The Earth can be divided into four main layers: the solid crust on the outside, the mantle, the outer core and the inner core. Out of them, the crust is the thinnest layer of the Earth, amounting for less than 1% of our planet’s volume.
The Earth’s structure can be classified in two ways – based on mechanical properties, and based on the chemistry. Here, we’re just going to discuss a basic classification, without going into most details. The main focus here is to understand how the Earth’s crust really is, and why it is the thinnest layer.
The crust ranges from 5–70 km (~3–44 miles) in depth and is the outermost layer. The thinnest parts are oceanic crust, while the thicker parts are continental crust. Most rocks at the Earth’s surface are relatively young (less than 100 million years old, compared to the Earth’s age, which is approximately 4,4 billion years), but since we found some rocks which are much older, we know that Earth has had a solid crust for at least 4.4 billion years.
The mantle extends from where the crust ends to about 2,890 km, making it the thickest layer of Earth. The mantle is also composed of silicate rocks, but the mantle as a whole is very viscous – the high temperatures there cause the silicate material to be sufficiently ductile that it can flow (in a very long time). The mantle is generally divided into the upper and the lower mantle.
The core, typically divided into the outer core and the inner core. The outer core is regarded as viscous, though much less so than the mantle, while the inner core is solid.