596
October 2018
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
PHOTOGRAMMETRIC ENGINEERING
& REMOTE SENSING
J
ournal
S
taff
Publisher ASPRS
Editor-In-Chief Alper Yilmaz
Technical Editor Michael S. Renslow
Assistant Editor Jie Shan
Assistant Director — Publications Rae Kelley
Electronic Publications Manager/Graphic Artist Matthew Austin
Photogrammetric Engineering & Remote Sensing
is the official journal of the
American Society for Photogrammetry and Remote Sensing. It is devoted to the
exchange of ideas and information about the applications of photogrammetry,
remote sensing, and geographic information systems. The technical activities of
the Society are conducted through the following Technical Divisions: Geographic
Information Systems, Photogrammetric Applications, Lidar, Primary Data
Acquisition, Professional Practice, and Remote Sensing Applications. Additional
information on the functioning of the Technical Divisions and the Society can
be found in the Yearbook issue of
PE&RS.
Correspondence relating to all business and editorial matters pertaining to this
and other Society publications should be directed to the American Society for
Photogrammetry and Remote Sensing, 425 Barlow Place, Suite 210, Bethesda,
Maryland 20814-2144, including inquiries, memberships, subscriptions, changes
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tions. The telephone number of the Society Headquarters is 301-493-0290; the
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PE&RS
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Copyright by the American Society for Photogrammetry and
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About two billion years ago, an asteroid measuring at least 10 kilometers across hur-
tled toward Earth. The impact occurred southwest of what is now Johannesburg, South
Africa, and temporarily made a 40-kilometer-deep and 100-kilometer-wide dent in the
surface. Almost immediately after impact, the crater widened and shallowed as the rock
below started to rebound and the walls collapsed. The world’s oldest and largest known
impact structure was formed.
Scientists estimate that when the rebound and collapse ceased, Vredefort Crater mea-
sured somewhere between 180 and 300 kilometers wide. But more than 2 billion years
of erosion has made the exact size hard to pin down.
“If you consider that the original impact crater was a shallow bowl like you would serve
food in, and you were able to slice horizontally through the bowl progressively, you
would see that the bowl’s diameter will decrease with each slice you take off,” said
Roger Gibson of University of the Witwatersrand and an expert on impact processes.
“For this reason, we are unable to categorically fix where the edge now lies.”
According to Gibson, the uplift at the center of the impact was so strong that a 25-ki-
lometer section of Earth’s crust was turned on end. The various layers of upturned rock
eroded at different rates and produced the concentric pattern still visible today. Vrede-
fort Dome, which measures about 90 kilometers across, was observed on June 27, 2018,
by the Operational Land Imager (OLI) on Landsat 8.
Notice that only part of the ring is visible. That’s because areas to the south have been
paved over by rock formations that are less than 300 million years old. The young rock
formations have begotten fertile soils that are intensely cultivated.
The darker ring in the center of this image, known as the Vredefort Mountainland, has
shallow soils with steep terrain not suitable for farming, so the area remains naturally
forested. Along the ridges in the Mountainland you can see white lines: these are the
hardest layers of rock, such as quartzite, which resist erosion. The outer part of Moun-
tainland has exposed rocks that are roughly 2.8 billion years old; this is the Central Rand
Group, source of more than one-third of all gold mined on Earth.
Visitors to the impact site today can witness geologic time by traversing just 50 kilo-
meters from Potchefstroom toward Vredefort. The journey would take you from shallow
crustal sedimentary rocks deposited between 2.5 and 2.1 billion years ago, ending with
3.1- to 3.5-billion-year-old granites and remnants of ocean crust that were once about
25 kilometers below Earth’s surface.
“Such exposed crustal sections are incredibly rare on Earth,” Gibson said. “The added
bonus here is that the rocks preserve an almost continuous record spanning almost
one-third of Earth’s history.”
For more information, visit
