Thursday, March 6, 2008

The Hohentwiel

In the last Weekend Fun I demonstrated a sample of volcanic ash tuff from the Singen-Hohentwiel former volcanoe complex in southern Germany. Today the actually volcanoe can no longer be observed, it has been eroded during the last periods of glaciation. Volcanic activity was seperated into two phases. First there was an extensive ash production phase that created the very extensive tuff cover of much of the modern Hegau-region. Second was a phase of magma production that, with one exception, never reached the surface and created a number of intrusions into the tuff layers. In the Hohentwiel this formed the central Phonolite body we see today. Volcanic activity lasted roughly from the Miocene to the Pliocene. In the post-volcanic phase from the Pliocene to Pleistocene the volcanoes were eroded by glaciers down to the current geologic level. The tuff used to surround the entire phonolite body, however, theory says that it could only be preserved in the "wind-shadow" of the hard phonolite where it was protected against glacial movements. Below you can see a schematic cross-section of the Hohentwiel.
At the end of the day I took a foto that - by chance - shows roughly the same section as the above schematics. It can easily be observed how the tuff in the SW has been shielded by the phonolite. Today it is home to a nature reserve and lovely wine gardens.

The tuff sample from Weekend Fun was taken from the tuff section. Actuall location is on the backside of the hill though. Below is a nice example of the wonderful volcanic bombs that can be encountered in the tuff cover when taking the long way down the castle through the wine gardens. I forgot a scale on these careless... :-(

And finally a wonderful and beautiful flower from the nature reserve. Unfortunately I don't have a clue what it is called but it looks very nice!

Except for the schematics fotos were all taken by myself.


  • Henningsen, D. & Katzung, G. (2006): Einf├╝hrung in die Geologie Deutschlands, 7. Auflage, Elsevier


Silver Fox said...

Aargh! For not guessing any kind of volcaniclastic at all. Aargh! I must be spending too much time in carbonate terranes.

The Lost Geologist said...

Don't worry! You are not alone with that!