|
GLOSSARY OF TERMS
Fingerprints of the blast
Pseudotachylite (pseudo – “as if” – tachylite – “volcanic” = as if volcanic but
not volcanic.): melt rock. Basically, solidified glass, a black rock known as
obsidian (volcanic type silica glass) which was molten and has been cooled very
quickly. Veins of pseudotachylite are found all over the Vredefort Dome and go
down very deep. Friction in the cooling granite which rushed upwards to form the
Dome melted the rock, while picking up debris (breccia).
Shatter cone: Shatter cones are rare geological features that are only known to
form in the bedrock beneath large meteorite impact craters. This rock did not
melt but was shocked by a blast. It has distinctive “horsehair” or “fir leaf”
markings radiating from the top (apex) of the cone The azimuths of the cones's
axes typically radiate outwards from the point of impact, with the cones
pointing towards the center of the impact crater. The first man-made shatter
cone was produced in 1959 during an underground nuclear explosion.
Planar deformation features (PDFs): Distortions found in quartz which represent
impact features can only be seen under a microscope. Planar features of shock
origin and the so-called “strawberry texture” of high-temperature origin were
described from zircons in Vredefort rocks.
Other terms
Breccia: a rock composed of angular fragments of rocks or minerals in a matrix,
that is a cementing material, that may be similar or different in composition to
the fragments. Breccias are sometimes called clastic rocks. Volcanic pyroclastic
rocks are formed by explosive eruption that picks up rocks within the eruptive
column.
Stromatolite: samples that we show are fossilized evidence of the earliest forms
of life on Earth. Stromatolites are commonly thought to have been formed by the
trapping, binding, and cementation of sedimentary grains by microorganisms,
especially cyanobacteria (formerly known as blue-green algae).
Craton: nothing to do with a crater! The Kaapvaal Craton is an immense shield of
rocks which has remained relatively undisturbed for thousands of millions of
years, which is why we can still see the (eroded) Vredefort Dome structure in
the middle. The Kaapvaal craton along with the Pilbara craton of Western
Australia, are the only remaining areas of pristine 3.6-2.5 Ga crust on Earth.
Similarities of rock records from both, especially of the overlying Archaen
(ancient) rock sequences, suggest that they were once part of the Vaalbara
supercontinent.
Plucking: of rocks in the river. the process by which solid rock masses become
fragmented (due to weathering, temperature, lightning, chemical or plant
erosion) and are “plucked” off as boulders by hydraulic force. The plucked rocks
fragment further, ending up as river gravel, which picks up organics and becomes
alluvial soil.
Anabranched
or anastomosed riverbed: a term drawn from medical science
describing connections between blood vessels. In geology, anastomosis is used
to refer to quartz (or other) veins displaying this property, which is often
related to shearing in metamorphic regions. There is also a type of stream
called anastomosing, which consists of a wide strath and multiple thalwegs
divided by vegetated stabilized islands within subparallel banks.
|
I'm
a writer of popular science and technology books and
articles, a former Professor of Communication whose
hobby is now "backyard astronomy" with history thrown
in! I'm a keen mountain-biker and kayaker, so the
Dome and Vaal River are my playground.
I'm
a former teacher who has worked with Graeme on many
aspects of science & local history research, and I have
got to know the Dome by preparing the maps and data used
in our various presentations. My passions are mountain
running and education. I manage Otters' Haunt.