The universe is shrouded in mystery, and part of this mystery involves our own little planet. Although there have been countless research, the scientists and experts have only been able to barely scratch the surface (both figuratively and literally) in the case of planet earth.

However, the studies in Seismology have allowed us to gather sufficient information about the Earth and its structure. The planet comprises several layers, which have their own attributes, and composition. So let’s dive in further to discover some enlightening aspects.

The composition of the Earth’s Layers- An overview

Earth's structure
Earth’s structure. Image: Wikipedia

The structure of the Earth can be determined in two ways, chemically and mechanically. Mechanically or rheologically means the study of the liquid states, which can be further categorized into the lithosphere, asthenosphere, mesosphere, mantle, and core. However, chemically speaking, the earth can be categorized into the crust, the mantle (which can be classified into the upper and lower regions), and the core (which can also be subcategorized into the outer core, and the inner core).

While the inner core tends to be solid, the outer core is liquid. This is owing to the relative melting points of the various layers (silicate crust, nickel-iron core, and mantle) and the rise in temperature and pressure as depth expands. At the surface, the silicates and nickel-iron alloys are cool enough to form a solid state. In the upper mantle, the silicates are usually solid, but localized regions of melt are present, resulting in limited viscosity.

On the contrary, the lower mantle goes through intense pressure and hence has a lower viscosity as compared to the upper mantle. The metallic nickel-iron elements within the outer core are liquid because of the extremely high temperature. However, the tremendous pressure, which mounts up towards the inner core, dramatically alters the melting point of the nickel-iron, turning it into solid.

The distinction between all the layers is owing to the processes that took place during the initial stages of the formation of the earth (approximately 4.5 billion years ago) as we know it today. At this time, melting would have resulted in the denser elements moving toward the center while comparatively less dense materials would have shifted to the crust. Thus, the core is believed to be primarily composed of iron, other than nickel and some lighter elements, while less dense substances moved to the surface, along with the silicate rock.


The outermost layer of the earth is known as the crust. This is the hardened and cool part of the Earth that varies in terms of depth from nearly 5-70 km (3-44 miles). This layer actually consists of only 1% of the entire volume of the Earth, though it encompasses the entire surface (the ocean and the continents floor).

The oceanic crust tends to be the thinner part, which lies underneath the ocean basins and runs 5-10 km (3-6 miles) deep, while the continental crust is comparatively thicker. This oceanic crust constitutes dense substances, for instance, iron, silicate magnesium, and igneous rocks (like basalt), while continental crust is less dense and comprises aluminum, potassium, and sodium silicate rocks, like granite.

The upper section of the mantle, along with the crust, forms the lithosphere, an irregular layer with the most thickness which goes up to 200 km (120 mi). Many different rocks now sitting in the Earth’s crust were developed less than 100 million years ago. However, the earliest known mineral substances are 4.4 billion years old, demonstrating that the Earth has had a solid crust for a prolonged span of time.

Upper and Lower Mantle

The mantle constitutes about 84% of Earth’s volume. It is primarily solid but comes off as an extremely viscous fluid as per geological research. The upper section of the mantle and the overlying crust is known as the lithosphere, which is significantly rigid at the top but becomes more plastic underneath.

There is plenty of information on the upper mantle as compared to other layers of the earth. Such details have been uncovered after several seismic studies and direct investigations using geological and mineralogical surveys. The shifts in the mantle (i.e. convection) are indicated through the motions of tectonic plates. The heat coming from the interior of the mantle is known to be responsible for earthquakes, Continental Drift, the development of mountain chains, and a number of other geological phenomena.

The mantle also tends to be chemically distinctive from the crust, other than being different in terms of seismic attributes and rock types. This is because of the fact that the crust consists of solidified elements derived from the mantle, where the mantle elements are partially melted and viscous. This prompts the incompatible elements to deviate from the mantle, as the less dense materials float upward and ultimately become solid at the surface.

The crystallized melt substances close to the surface where we live are ideally known to contain lower levels of magnesium to iron ratio and a higher amount of silicon and aluminum. Such shifts in mineralogy may impact mantle convection, as they lead to density changes.

The lower mantle is approximately 660-2,891 km (410-1,796 miles) deep. The temperatures in this part of the planet can reach up to 4,000 °C (7,230 °F) specifically along its boundary adjacent to the core, thus, considerably expanding the melting points of mantle rocks. However, owing to the humongous pressure wielded on the mantle, the viscosity of this part is lower as compared to the upper mantle. There is very little information about the lower mantle aside from the fact that it comes across as seismically homogeneous.

The Inner and the Outer Core

The inner core consists essentially of nickel and iron and has a radius of 1,220 km. The density in the core is indicative of the fact that there must be heavy substances like gold, palladium, platinum, tungsten, and silver.
The temperature of the inner core is anticipated to be about 5,700 K (5,400 °C; 9,800 °F). Now, the reason why iron and other metals can remain solid at such high temperatures is that their melting temperatures drastically elevate at the pressure present at the core, which can roughly hover around 330 to 360 gigapascals.

The outer core, which has been found to be liquid (as per the seismic research), is 2300 km thick, and the radius extends around 3,400 km. The density is anticipated to be much higher than the crust or mantle, which varies between 9,900 and 12,200 kg/m3. The outer core is considered to be developed with 80% iron, along with nickel and some other lighter elements.

Due to its high temperature, the outer core remains in a low-viscosity fluid state that ensures turbulent convection and rotates quicker than the rest of the planet. The average strength of the magnetic field in Earth’s outer core is said to be 50 times the strength of the magnetic field recorded on the Earth’s surface.

Like other significant elements of the Universe, the Earth isn’t a finished product, but rather an entity that’s constantly evolving. And the knowledge that has so far been accumulated is still largely dependent on theories and guesswork, considering the fact that we can’t examine the earth’s interior closely enough.

Now, as the Earth’s tectonic plates keep colliding and drifting, its interiors sustain convections, and its core continues to expand. Who knows what it will evolve into eons from now? At the end of the day, the Earth was here long before we came into existence, and is likely to thrive long after we are gone.

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