Interior of the Earth

1. Understand the sources of information about Earth’s interior, both direct and indirect.
2. Comprehend the mechanics of earthquakes, including how seismic waves propagate and create shadow zones.
3. Gain knowledge of the Earth’s structure: crust, mantle, and core, as well as volcanic landforms and intrusive forms.

Sources of Information About the Interior

The Earth’s radius measures about 6,378 km. Since reaching the Earth’s center is impossible, scientists have devised indirect methods to study its interior. Our knowledge stems from both direct observations and indirect inferences based on various phenomena.

Direct Sources:

1. Surface Rocks: The most accessible materials come from surface rocks or mining activities. Mines, such as gold mines in South Africa, have reached depths of 3-4 km. However, exploring deeper is difficult due to extreme heat.
2. Drilling Projects: Projects like the Deep Ocean Drilling Project and the Integrated Ocean Drilling Project have provided crucial information. The deepest drill, at Kola in the Arctic Ocean, reached 12 km. These projects collect samples at different depths for analysis.
3. Volcanic Eruptions: The magma brought to the surface during eruptions offers another source of data. However, it’s hard to pinpoint the exact depth of magma origin.

Indirect Sources:

1. Temperature and Pressure: Through mining activities, scientists know that temperature and pressure increase with depth. They estimate these characteristics throughout the Earth.
2. Meteors: Although not from Earth’s interior, meteors share a similar structure and composition with Earth, providing valuable insights.
3. Gravitation and Seismic Activity: Gravity varies by latitude and the material’s mass within the Earth, leading to gravity anomalies that reveal mass distribution. Seismic activity is the most crucial indirect source for understanding Earth’s interior, as earthquake waves reveal the structure beneath the surface.

Important Note: Earth’s materials vary in density, and seismic waves travel at different speeds depending on this, making seismic wave study vital for understanding Earth’s inner layers.

Earthquake: Causes and Mechanics

An earthquake occurs when energy is released along a fault line, a fracture in the Earth’s crust where two blocks of rock move against each other. The friction along the fault prevents movement until enough stress builds up, causing a sudden shift and release of energy, generating seismic waves.

1. Focus: The point within the Earth where the energy is released.
2. Epicenter: The point on Earth’s surface directly above the focus, experiencing the strongest shaking.

Earthquake Waves

All earthquakes occur in the lithosphere, the outer layer extending up to 200 km from the surface. Seismographs record the waves, producing curves that show different wave patterns. Earthquake waves are categorized into two types:

1. Body Waves: These travel through Earth’s interior, and there are two types:

• P-Waves: Also known as primary waves, they move the fastest and travel through solids, liquids, and gases.
• S-Waves: These slower secondary waves can only travel through solids. Their inability to move through liquids helps scientists infer Earth’s inner layers.

1. Surface Waves: Generated when body waves interact with surface rocks, these are the slowest but most destructive waves, causing rock displacement and structural collapse.

Propagation of Earthquake Waves

As P-waves and S-waves travel, they cause different types of vibrations. P-waves compress and expand material in the direction of wave travel, while S-waves create perpendicular vibrations, leading to crests and troughs.

Emergence of Shadow Zones

Seismographs detect waves from earthquakes around the globe, but certain areas, known as shadow zones, do not record these waves. For example, S-waves do not travel through liquids, creating large shadow zones that cover 40% of Earth’s surface. P-waves, although traveling through liquids, create a smaller shadow zone between 105° and 145° from the epicenter.

Types of Earthquakes

1. Tectonic Earthquakes: These are the most common and occur due to the sliding of rocks along a fault.
2. Volcanic Earthquakes: Occurring in volcanic areas, these earthquakes are triggered by magma movement.
3. Collapse Earthquakes: These result from the collapse of underground mines.
4. Explosion Earthquakes: Triggered by the detonation of nuclear or chemical devices.
5. Reservoir-Induced Earthquakes: Caused by the filling of large reservoirs, leading to tremors in the surrounding area.

Measuring Earthquakes

1. Magnitude: Measured on the Richter scale, earthquake magnitude reflects the energy released, with values ranging from 0 to 10.
2. Intensity: The Mercalli scale measures earthquake intensity based on visible damage, ranging from 1 to 12.

Effects of Earthquakes

Earthquakes can cause numerous hazards:

1. Ground Shaking: The most immediate effect, it damages buildings and infrastructure.
2. Differential Ground Settlement: Variations in how the ground settles lead to structural instability.
3. Landslides: Earthquakes trigger landslides in hilly areas.
4. Soil Liquefaction: Soil temporarily behaves like a liquid, compromising the foundations of buildings.
5. Ground Lurching: Sudden shifts in the ground affect roads and railways.
6. Avalanches: Triggered in mountainous regions.
7. Ground Displacement: Faults displace the land, breaking roads and pipelines.
8. Dam Failures: Earthquakes can rupture dams, causing catastrophic flooding.
9. Fires: Gas lines break, and electrical systems malfunction, sparking fires.
10. Tsunamis: Quakes below oceanic waters can generate massive waves, devastating coastal areas.

Important Note: Tsunamis are not caused by earthquakes themselves but by the movement of the seafloor during an earthquake.

Frequency of Earthquake Occurrences

Major earthquakes of magnitude 8 or higher are rare, occurring once every 1-2 years, while smaller tremors happen almost every minute. Despite their unpredictability, certain areas are more prone to frequent seismic activity, as discussed in future chapters.

Structure of the Earth

The Earth is composed of three main layers: the crust, mantle, and core.

The Crust

1. The crust is the outermost layer, and its thickness varies. The oceanic crust is thinner, around 5 km, while the continental crust can be as thick as 70 km under major mountain ranges like the Himalayas.

The Mantle

2. Below the crust lies the mantle, extending to a depth of 2,900 km. The upper part is called the asthenosphere, a weak, plastic layer where magma originates. The lower mantle remains solid.

The Core

3. The core lies beneath the mantle, divided into the liquid outer core and the solid inner core, primarily composed of nickel and iron. These materials give rise to the Earth’s magnetic field.

Volcanoes and Volcanic Landforms

Volcanoes occur when molten rock, gases, and ash escape from below the Earth’s surface. The molten rock below the crust is called magma, and when it reaches the surface, it becomes lava.

Types of Volcanoes

1. Shield Volcanoes: These are the largest volcanoes and have low-explosivity. Formed mostly of basalt, their fluid lava forms gentle slopes.
2. Composite Volcanoes: Made of more viscous lava, these volcanoes can be highly explosive, releasing both lava and large quantities of pyroclastic material.
3. Calderas: The most explosive type of volcanoes, calderas often collapse into themselves after large eruptions.
4. Flood Basalt Provinces: Known for massive outpourings of fluid lava, these areas are covered by vast basalt flows, such as the Deccan Traps in India.
5. Mid-Ocean Ridge Volcanoes: Occurring along the mid-ocean ridges, these volcanoes are part of an extensive system stretching over 70,000 km beneath the oceans.

Volcanic Landforms

When lava cools and solidifies, it forms igneous rocks. If this occurs beneath the surface, it results in intrusive forms, such as batholiths, laccoliths, and dykes.

Table Comparison: Intrusive Forms of Igneous Rocks

Important Note: The intrusive forms beneath the surface play a critical role in shaping the Earth’s landscape, especially when exposed through erosion.

MCQ

Which type of volcanic eruptions have caused Deccan Trap formations?
a) Shield
b)

Flood (Answer)

c) Composite
d) Caldera