!Journey to the center of the Earth

Before we go and see the first amazing outcrops in Oman it would probably be good to understand how our mother Earth is constructed.

Earth's Guts

Planet Earth is like a giant, inedible layered cake. It is composed of several layers which are all different because they are composed of different minerals and lie at different depths The minerals combined together form rocks. In the first field trip we want to suggest, in a wonderful dry valley of Wadi Far (wadi - in Arabic valley), you will see some of the most peculiar rock types you can see at the Earth's surface - peridotites and gabbros.

So lets start with some SCIENCE (bueh!)

Well, in the centre of the Earth is the core, and more specifically the inner core. No one exactly knows what it is composed of, since no one’s ever been there, but there are several theories based on the indirect observations of the seismic waves (will explain at some point, do not be afraid). Geophysicists figured out that it is probably metallic (Iron-Nickel Alloy). Some hints about Earth’s inner core were also brought by meteorites - but this is a different story, probably for another post. We know that above the inner core there is a rather non-solid (more likely liquid) outer core. Outer core is responsible for generation of our protective shield against solar wind known as magnetic field. Again, we could write books about outer core. Anyway, we do not have outcrops of inner or outer core at the Earth surface.

And there we go, above the outer core we have the biggest part of our planet known as the mantle. It contains about 85% of Earth's volume. Yet, we do not know much about the mantle because the top of the mantle is located approx. 7-35 km beneath us. But in some places (e.g. in Oman) due to some quirky tectonic processes, the mantle was exposed on the surface. The top of the mantle has been defined by geologists at the boundary where seismic waves suddenly increase their velocity (this zone is known as a Mohorovicic Boundary, often abbreviated as MOHO boundary). This sudden change in seismic wave velocity can be explained only be significant change of rock composition (which is in fact true). The mantle is not composed of only one layer and geologists found a few distinct regions within it (so we can divide the mantle into lower and upper parts, also there is a few significant boundaries within - these are related to some mineral transformations).

The crust is the topmost part of the Earth. It is where we live. There are two kinds of crust: oceanic and continental (well, apparently there are more but we can ignore this for now) and they have different chemical composition and densities. The continental crust is composed mainly of relatively "light" or, more precise, less dense rocks (e.g. granite) whereas oceanic crust is much more dense and is made of e.g. basalts. Also, chemically these two types are totally different.

It may be a bit confusing but there are a couple more terms commonly used in geology, known as a lithosphere and asthenosphere. This classification is based not on rock composition but on how rocks behave in the certain conditions (the science of rheology). Lithosphere seems to be more brittle (easy to fracture) whereas astenosphere is weak and ductile (more plastic). This is a very important difference as it controls mountains building, creating sedimentary basins etc. Astenosphere is in the upper part of the mantle. The lithosphere-asthenosphere boundary is defined at the depth where temperature is 1300°C (isotherm), above which the upper mantle becomes rigid, whilst the rocks below behave elastically.

Over 60 million years ago (in the Cretaceous times, Cenomanian) a big slab of oceanic crust has been pushed onto an otherwise stable Arabian Peninsula. This process is known as obduction and the oceanic crust at the surface is called an ophiolite sequence (see below). The Earth forces were so big that they pushed also some mantle rock. Geologists can now examine these rocks at the surface.

Now let's go and see that badass sequence (well, maybe it is not as spectacular as you may think but it is still nice to go and enjoy some 4WD driving in a beautiful valley)...

#crust #mantle #Earth #MOHO

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