints parallel to the surface. The granite then erodes in
concentric layers (similar to how an onion peels) forming rounded
masses called exfoliation domes.
Sources:
Wikipedia
Also a big thank you to team GPS Storm for the information
supplied.
To get to this EC you have to park at one of the parking
co-ordinates below. Your decision will determine the difficulty of
the cache. For the first co-ordinates it is 2½ stars and for the
second co-ordinates it is 3½. Both routes however will give you
great scenery. Just follow the trails and markings and the
additional waypoint to show you the right track if you get
stuck.
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. These exposed granite domes were formed and sculpted over the
years by weathering. In the vicinity of this EarthCache you will
find different kinds of weathering. Take a photo of you/your team
at one of these weathering processes and place it in your
log.
2. What is the difference between a dome, a tor and a cupola?
3. What is the approximate height of the unnatural extrusion at the
co-ordinates?
Uploading photos to the cache page is the best way to say thank
you to the cache developer and to encourage others to visit the
location but is only optional.
You can send your answers to me in either Afrikaans or
English.