We cannot directly see Earth's interior. However, by studying how seismic waves travel, we know that the interior is divided into layers.
Basic Knowledge
From the surface inward, Earth's interior is divided into the following four layers.
① Crust: From the surface to the Mohorovičić discontinuity (Moho). Continental crust is approximately 30〜60 km thick, and oceanic crust is approximately 5〜10 km thick. It is mainly composed of rocks (granite and basaltic rocks). Density is approximately 2.7〜3.0 g/cm³.
② Mantle: From the Moho to a depth of approximately 2900 km. It is the largest layer, accounting for approximately 80% of Earth's total volume. It is peridotitic. Density is 3.3〜5.6 g/cm³. Although solid, it flows over a timescale of several million years because of its extremely high temperature and pressure (convection).
③ Outer core: Depth of approximately 2900〜5100 km. Liquid iron (Fe) and nickel (Ni). Density is 10〜12 g/cm³. It was determined to be liquid because S waves cannot pass through it.
④ Inner core: From a depth of approximately 5100 km to Earth's center (6371 km). Solid iron and nickel. Density is approximately 13 g/cm³.
The rigid rocky layer consisting of the crust and uppermost mantle is called the lithosphere, and the softer layer beneath it is called the asthenosphere.
Crust（outermost layer. Continental: 30〜60 km / oceanic: 5〜10 km）
Mohorovičić discontinuity (Moho)（boundary between the crust and mantle）
Mantle（to a depth of〜2900 km. Peridotitic; solid but flowing）
Outer core（depth 2900〜5100 km. Liquid Fe-Ni. S waves cannot pass through）
Inner core（depth 5100 km〜the center. Solid Fe-Ni）
Lithosphere（crust + uppermost mantle. Approximately 100 km thick）
Asthenosphere（beneath the lithosphere. Soft and prone to flow）
Further Exploration
Seismic waves enabled scientists to clarify Earth's internal structure. In 1909, Croatian seismologist Andrija Mohorovičić discovered the boundary between the crust and mantle (the Moho). In 1914, Beno Gutenberg discovered the boundary with the outer core (the Gutenberg discontinuity, at a depth of 2900 km), and in 1936, Inge Lehmann discovered the existence of the inner core.
The Earth's magnetic field is thought to be generated by convection of liquid iron in the outer core (the dynamo theory). The fact that the outer core is liquid was determined from the observation that S waves (transverse waves) cannot pass through it.
- Earth's interior: crust → mantle → outer core → inner core
- Moho = boundary between crust and mantle
- Outer core = liquid Fe-Ni (S waves cannot pass through)
- Inner core = solid Fe-Ni
- Lithosphere = crust + uppermost mantle (approximately 100 km thick)
- The mantle is solid but flows over long periods
Points to Note
① The outer core is liquid and the inner core is solid. Do not reverse them. ② The mantle is solid, but it flows over an extremely long timescale (several million years). “The mantle is liquid” is incorrect. ③ The Moho is the lower surface of the crust, and its depth differs greatly between continents and oceans.
Practice
- What is the boundary between the crust and mantle called?
- From what property of which seismic wave can we determine that the outer core is liquid?
- What is the approximate thickness of continental crust, in km?