2.1: Earth’s Interior

As shown on Figure 2.1.1 the interior of the Earth is comprised of several concentric layers, starting with the crust at surface, down through the different parts of the mantle, and then into the outer and inner core.

Figure \(\PageIndex{1}\): The Parts of the Earth’s Interior

The crust is the Earth’s thin, brittle, outer layer, and is shown in more detail in the inset of Figure 2.3.1. It makes up approximately 0.2% of the Earth’s radius, and therefore is proportionally no thicker than the skin on an apple. There are two types of crust: continental (making up the land) and oceanic (found at the bottom of oceans). The continental crust is dominated by rocks formed as magma (molten rock below the surface) cools and forms rocks like granite. There are also lesser amounts of various types of rocks that form when lava cools (like you see in Hawaii), sedimentary rocks (like you see in the Grand Canyon), and metamorphic rocks (a rock type you will learn about soon). Overall, it has a composition high in silica (silicon and oxygen) which has a relatively low density and low in metals making it relatively "light". It is 30 to 40 km thick in most areas (thicker in mountainous areas, and thinner near to the edges of the continents). The oceanic crust is also made from rocks formed as magma or lava cools, but it is much thinner at 5 to 6 km thick in most areas. These rocks contain more metals than the continental crust, so they are denser.

The key takeaways about the crust are:

• The crust is the thin, brittle, outer layer of Earth.
• There are two types of crust.
• Continental crust is thick and "light" or less dense. Oceanic crust is thin and "heavy" or denser.

The crust rests on the mantle, which is made up of rocks that are high in metals. The upper part of the mantle is rigid or brittle, just like the crust. That rigid part of the mantle plus the crust is called the lithosphere. The temperature within the Earth increases with depth, from close to 0⁰ C at surface to almost 5000⁰ C in the center of the core, but that rate of increase isn’t linear (Figure 2.1.2). The temperature rises quite rapidly within the lithosphere, but the rate slows within the lower mantle. At between about 100 and 250 km depth the temperature is very close to, or even slightly above, the melting temperature for mantle rock and so the mantle rock within that depth range is partly molten. This layer is known as the asthenosphere because the rock there is weaker (softer, or more plastic-like) than in the rest of the mantle.

Figure \(\PageIndex{2}\): Temperature and Partial Melting in the Upper Part of the Mantle

The asthenosphere, due to heat transfer from the core, is slowly convecting. As we’ll see in the next section, that convection is critical to the process of plate tectonics.

The key takeaways about the mantle are:

• The mantle is a thick middle part of the Earth's interior that is high in metals.
• The upper part of the mantle is solid, so we combine it with the solid crust and call this the lithosphere.
• Below the solid part of the mantle is a plasticy or waxy layer of the mantle we call the asthenosphere.
• Convection occurs in the asthenosphere which will be important later.

The Earth’s core is mostly made of iron, with up to 10% nickel and a small proportion of other elements. The temperature at the core-mantle boundary is close to 4000⁰ C and that gradually increases to about 5000⁰ C at the Earth’s center. In the outer part of the core that temperature is high enough for the iron-nickel alloy to be liquid, but the pressure is so extreme at greater depth that the inner core is solid. The transfer of heat from the core leads to convection in the outer core, and, because the core is metallic, that convection generates the Earth’s magnetic field.

The key takeaways about the core are:

• The core is made of iron.
• The outer core is liquid, the inner core is solid.
• Since the core is so hot, the heat transfer causes convection in the outer core which generates the Earth's magnetic field.

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{2}\): Steven Earle, CC BY 4.0