Selected Article from the BHAS Bi-Annual magazine "Flint"
The Geology of the
Mediterranean: BHAS Day School
Tony Corrigan made an
excellent job of explaining the complex geology of the Mediterranean
Basin and the way in which its current configuration came into being.
As archaeologists we are used to dealing with events that happened
thousands of years ago but the time frame of geological processes
extends many millions of years into the past and it can be difficult
for the unini-tiated to get their minds around this.
Tony began the day with a
description of the make-up of Earth, as it is understood now. The
outer crust on which we live lies over the mantle, which Is divided
into two zones and reaches 2,900 kilometres down to the core. The
ocean crust, mainly bas-alt, is about 5 km thick while the
continental crust, mainly sili-cates and limestone is mostly 30-40 km
thick but can extend to 100km in places. The crust and upper mantle
are made up of tectonic plates which move over the mantle and faults
in be-tween these plates allow for molten magma from the lower
lev-els to erupt into the atmosphere as volcanoes.
He then described what is
believed to have been the way in which the Earths crust has
changed over those millions of years: tectonic plates meet and
coalesce into one, creating con-tinents and oceans; then break up and
create new configura-tions. About 250 million years ago (mya) the
giant continent Pangea broke up and formed two land masses separated
by the huge Tethys Ocean but they are now moving together again.
About 5.96 mya the nascent
Mediterranean Sea was closed off from the oceans and dried up in an
episode known as the Mes-sinian Salinity Crisis. About 5.33 mya the
Gibraltar Straits reo-pened and the Mediterranean Sea filled up.
This event, known as the
Zanclean Flood, could have taken place over as short a time as one to
two years. It is, however, still an area of negative precipitation
and needs its link to the Atlantic Ocean to maintain its existence.
Meanwhile, the tectonic
plates which have been moving to-gether and created the more or less
enclosed basin of the Mediterranean continue to push against each
other as they have done over the millions of years, causing the
formation of the mountain ranges which surround the Mediterranean
Sea. Some of these mountain ranges and parts of the sea-bed have
become very distorted by these tectonic pressures and the depth of
the sea bed varies greatly. A major fault line runs more or less the
length of Italy and extends south in to the sea. This has resulted in
the many volcanic eruptions and earthquakes which both the mainland
and islands experience.
So the next time you visit
Italy or other Mediterranean lands and islands try to imagine what
will happen as the surrounding land masses move ever closer together,
eventually strangling the Mediterranean Sea again and creating a new continent!