Friday Non-Lifeform Blogging
(Non-lifeform, that is unless you believe that the Earth is a living entity in and of itself, a la the Gaia hypothesis)
Grabens are interesting. Imagine you’re making an apple pie, and have rolled the top crust too thin. Then try stretching it over the pie dish. What you’ll see first are stretch marks , then the stretch marks will split along the lines of strain. The apple will come up through the slits., as does new unexposed rock or magma in a graben. In this image the fault lines are the equivalent of stretch marks that have split:
The geological process is almost exactly the same – ‘graben’ is a German word meaning ditch or grave. In the geological sense it is a collapsed or down-dropped block of rock that is bordered on its long sides by faults caused by stretching. (Grabens are normally associated with “horsts,” which are the up-thrown blocks of rock. In German, Horst means aerie, referring to the high nesting sites of predatory birds)
Grabens can be found wherever there are tectonic forces acting on a planet, even on the Moon:
Willis, K.J., Dickerson, P. W., and McRay, B.H. 30 Sep. 2004. ” Grabens on Earth and Moon .” http://earth.jsc.nasa.gov/newsletter/planetary/sld005.htm (16 Dec. 2005).
There are grabens on Mars: fracture belts surrounding the Tharsis area, supporting the theory of volcanic uprising at the edge of this upswelling dome. Tharsis and also the Elysium mountains are surrounded by a lot of radial grabens, which shows that there was initially some dome-like uplifting andthat the area isn’t just a thicker part of the lithosphere.
Image of Claritas Fossae, Mars, from European Space Agency/Mars Express.
Many Martian volcanoes show signs of radial graben formation as a result of updoming, as in the above image, but some of the most beautiful grabens of all are here on Earth at Canyonlands National Park in Utah. A system of geologically young grabens has developed in the Needles District, which have since filled with sediment up to 100 metres deep:
Image courtesy Tara Gregg
Whitman College, Walla Walla, WA 99362
Which brings me right back again to the Great Rift Valley, of which I am inordinately fond.
Here in Tanzania grabens can be seen in the process of forming (also see previous geology post re the Afar Depression).
Before satellite imagery and the internet one could only visualise these geological processes. Now we can see them whenever we want, and watch this and other planets in the continuing process of reforming themselves. How cool is that?
