Inside the Earth: Crust, Mantle, Core and the Rocks

1.1 What is the Earth made of?

USGS says our planet has three main layers: crust, mantle and core. The crust is the outermost layer. Like an eggshell, it is brittle and can break. Under the oceans it is only about 5 km thick. Under continents it averages about 30 km.

Under big mountain ranges the crust can reach 100 km.

1.2 What are the mantle and core?

Below the crust is the mantle. It is a dense, hot layer of semi-solid rock about 2,900 km thick. At the centre lies the core. It is nearly twice as dense as the mantle, because it is a metal alloy of iron and nickel.

Source: U.S. Geological Survey, This Dynamic Earth.

The core has two parts. The outer core is liquid and 2,200 km thick. The inner core is solid and 1,250 km thick. NASA says the inner core can reach 5,400 degrees Celsius.

1.3 Why does the core matter?

As the Earth rotates, the liquid outer core spins. That creates the Earth’s magnetic field.

2.1 What is the lithosphere?

The upper part of the mantle is cooler and more rigid than the deep mantle. Together with the crust it forms a rigid layer called the lithosphere. It averages at least 80 km in thickness. It is broken into the moving plates that carry the continents and oceans.

2.2 Why does the asthenosphere matter?

Below the lithosphere is the asthenosphere. It is a narrow, mobile zone in the mantle. It is hot, semi-solid material that can soften and flow. The rigid lithosphere is thought to float on this slowly flowing layer. That is how plates move.

• Lithosphere: rigid, includes the crust.
• Asthenosphere: soft, flowing, sits below.

3.1 Can we dig to the core?

No. The Kola Superdeep Borehole in Russia started on 24 May 1970. Its deepest hole reached 12,262 metres. That is a tiny scratch on a planet about 12,750 km across.

So scientists use earthquake waves. These waves change speed when they cross a boundary between materials.

3.2 What is the Moho?

On 8 October 1909 the Kupa Valley earthquake shook Croatia. Andrija Mohorovicic studied its seismograms, along with records from across Europe. He proved there is a boundary between the crust and the mantle. It is now called the Mohorovicic discontinuity, or the Moho.

A teaching lab at Tennessee Tech says P-waves speed up suddenly at about 50 km depth. That is the Moho.

3.3 What are the deeper boundaries?

• Gutenberg discontinuity: mantle to outer core, at about 2,900 km. P-waves slow sharply here.
• Lehmann discontinuity: outer core to inner core, at about 5,100 km. P-waves speed up here.

4.1 What is an earthquake wave?

When an earthquake strikes, shaking waves travel out from the focus, the point of origin. The point on the surface directly above it is the epicentre.

Two kinds of waves travel through the Earth. They are called P-waves and S-waves. They are body waves. The third kind, the surface wave, starts when P- and S-waves reach the surface.

4.2 How do P-waves and S-waves differ?

• P-waves travel through solids, liquids and gases. They move faster through dense material.
• S-waves travel through solid material only.

4.3 Why are surface waves a problem?

Surface waves travel outward along the surface from the epicentre. They are born when body waves reach the top layer of rock.

5.1 What is the shadow zone?

Seismograph stations far from an earthquake can miss its waves. P- and S-waves are not detected beyond about 103 degrees from the epicentre. P-waves show up again at about 143 degrees or more. So a ring between 103 and 143 degrees gets no waves at all.

Books give slightly different angles, such as 105 and 145 degrees. Learn the idea, not one exact number.

5.2 What does it tell us about the core?

S-waves cannot travel through liquids. They fail to arrive on the far side of the Earth. So scientists think the outer core is fluid. P-waves bend sharply where the mantle meets the core. That bend creates the P-wave gap.

6.1 What are the main kinds?

USGS groups volcanoes into four kinds: cinder cones, composite volcanoes, shield volcanoes and lava domes. Three of these matter most for exams.

6.2 What is a shield volcano?

Shield volcanoes are built almost entirely of fluid lava flows. They form a broad, gently sloping cone like a warrior’s shield. The Hawaiian Islands are chains of shield volcanoes. Kilauea and Mauna Loa are examples.

6.3 What is a composite volcano?

Composite volcanoes are steep-sided cones. They are built of alternating layers of lava, ash, cinders, blocks and bombs. Mount Fuji is one.

Their thick lava traps gas. Pressure builds, so eruptions are often explosive. They are about half lava and half pyroclastic material. Pyroclastic material is the loose ash, cinders, blocks and bombs thrown out by an eruption.

6.4 What is a caldera?

A caldera is a depression formed when a volcano collapses. Crater Lake in Oregon is one. After tremendous explosions about 6,800 years ago, Mount Mazama lost its top.

7.1 What are flood basalts?

In some eruptions basalt lava pours out quietly from long fissures, not from a central vent. It floods the countryside, flow after flow, and forms broad plateaus. These are called flood basalts. They are thick successions of basaltic flows erupted over short time spans.

7.2 What are the Deccan Traps?

The Deccan Traps are India’s great flood basalt. ‘Traps’ means step-like features. ‘Deccan’ comes from the Sanskrit Dakshin, meaning south. The exposed lava covers about 500,000 square kilometres. It is about 3,000 metres thick. It erupted from 67 to 60 million years ago, in the Late Cretaceous.

Read the Deccan Traps article on this site for black soil and the Western Ghats.

7.3 What are dikes?

Lava that cools in a fissure forms a dike. In composite volcanoes the dikes act as ribs and strengthen the cone.

8.1 Which mix-ups come up?

• Moho is crust to mantle. Gutenberg is mantle to outer core.
• S-waves need solids. That is why they show the outer core is liquid.
• Surface waves come from body waves reaching the surface.
• Shield is broad; composite is steep and explosive.
• Lithosphere is rigid; asthenosphere flows.