| FACT
FILE Also visit the
Frequently Asked Questions.
Here are some quick planetary science facts about the
Vredefort Dome. Keep in mind that many scientific
observations and deductions remain controversial - it is
not even 100% agreed among experts that the Dome was caused
by an asteroid strike (although it is 99% certain that it
was!).
The facts here are purely about the pyrotechnics of
the event and not about human history, archaeology,
ecology or cultural understandings of the Dome. For more
general information about Dome themes outside of geology, go
to the
introductory page.
On our BLAST!
Talks and Trails from Otters' Haunt we offer explanation
of the Dome's possible origins and current structure. All
the facts below need to be explored in more detail by
visiting the site and learning about the debates.
SIZE
AND SPEED OF THE ASTEROID
The asteroid, or rock from space, was
at least 10-15km across, travelling at a speed
of around 10-20km per second.
Vaporised in the impact were 70
cubic kilometres of rock. The impact also
completely vaporised the asteroid itself, melted most
of the adjacent crustal rocks, and sent a
compressive shock wave circling around the Earth.
TOTAL
DIAMETER OF THE CRATER
Originally: about 360km across
(from north of Pretoria to near Harrismith)
Now (after two billion years of
erosion): about 230km at its widest (from
Johannesburg to Welkom)
Because of its size the crater
cannot be seen as a whole from ground level, or even
from an airliner. The best images we have were taken
from the Space Shuttle and satellites.
DIAMETER OF THE "DOME" OR RING STRUCTURE
Between 30-60km across. It is
somewhat narrowed at the top and pear-shaped, being most visible on the
northwestern semicircle of the Bergland, and least
visible in the southeast. The Dome is the core and
collar of the upheaval dome in the middle of the crater.
DEPTH
OF THE CRATER
Originally: more than 50km deep.
There was also a hole in the atmosphere
through which massive quantities of debris were probably
flung into space, generating Earth-origin meteorites.
Now (after two billion years of
erosion): about 35km. The surface has been scraped
away and what we can now see is the eroded deep
structure of a highly complex crater.
FORCE
OF THE BLAST
A 10km object produces an explosion of
6 × 107 megatons of TNT (equivalent to
an earthquake of magnitude 12.4 on the Richter scale,
which has never been measured in human times).
One popular estimate puts it like
this: "When it struck, the kinetic energy of the
projectile released the equivalent of 100-million
megatons of dynamite - that's 100-trillion tons of
explosive force. By comparison, the most powerful
nuclear bomb ever detonated only yielded 50 megatons." -
David Fleminger in Vredefort Dome (Southbound
Pocket Guide)
EFFECTS
OF THE BLAST
SHOCK WAVE: This would have
travelled several times around the Earth, sweeping
all before it. On colliding with itself on the opposite
side of the planet, the shock wave would likely have
caused the crust to crack, letting forth volcanic
eruptions.
TSUNAMI:
It has been estimated that an asteroid 2km in size
would cause a tsunami 230metres high. If this figure is
extrapolated to an asteroid like the Vredefort one, say
(at its smallest) 10km across, then a tsunami some 230m
X 10, or 2.3km high could be generated. Compare
this with the Boxing Day 2004 tsunami whose waves were
at most 20m high!
GLOBAL FIRESTORM: The material
ejected from the impact through the hole in the
atmosphere will re-enter all over the globe and heat up
from the friction with the atmosphere. The chunks of
material will be hot enough to produce a lot of infrared
light. The heat from the glowing material will start
fires around the globe. Global fires will
put about 7 × 1010 tons of soot into the air.
This would ``aggravate environmental stresses associated
with the ... impact.'' -
Nick Strobel's Astronomy Notes.
EXTINCTION OF LIFE
Mass extinction is the most likely
outcome of such a blast. The probability of a
Vredefort-sized asteroid hitting the Earth is about 1
in 1000 million years. The last big one we know of
was at Chicxulub, Yucatan, 65 million years ago, which
contributed to the death of the dinosaurs. "A global
event is one where impact fallout (dust lofted into the
stratosphere, smoke from wildfires, and so on) causes
global climate change sufficient to disrupt worldwide
agriculture and threaten mass starvation." -
The Planetary Society.
Got anything to add?
Send it to me. |
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.