In my last post I tried to explain why I study rocks. In my next few posts I will describe my research focus and exactly what I am doing for my PhD thesis. However, in order to better explain this I should first tell you a little about the design of the Earth.
The Earth is a differentiated body: it has four major layers – the inner core, the outer core, the mantle and the crust. These layers developed due to a difference in density of the Earth’s constituent materials: dense metals such as iron and nickel ‘sank’ to the core and lighter elements such as silicon and oxygen ‘floated’ to the top and cooled forming crystals and the outermost layer of the Earth, the crust.
The crust is made of two categories of solid rock: oceanic crust and continental crust. Oceanic crust is thin (<10km) and typically forms the rock under the deep oceans. Continental crust is thicker (<70km), less dense and typically forms the rocks of the continents and rocks under shallow seas: continental crust is developed from oceanic crust over considerable amounts of time.
The crust is also broken up into pieces called tectonic plates which move very slowly around the Earth (<10cm per year – the average is similar to the rate of fingernail growth). This movement is driven by processes in the layer below the crust – the mantle. The thermodynamic forces of the mantle cause the tectonic plates to move in a variety of relative movements which lead to a variety of crustal processes.
Where plates move away from each other, new oceanic crust is produced; this is known as a ‘divergent’ or ‘constructive’ margin e.g. the Mid-Atlantic Ridge. Where plates move toward each other, old oceanic crust is destroyed and new continental crust is developed; this is known as a ‘convergent’ or ‘destructive’ margin e.g. the west coast of South America. Lastly, where the plates moves parallel to each other, a ‘transform’ margin, crust is mostly conserved e.g. the San Andreas Fault, California.
At convergent margins, where two plates move towards each other, the denser plate (generally that with oceanic crust) is forced below the other in a process termed subduction and the subducted crust enters the mantle. Over time, subduction leads to the closure of oceans (as oceanic crust is being destroyed) and ultimately results in the collision of continental crust and mountain-building events producing mountain belts such as the Himalayas and the Alps.
There are three main rock types: igneous, sedimentary and metamorphic. Igneous rocks are new rocks formed from molten rock or ‘magma’ from the mantle and form mainly at divergent plate margins and volcanoes. Sedimentary rocks are made of the eroded pieces of previous rocks such as those which form beaches and deserts and are found in rivers. Metamorphic rocks are igneous and sedimentary rocks which have been subject to the process of metamorphism.
Metamorphism is the change which occurs to a rock when it is exerted to heat and pressure; which are both found with increasing depth. The minerals within the rocks change so that it is more stable under the new pressure-temperature conditions i.e. to exist at a lower energy state: nature is driven by attempts to attain a lower energy state.
Now I’ve given the background, in my next post I will explain exactly what I am studying for my PhD research project.