Background
plate tectonics
Plate tectonics is the unifying explanation of what drives the Earth's geology, and therefore a brief overview of it is relevant before we get into the specifics of Southern Spain. We start with the Earth's layers:
The Earth is composed of three basic layers - the Core, the Mantle and the Crust. The Crust is the planet's very outer wrapping, made of hard, brittle rock. Under the continental land masses, where it's known as "Continental Crust", it is relatively thick - on average about 40km - and consists of a complex amalgam of sedimentary, metamorphic and igneous rocks that have formed and re-formed ever since the Earth began. Beneath the oceans, where the Crust is known as "Oceanic Crust", it is much thinner - up to 10km thick - and is made primarily of a dark, dense igneous rock called basalt. For us standing on its surface, at 40km or more thick, the Crust seems pretty substantial. However, in the context of the planet as a whole, the Crust is very, very thin. Relatively, it is not even as thick as the rind on an orange - think of it more like the thickness of an eggshell on a large hen’s egg, or the skin of an apple. Beneath the Crust is the Mantle. At about 2850km thick, by volume it makes up most of the Earth. The Mantle is very hot (about 4000 degrees centigrade at the boundary with the Core, and 500-1000 degrees centigrade in the upper levels below the Crust). These temperatures would normally be sufficient to melt rock of the Mantle's composition, but there is a competing force – pressure – which acts to try and keep the Mantle in a solid state. The amazing outcome of this temperature vs pressure arm-wrestling match is a Mantle that is a solid – i.e. ‘rock-like’, but still sufficiently soft to be able to flow in a plastic manner over geological timeframes. Imagine a tar-like substance under conditions not quite hot enough to melt but sufficiently ductile to be able to flow given enough time - except that the Mantle is at least a million times more viscous than the road tar we are familiar with! Even more incredibly, this grudging ability to flow, when combined with the intense heat from the Core, is enough to create giant convection currents of plasticised rock within the Mantle, rising from the boundary with the Outer Core, up to the outer layers, where they spread sideways, cool and descend again. Recall those old school experiments with currents of coloured water heated in a beaker by a bunsen burner below - rising, then spreading sideways just below the water's surface before sinking again as they cool - well, the Mantle convection currents are similar in concept, except that these are currents of rock moving at just a few centimetres per year! At those speeds it would take around 100 million years just to flow from the base of the Mantle to the very top, let alone complete a full cycle ! Although most of the Mantle is this “flowing rock”, its very outer part is actually brittle rock – like the Crust above it – and doesn’t flow. In fact together, the Crust and this very upper part of the Mantle act like a single rigid layer, afloat above the circulating Mantle below. This combined layer of the Crust and very upper Mantle is called the Lithosphere – from the Greek ‘lithos’ meaning 'stone'. The ductile Mantle below, with its circulating convection currents, is called the Asthenosphere – from the Greek ‘asthenḗs’ meaning 'weak'. The Lithosphere is quite variable in thickness, but a good working average for our purposes is about 80km thick under the oceans and 150km thick under the continents. Not surprisingly, given its brittle nature and the chaotic forces churning below, the Lithosphere is not a uniform, unbroken layer wrapped around the globe. Instead it is broken up into a mosaic of gigantic fragments that ride on, and are dragged around by, the Mantle currents below – a bit like giant ice floes atop circulating ocean currents beneath. These massive fragments of the Earth’s outer 100km or so, are called Plates, and are the force behind our planet’s marvelous geology. You could almost imagine the Plates to be like the panels on a football, creeping slowly around the ball's surface, jostling for space at the edges of the panels - the 'Plate margins'.
Of course, unlike the football analogy, the Earth's real plates are not uniform in size or shape. The Earth’s skin is currently composed of 70 or so Plates in total, looking a bit like the cracked shell of a hard boiled egg! This mosaic of Plates ranges in size from the so-called “microplates” less than 1 million square kilometers in area, to the largest, the Pacific plate, which is a gigantic 103 million square kilometers. Fourteen of the most significant plates are shown opposite. The speed at which the Plates are moving is typically in the range of 1-5 cm/year, which is about the same rate as your fingernails grow and similar to the average speed of the Mantle's convection currents below. But they can reach speeds up to 16 cm/year (about the same rate as your hair grows). Whilst each Plate margin typically has a dominant character - divergent, convergent or transform - in detail, the margins often exhibit combined characteristics as they jostle each other for room in a complex battle for space. As you might imagine, the most dramatic geology happens at the margins, literally shaping our world in the process. Most mountain building, volcanic activity and the world’s major earthquakes are associated with the plate margins, both today and for hundreds of millions of years in the past. The three most dominant types of margin, and the ones most relevant to our Spanish story, are as follows: 1) Divergent margin: Ocean Spreading Centres Ocean spreading centres actually start life in the middle of continental crust, above a line of upwelling asthenospheric currents. The hot, upwelling asthenosphere partially melts the overlying lithosphere and crust, and the sideways currents, pulling in opposite directions, begin to stretch and thin the Plate - a bit like pulling soft toffee apart. Above surface, new ocean floods into the topographic low caused by the thinning and stretching. Below ground, as the continental Plate eventually ruptures, molten rock ("magma"), erupts along the line of the upwelling. It erupts underwater as lava, which cools rapidly on contact with the seawater to form new rock and new crust - 'oceanic crust'. The ocean spreading phase then takes firm hold (schematic opposite) with new Plates either side of the spreading centre continuing to be pulled and pushed further and further apart, as a conveyor belt of new oceanic crust is constantly injected along the line of the rifting. The classic example of this type of margin is the Mid-Atlantic Ridge which runs down the centre of the Atlantic Ocean and is actively erupting new ocean crust along its entire length as Africa/Eurasia on one side drifts apart from South/North America on the other (more on this later). 2) Convergent margin: Subduction Where an oceanic plate converges with a continental plate, the oceanic slab will be dragged down under the continental lithosphere to be subsumed back into the Mantle, in a process called 'subduction'. Lots of things happen during subduction, but the key happenings for the purposes of our story are:
3) Convergent margin: Continent-Continent collision When a continental plate meets another continental plate, both are strong and buoyant and neither is willing to be subducted. So like two giant sumo wrestlers they collide and thrust the crust and lithosphere on each side upwards and over each other. Key elements of this type of collision are:
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