Interior of the EarthClass 11 Fundamentals of Physical Geography Notes

Sources of Information about the Interior

Have you ever wondered what's deep inside the Earth? Since the Earth's radius is about 6,378 km, and humans cannot travel to its center due to extreme heat and pressure, scientists rely on various clues to understand its structure. Our knowledge comes from a mix of direct observations and indirect evidence, like piecing together a puzzle.

Direct Sources

These are sources where we can physically observe or analyze materials from within the Earth.

• Rocks from Mining and Drilling: The most accessible materials are rocks from the surface or from deep mines, like the gold mines in South Africa which go down 3-4 km. To go even deeper, scientists have started projects like the “Deep Ocean Drilling Project” and the “Integrated Ocean Drilling Project”. The deepest drill, located at Kola in the Arctic Ocean, has reached a depth of 12 km, providing valuable material for analysis.
• Volcanic Eruptions: When a volcano erupts, it brings molten material called magma from the Earth's interior to the surface. Once on the surface, this material can be collected and studied in a lab. However, it's often difficult to know the exact depth from which the magma originated.

Indirect Sources

These sources involve studying the properties of the Earth and other celestial bodies to infer what the interior is like.

• Temperature, Pressure, and Density: From mining, we know that temperature and pressure increase the deeper we go. The density of materials also increases with depth. By calculating the rate of these changes, scientists can estimate the conditions and types of materials at different layers of the Earth.
• Meteors: These are objects from space that sometimes reach the Earth. While not from our planet's interior, they are believed to be made of similar materials as the Earth. Studying their structure gives us clues about what our own planet's core might be like.
• Gravitation: The force of gravity is not the same everywhere on the Earth's surface. It's slightly stronger at the poles and weaker at the equator. This variation is partly due to the uneven distribution of mass within the Earth. Differences between expected and observed gravity readings, known as gravity anomaly, help scientists understand the distribution of mass in the crust.
• Magnetic Field: Magnetic surveys reveal the distribution of magnetic materials in the crust, giving another layer of information about the composition of the Earth's interior.
• Seismic Activity: The study of earthquake waves is one of the most important sources of information. It provides a detailed picture of the Earth's layered structure.

Earthquake

An earthquake is simply the shaking of the Earth. It's a natural event caused by the release of energy deep within the planet, which creates waves that travel in all directions.

The release of energy happens along a fault, which is a sharp break in the crustal rocks. The rocks on either side of a fault are pushed in opposite directions, but friction holds them in place. Over time, the pressure builds until it overcomes the friction, causing the rocks to suddenly slip past one another. This abrupt movement releases a massive amount of energy.

• The point where the energy is released is called the focus or hypocentre.
• The point on the Earth's surface directly above the focus is the epicentre. The epicentre is the first place to experience the earthquake waves.

Earthquake Waves

All natural earthquakes occur in the lithosphere, which is the portion of the Earth extending up to 200 km from the surface. The waves generated by an earthquake are recorded by an instrument called a seismograph.

There are two main types of earthquake waves:

1. Body Waves: These are generated at the focus and travel through the Earth's interior, or "body."
2. Surface Waves: These are created when body waves interact with the surface rocks. They travel along the surface and are the most destructive.

The velocity of these waves changes as they pass through materials of different densities—the denser the material, the faster the waves travel. Their direction can also change, either by reflecting (bouncing off) or refracting (bending) when they encounter different layers.

There are two types of body waves:

• P-waves (Primary waves): These are the fastest waves and the first to be recorded by a seismograph. They are similar to sound waves and can travel through solid, liquid, and gaseous materials.
• S-waves (Secondary waves): These arrive after the P-waves. They have a unique and important characteristic: they can only travel through solid materials.

Propagation of Earthquake Waves

Earthquake waves cause vibrations in the rocks they pass through in different ways:

• P-waves vibrate parallel to the direction of wave movement. This causes the rock material to be stretched and squeezed.
• S-waves vibrate perpendicular to the direction of wave movement. This creates crests and troughs in the material, similar to waves on water.

Emergence of Shadow Zone

A shadow zone is a specific area on the Earth's surface where seismographs do not detect the waves from an earthquake. The location of the shadow zone is different for every earthquake.

• Seismographs within 105° from the epicentre record both P-waves and S-waves.
• Seismographs beyond 145° from the epicentre record P-waves, but not S-waves.
• The area between 105° and 145° from the epicentre is the shadow zone for both P-waves and S-waves.
• The shadow zone for S-waves is much larger, covering the entire area beyond 105°. This is because S-waves cannot pass through the liquid outer core. The shadow zone for S-waves covers over 40% of the Earth's surface.

Types of Earthquakes

Earthquakes can be classified based on their cause:

• Tectonic earthquakes: The most common type, caused by the sliding of rocks along a fault.
• Volcanic earthquakes: A special type of tectonic earthquake confined to areas with active volcanoes.
• Collapse earthquakes: Minor tremors caused by the collapse of roofs in underground mines.
• Explosion earthquakes: Ground shaking caused by the explosion of chemical or nuclear devices.
• Reservoir induced earthquakes: Occur in areas with large reservoirs.

Measuring Earthquakes

Earthquakes are measured using two main scales:

1. Magnitude (Richter scale): This scale measures the amount of energy released during the earthquake. It is expressed in numbers from 0-10.
2. Intensity (Mercalli scale): This scale measures the visible damage caused by the earthquake. The range of intensity is from 1-12.

Effects of Earthquake

Earthquakes are natural hazards with many immediate and dangerous effects, including:

• Ground Shaking
• Land and mud slides
• Avalanches
• Floods from dam failures
• Fires
• Structural collapse
• Tsunami: These are large waves generated by an earthquake if the epicentre is under oceanic waters and the magnitude is high. A tsunami is a result of an earthquake, not the earthquake itself.

Structure of the Earth

The study of seismic waves has allowed scientists to map out the layered structure of the Earth's interior.

The Crust

• This is the outermost, solid part of the Earth. It is brittle in nature.
• Its thickness varies:
  • Oceanic crust: Thinner, with a mean thickness of 5 km.
  • Continental crust: Thicker, with a mean thickness of around 30 km. It can be as thick as 70 km in areas with major mountain systems like the Himalayas.

The Mantle

• Located beneath the crust, the mantle extends to a depth of 2,900 km from a boundary called Moho's discontinuity.
• The upper portion of the mantle is called the asthenosphere (from the word astheno, meaning weak). It extends up to 400 km and is the main source of the magma that erupts from volcanoes.
• The lithosphere is composed of the crust and the uppermost, solid part of the mantle. Its thickness ranges from 10-200 km.
• The lower mantle, below the asthenosphere, is in a solid state.

The Core

• The boundary between the mantle and the core is at a depth of 2,900 km.
• Outer Core: This layer is in a liquid state.
• Inner Core: This layer is in a solid state.
• The core is made of very heavy materials, mostly nickel and iron. For this reason, it is sometimes called the nife layer (Ni for nickel, Fe for iron).

Volcanoes and Volcanic Landforms

A volcano is a place where gases, ashes, and/or molten rock material (lava) escape to the ground from the Earth's interior. The molten material comes from the asthenosphere in the upper mantle.

• Magma: Molten rock material while it is still inside the Earth.
• Lava: Molten rock material once it reaches the surface.

Volcanoes

Volcanoes are classified based on the nature of their eruption and the landform they create.

Shield Volcanoes

• These are the largest volcanoes on Earth (excluding massive basalt flows).
• They are made of basalt, a type of lava that is very fluid when it erupts. This fluidity means the lava spreads out over large areas, creating wide, gently sloping volcanoes that look like a warrior's shield.
• Eruptions are generally low-explosivity unless water gets into the vent.
• The Hawaiian volcanoes are famous examples.

Composite Volcanoes

• These volcanoes erupt cooler, more viscous (thicker) lavas compared to basalt.
• Their eruptions are often explosive, throwing out large quantities of pyroclastic material and ash along with lava.
• This material builds up in layers around the vent, creating steep-sided, cone-shaped mountains.

Caldera

• These are the most explosive volcanoes on Earth.
• They are so explosive that they tend to collapse on themselves during an eruption rather than building a tall structure.
• The collapsed depressions are called calderas. Their extreme explosiveness suggests a very large magma chamber located close to the surface.

Flood Basalt Provinces

• These volcanoes pour out highly fluid lava that flows for very long distances, covering thousands of square kilometers in thick sheets.
• Individual flows can be over 50 meters thick and extend for hundreds of kilometers.
• The Deccan Traps in India, which cover most of the Maharashtra plateau, are a well-known example.

Mid-Ocean Ridge Volcanoes

• These volcanoes are found in oceanic areas.
• They occur along a system of mid-ocean ridges that stretches for more than 70,000 km through all the world's ocean basins. Frequent eruptions occur along the central part of these ridges.

Volcanic Landforms

When lava cools, it solidifies into igneous rocks.

• Volcanic rocks form when lava cools on the Earth's surface.
• Plutonic rocks form when magma cools while still inside the Earth's crust.

Intrusive Forms

These are the various shapes and structures formed when magma cools and solidifies within the crust. They are only visible on the surface after the overlying rock has been eroded away.

• Batholiths: A very large mass of magmatic material that cools deep within the crust, forming large domes. They are essentially the cooled portions of magma chambers.
• Lacoliths: Large, dome-shaped intrusive bodies with a flat base, connected to a magma source below by a pipe-like conduit.
• Lapolith: A saucer-shaped intrusive body that is concave (curved inward) towards the sky.
• Phacolith: A wavy mass of intrusive rock found at the base of troughs (synclines) or the crest of arches (anticlines) in folded rock layers.
• Sills and Sheets: Near-horizontal bodies of intrusive rock. Sills are the thick deposits, while sheets are thinner.
• Dykes: Wall-like structures that form when lava makes its way through vertical cracks or fissures in the land and solidifies there. Dykes are common in the western Maharashtra area and are thought to be the "feeders" for the eruptions that formed the Deccan Traps.