QUESTION

# Which of the following is called the outermost layer of the Earth?${\text{A}}{\text{.}}$ crust${\text{B}}{\text{.}}$ mantle${\text{C}}{\text{.}}$ core${\text{D}}{\text{.}}$ none of these

Hint: Here, we will proceed by mentioning all the layers of which the Earth is made up of and we will be discussing the features of all these layers in order to get which one of these layers is the outermost layer of the Earth.

The Earth is made up of many rocky layers. The solid, outer layer is called the crust. Below the layer known as crust lies a layer of very hot, almost solid rock called the mantle. Beneath the mantle lies the core. The outer core is a liquid mix of iron and nickel, but the inner core is solid metal. Sometimes, hot molten rock which is called magma, bursts through Earth’s surface in the form of a volcano.

Earth's Mantle
The mantle is the layer of the earth that lies below the crust and is by far the largest layer making up 84% of Earth's volume. The mantle starts at the Mohorovicic Discontinuity, also known as the Moho. The Moho is defined as the density contrast from less dense crust to denser mantle and where seismic wave velocities increase.

Earth's Outer Core
The outer core is the liquid largely iron layer of the earth that lies below the mantle. Geologists have confirmed that the outer core is liquid due to seismic surveys of Earth's interior. The outer core is 2,300 km thick and goes down to approximately 3,400 km into the earth. No one has ever seen the outer core but based on a number of indicators, geologists believe the outer core is 80% iron, some nickel and a number of different lighter elements.

Earth's Inner Core
The inner core is the center most layer of Earth and is in many ways similar to the outer core. It is also primarily iron and nickel and has a radius of about 1,220 km. The differentiation between the outer core and inner core is density driven.

Therefore, the outermost layer of the Earth is known as crust.
Hence, option A is correct.

Note: Earth's Crust
The crust is what you and I live on and is by far the thinnest of the layers of earth. The thickness varies depending on where you are on earth, with the oceanic crust being 5-10 km and continental mountain ranges being up to 30-45 km thick. Thin oceanic crust is denser than the thicker continental crust and therefore 'floats' lower in the mantle as compared to continental crust.