The earth has a spherical shape with a diameter of 7926 miles (12756 kilometers). The rotation of the earth has distorted its spherical figure, giving it a slightly flattened shape. As a result the polar diameter of 7899.83 miles (12713.54 kilometers) is about 26.5 miles (42.7 kilometers) less than the equatorial diameter.

Magnetic and gravitational field or seismic waves are used to study the internal structure of the earth. In this way layers or boundaries of the earth have been divided into crust, mantle and core.

The outermost layer of the earth is called the crust of the earth. It constitutes 0.6 percent of the volume of the earth and its thickness under the oceans is around 5 to 9 kilometers. However, the continental crust varies in thickness and is 30 to 40 kilometers. It is very thick beneath the mountain ranges like the Himalayan range where its thickness is 80 km. The continental crust is composed of rocks made by lava flow, granite and sediments. The composition of the rocks is roughly uniform. The most abundant elements in these rocks are oxygen, silicon and aluminum. The oceanic crust is relatively more uniform consisting largely of basalt rocks underlined by gabbro. Gabbro has the same composition as basalt but has coarser grains. Major elements found in these rocks are oxygen, silicon, aluminum while magnesium and calcium are also more as compared to continental rocks.

The mantle lies beneath the crust and extends towards the center of the earth to a depth of about 2900 km. It comprises of about 82 percent of the volume of the earth. The sharp boundary between the crust and the mantle was discovered by Andrija Nohorovicic in 1909 and is called the Mohorovicic Discontinuity. The mantle is supposed to be mainly composed of the rock peridotite. Major components of this layer are oxygen, iron, silicon and magnesium. Mantle is mostly solid but contains a partially molten layer.

The innermost part of the earth is called the core and it extends from the mantle to the center of the earth. It makes roughly about 17 percent of the volume of the earth. The boundary between the mantle and the core is called the Gutenberg Discontinuity after its discoverer Beno Gutenberg. The core is demarcated into two regions. The outer core extends to a depth of 5155 km and is in liquid state while the inner core in solid. The core is mainly composed of iron with other elements like sulphur, silicon, carbon, hydrogen and oxygen.