Part II- THE INTRICATE LAYERED INNER STRUCTURE OF EARTH.
We take for granted the shape of our world and the position of the continents— the familiar geography that seems as eternal as the order of the planets. But this arrangement is temporary: it isn’t how the planet has been and it isn’t how it will be.
By Peter Brannen.
To many of us it may surprise us that despite having the capability to explore the exoplanets using various satellites and technology, we still do not understand completely what goes on and exist in the interior of the earth. Earth is much more intricate than what we see just at the surface. In fact, if we were able to hold the Earth in our hand and slice it in half, we see that it has multiple layers. But of course, the interior of our world continues to hold some mysteries for us and the inner layers of our planet still remain off-limit to the explorers.
The advances in seismology have allowed us to learn a great deal about the Earth and the many layers that make it up.
Each layer has its own properties, composition, and characteristics that affect many of the key processes that keep going on in the planet. They are, in order from the exterior to the interior – the crust, the mantle, the outer core, and the inner core.
Like all terrestrial planets, the Earth’s interior is not homogeneous but differentiated. This means that it has an internal structure consisting of layers, arranged like the skin of an onion. Peel back one, and you find another, distinguished from the last by its chemical and geological properties, as well as vast differences in temperature and pressure.
Our modern, scientific understanding of the Earth’s interior structure is based on inferences made with the help of seismic monitoring. In essence, this involves measuring sound waves generated by earthquakes, and examining how passing through the different layers of the Earth causes them to slow down. The changes in seismic velocity cause refraction which is calculated (in accordance with Snell’s Law) to determine differences in density.
These are used, along with measurements of the gravitational and magnetic fields of the Earth and experiments with crystalline solids at pressures and temperatures characteristic of the Earth’s deep interior, to determine what Earth’s layers look like. In addition, it is understood that the differences in temperature and pressure are due to leftover heat from the planet’s initial formation, the decay of radioactive elements, and the freezing of the inner core due to intense pressure.
CRUST
The crust is the outermost layer of the planet, the cooled and hardened part of the Earth that ranges in depth from approximately 5-70 km (~3-44 miles). This layer makes up only 1% of the entire volume of the Earth, though it makes up the entire surface (the continents and the ocean floor).
The thinner parts are the oceanic crust, which underlies the ocean basins at a depth of 5-10 km (~3-6 miles), while the thicker crust is the continental crust. Whereas the oceanic crust is composed of dense material such as iron magnesium silicate igneous rocks (like basalt), the continental crust is less dense and composed of sodium potassium aluminum silicate rocks, like granite.
UPPER MANTLE
The mantle, which makes up about 84% of Earth’s volume, is predominantly solid but behaves as a very viscous fluid in geological time. The upper mantle, which starts at the “Mohorovicic Discontinuity” (aka. the “Moho” – the base of the crust) extends from a depth of 7 to 35 km (4.3 to 21.7 mi) downwards to a depth of 410 km (250 mi). The uppermost mantle and the overlying crust form the lithosphere, which is relatively rigid at the top but becomes noticeably more plastic beneath.
Compared to other strata, much is known about the upper mantle, thanks to seismic studies and direct investigations using mineralogical and geological surveys. Movement in the mantle (i.e. convection) is expressed at the surface through the motions of tectonic plates. Driven by heat from deeper in the interior, this process is responsible for Continental Drift, earthquakes, the formation of mountain chains, and a number of other geological processes.
LOWER MANTLE
The lower mantle lies between 660-2,891 km (410-1,796 miles) in depth. Temperatures in this region of the planet can reach over 4,000 °C (7,230 °F) at the boundary with the core, vastly exceeding the melting points of mantle rocks. However, due to the enormous pressure exerted on the mantle, viscosity and melting are very limited compared to the upper mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous.
OUTER CORE
The outer core, which has been confirmed to be liquid (based on seismic investigations), is 2300 km thick, extending to a radius of ~3,400 km. In this region, the density is estimated to be much higher than the mantle or crust, ranging between 9,900 and 12,200 kg/m3. The outer core is believed to be composed of 80% iron, along with nickel and some other lighter elements.
Denser elements, like lead and uranium, are either too rare to be significant or tend to bind to lighter elements and thus remain in the crust. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core.
INNER CORE
Like the outer core, the inner core is composed primarily of iron and nickel and has a radius of ~1,220 km. Density in the core ranges between 12,600-13,000 kg/m3, which suggests that there must also be plenty of heavy elements there as well – such as gold, platinum, palladium, silver, and tungsten.
The temperature of the inner core is estimated to be about 5,700 K (~5,400 °C; 9,800 °F). The only reason why iron and other heavy metals can be solid at such high temperatures is that their melting temperatures dramatically increase at the pressures present there, which range from about 330 to 360 gigapascals.
In the next article, we’ll start discussing major natural disasters like earthquake, tsunami and their impact and later on how to mitigate.