Salpeterkop EarthCache

Salpeterkop as seen from the north (the opposite side from where you will be) with Springbok grazing on the grounds of the Observatory.

Salpeterkop (Saltpetre Hill) was an active surface volcano about 66 million years ago – believed to be the last one in South Africa, making it the youngest known active volcano in Africa south of the Equator. The volcano pushed through the sedimentary layers during the late Cretaceous period, which is later than the Jurassic dolerite intrusions common around this area (the focus of my nearby RockStars EarthCache).

A study, done in 1988 by W.J. Verwoerd of the Geology Department, University of Stellenbosch found that Salpeterkop forms part of a dome with radial fractures extending to a distance of at least 16 km. Four concentric zones of decreasing megascopic deformation were distinguished, including a central crater 1 km in diameter, but no unequivocal shock features could be found. The Salpeterkop structure is interpreted as a dome overlying an alkaline-carbonatite ring complex, obscured by a volcanic superstructure that has been only partly eroded.

Salpeterkop on a geological map, where:
Ksa=Agglomerate, tuff, breccia
Kst=Trachyte
Ksm=Melilite basalt
Ksk=Carbonatite
Jd=Dolerite

When viewing this structure today, it must be remembered that about 2 km of rock has been eroded away, so we are looking at a level deep in the throat of the volcano. Its present summit is reached via a kilometre-wide lava ash field. When climbing it (for which permission is needed) you cannot help to be intrigued by the striking circular strata and caves where walls are stained orange, yellow, red and black from volcanic sulphur elements – quite obvious where the name originated.

A close-up view of the rocks of Salpeterkop shows various colours and evidence of their heated past.
Pictures: dr. Steve Potter

There are natural caves around Salpeterkop with evidence of former occupancy by Khoisan people, but are also sites of significant fossil finds such as the remains of lions, eland and the extinct brown hyena. More recent human exploration activities are visible as horizontal shafts dug into the side of the mountain. More modern drilling activity is also evident when a series of some 75 prospecting boreholes were sunk in and around Salpeterkop during the 1960s. An intensive study was made by the University of Cape town in the early 1970s. They found traces of 77 minerals, mainly Carbonatites, trachytes and olivine melilitites, but only one, Barite, was worth exploring.

For an area once vigorously shaken by explosive eruptions, it is today regarded as one of the most seismographically quiet in the world, enabling a world-class Observatory to be built nearby.

To claim "Found it" you must email me satisfactory responses to the following:

Any logs not accompanied by an email will be deleted.

1. OPTIONAL: Send me a picture of you and/or your navigation device, taken from the designated position with Salpeterkop in the background.
   
2. Have a careful look to see if you can spot any signs of a crater visible at the summit of Salpeterkop. In your own words, explain what you see, but most importantly, why it is this way?
   
3. If you go on a night tour at the nearby Observatory you are likely to see people measure angles on the sky, using their hands, held at arm’s length, like in the picture below. Using this same technique, you must estimate the angle Ang of the summit above the true horizontal – remember to keep your arm straight and level with your eyes.
   Is Ang closer to 4°, 10° or 20°?
4. Given that the summit is 4 km away from here, using simple trigonometry, it is now easy to calculate its height (h) using the formula: h = tan (Ang) x 4000m
   What is h?
   
5. Finally, the height of the summit above sea level can now be determined by taking an altitude reading on your GPS (let's call it Alt) and add it to h.
   What do you get for Alt and Alt + h = ?
   

Note: Do not post any spoiler pictures or hints to this page, even if encrypted.