Fireball over the snowy Dolomites
A piece of fortuitously snapped space rock burning up in the atmosphere at dusk provides the background to a discussion of this Italian part of the Alpine chain, uplifted from the sea bottom into mountains as the Adriatic Plate grinds into Southern Europe while the Mediterranean closes. Many things are happening tectonically...Africa is colliding gently with Europe, while the Red Sea's opening pushes Arabia into Asia creating a great rotational movement through Turkey and Greece. This fireball on November 14th was widely seen and reported across Europe as it passed over Germany from its radiant in the constellation of Taurus. This meteor shower is associated with debris from the short period comet Enke.
The Dolomites took their name from the ubiquitous rock that forms the chain, a form of limestone where magnesium has replaced the calcium known as dolomite or dolostone (named after French mineralogist Deodat Gratet de Dolomieu who first described it and these mountains to the scientific world after an Italian journey in 1789-90), although they were traditionally known for obvious reasons as the pale mountains. The rock started off as ordinary limestone, but was transformed by magnesium rich fluids passing through somewhere around 230 million years ago and unlike limestone the resultant rock does not fizz on contact with acids.
The original sediments were laid down in warm tropical seas, filled with corals and other life, and spanning with good resolution the late Palaeozoic (from roughly 280- million years ago) through a large chunk of the Mesozoic (251-65 million years ago), starting when Africa and Europe were still united in the supercontinent Pangaea and spanning the opening and closing of the Tethys Ocean (see http://bit.ly/2yNcH7n) and varied sub basins thereof. The rocks here include some of the most complete fossil coral reef ecosystems on Earth, some up to 2km thick and complete fossil atolls (see http://bit.ly/2zdaUgh) displaying the complex interactions between reef and volcano in this distant past, and traceable from long gone beaches to sea floor, a thousand metres lower.
The great end Permian extinction and slow Triassic recovery are also well documented in these strata. Much of the marine life of these eras were also first classified here, as the world's fossils showed us in the 19th century how to divide geological time on the basis of their appearance and disappearance, with the lines being drawn at points of large scale faunal turnover. Sea level rose and fell, volcanism came and went and each phase of geological history left its readable imprint but during the Permian a large archipelago of coraline islands developed, followed by extensive volcanism between 240 and 230 million years ago repeatedly burying and fossilising reefs in the hot embrace of lava and ash followed by a new generation of islands.
Later still the area rose above sea level into a muddy plain, and the footprints of early dinosaurs have turned up in varied corners of the Dolomites. The sea rose again, and the entire Jurassic and Cretaceous saw huge amounts of marine sedimentation between 170 and 65 million years ago adding more layers of carbonate and marl (limey mudstone). Some 50 million years ago the growing tectonic pressure of the birthing Alps pushed the region above sea level without much folding or deformation, and started to uplift it into the current ranges in a slow process that is still ongoing. Much later still they were carved during the last 3 million years as the glaciers of the ice ages waxed and waned from their high altitude layers. One of the joys of all this geology is that large vertical (in time) and horizontal (in space) distances are available for study, giving us a clear idea of how a whole region evolved during a considerable span of geological time.
Image credit: Ollie Taylor via APOD
