The Earth's Interior, Plate Tectonics and Volcanoes
World geography begins with the structure of the Earth itself. To understand earthquakes, volcanoes, mountains and the movement of continents, a candidate must first know the internal layers of the Earth and the theory of plate tectonics. This topic is a favourite of Assam recruitment examinations because it combines direct facts, such as the thickness of a layer or the name of a scientist, with conceptual questions about why the plates move and what happens when they meet. Examiners repeatedly ask about the crust, mantle and core, the lithosphere and asthenosphere, the number and names of the major plates, the three types of plate boundaries, and the classification and examples of volcanoes.
The three layers of the Earth
The Earth is made of three main layers - the crust, the mantle and the core. The crust is the outermost and thinnest layer. Its average thickness is about 35 kilometres under the continents and about 5 kilometres under the oceans. Below the crust lies the mantle, which is the thickest layer of the Earth, extending to a depth of about 2,900 kilometres. The core is the innermost part. The outer core is liquid (molten iron and nickel) while the inner core is solid iron and nickel, kept solid by enormous pressure. The temperature of the core is about 5,000 to 6,000 degrees Celsius, and the average radius of the Earth is about 6,371 kilometres.
The crust together with the rigid uppermost part of the mantle forms the lithosphere. Below the lithosphere lies the asthenosphere, a semi-molten, plastic layer of the upper mantle on which the plates move. This distinction is important because it is the asthenosphere, not the solid crust, that allows the plates to drift.
Continental drift and plate tectonics
The theory of continental drift was proposed by Alfred Wegener in 1912. He argued that all the present continents were once joined in a single supercontinent called Pangaea, which existed about 200 million years ago and later broke apart and drifted to their present positions. Wegener's idea was later developed into the modern theory of plate tectonics, according to which the Earth's lithosphere is divided into a number of rigid plates that float on the asthenosphere and move slowly because of convection currents in the mantle. There are seven major plates - the Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic plates - along with several minor plates. The Pacific plate is the largest.
The three types of plate boundaries
Most geological activity happens at plate boundaries. At a convergent boundary two plates collide. Where an oceanic plate meets a continental plate, the denser oceanic plate sinks below the other in a process called subduction, producing deep ocean trenches, volcanic arcs and fold mountains. At a divergent boundary the plates move apart, and magma rises to form new crust, creating mid-oceanic ridges and rift valleys. At a transform boundary the plates slide horizontally past each other, releasing energy as earthquakes. The belt of volcanoes and earthquakes that encircles the Pacific Ocean is called the Ring of Fire, and the underwater mountain system formed at divergent boundaries is the mid-oceanic ridge, which is the longest mountain range in the world at about 65,000 kilometres.
Volcanoes and their types
A volcano is an opening in the Earth's crust through which molten rock, gases and ash escape. The molten rock below the surface is called magma; once it reaches the surface it is called lava. On the basis of activity, volcanoes are classified as active, dormant or extinct. On the basis of shape there are shield volcanoes, formed by low-viscosity lava that spreads widely (for example Mauna Loa in Hawaii, the largest active volcano in the world), and composite or stratovolcanoes, tall cones built of alternating layers of lava and ash. Other features include calderas (large collapse craters) and geysers. Mount Fuji in Japan, about 3,776 metres high, is a famous stratovolcano, and the highest volcano in the world is Nevado Ojos del Salado in the Andes.
Quick revision
For quick revision, remember that the Earth has three layers: the crust (about 35 km continental, 5 km oceanic), the mantle (about 2,900 km, the thickest) and the core (outer liquid, inner solid, about 5,000-6,000 degrees Celsius). The lithosphere is the crust plus the upper mantle, and it moves over the semi-molten asthenosphere. Wegener proposed continental drift in 1912 and named the supercontinent Pangaea, which existed about 200 million years ago. There are seven major plates, the Pacific being the largest. Convergent boundaries build mountains and trenches, divergent boundaries create mid-oceanic ridges, and transform boundaries cause earthquakes. Magma becomes lava at the surface, and volcanoes are active, dormant or extinct.
How to prepare this topic
- Memorise the three layers with their thickness, composition and temperature.
- Learn the difference between the lithosphere and the asthenosphere and why plates move.
- Fix the names - Alfred Wegener, Pangaea, plate tectonics - and the count of seven major plates.
- Draw a simple diagram of the three plate boundaries so that mountain, trench and ridge formation is clear.
- Keep the volcano examples ready: Mauna Loa (largest active), Mount Fuji (Japan) and Nevado Ojos del Salado (highest).
Common exam traps
- The mantle is the thickest layer, not the core.
- The inner core is solid, while the outer core is liquid.
- Magma is below the surface and lava is at the surface - do not interchange them.
- The Pacific plate is the largest plate; it is not a continent.
- The Ring of Fire is a belt around the Pacific Ocean, not a ring of volcanoes around the whole Earth.
Frequently Asked Questions
What are the three layers of the Earth and which is the thickest?
The Earth has three main layers - the crust, the mantle and the core. The crust is the outermost and thinnest layer, about 35 kilometres thick under the continents and about 5 kilometres under the oceans. The mantle is the thickest layer, extending to a depth of about 2,900 kilometres, and the core is the innermost part, with a liquid outer core and a solid inner core made of iron and nickel.
Who proposed the theory of continental drift and what is Pangaea?
Alfred Wegener proposed the theory of continental drift in 1912. He said that all the continents were once joined in a single supercontinent called Pangaea, which existed about 200 million years ago and later broke apart to form the present continents. His idea was later developed into the modern theory of plate tectonics, which explains the movement of the Earth's lithospheric plates over the asthenosphere.
What happens at convergent, divergent and transform plate boundaries?
At a convergent boundary two plates collide, producing fold mountains, deep ocean trenches and volcanoes through subduction. At a divergent boundary the plates move apart and magma rises to form new crust and mid-oceanic ridges. At a transform boundary the plates slide past each other horizontally, causing earthquakes. The belt of volcanic and seismic activity around the Pacific Ocean is known as the Ring of Fire.
What is the difference between magma and lava?
Magma is the molten rock, along with dissolved gases, that lies beneath the Earth's surface. When this molten rock reaches the surface through an opening in the crust it is called lava. Volcanoes are classified by activity as active, dormant or extinct, and by shape as shield volcanoes, such as the low-viscosity Mauna Loa, and composite or stratovolcanoes built of layers of lava and ash.
Which is the largest active volcano and the highest volcano in the world?
Mauna Loa in Hawaii is the largest active volcano in the world, formed by low-viscosity lava that gives it a broad shield shape. The highest volcano in the world is Nevado Ojos del Salado in the Andes mountains. Mount Fuji in Japan, about 3,776 metres high, is a well-known composite or stratovolcano and the highest peak of Japan.