Module 45. Earth Structure and Geology

The Earth, a dynamic planet within the solar system, possesses a complex internal structure and surface composition shaped by geological processes operating over billions of years. Understanding its structure and geology is fundamental to interpreting natural phenomena such as earthquakes, volcanic activity, mountain formation, and plate movements. The study of the Earth’s physical composition, materials, and processes is known as geology, which combines aspects of physics, chemistry, and biology to explain the planet’s evolution and ongoing transformations.

Internal Structure of the Earth

The Earth’s internal structure is divided into three primary layers: the crust, the mantle, and the core. Each layer differs in composition, density, and physical properties.

• Crust: The outermost layer, composed mainly of silicate rocks, forms the Earth’s solid surface. It is divided into two types:
  • Continental crust (thicker, about 35–70 km) made largely of granitic rocks.
  • Oceanic crust (thinner, about 5–10 km) made mainly of basaltic rocks.
• Mantle: Beneath the crust lies the mantle, extending to a depth of about 2,900 km. It consists of silicate minerals rich in iron and magnesium. The upper mantle includes the asthenosphere, a semi-molten zone that enables tectonic plates to move.
• Core: The central part of the Earth, extending from 2,900 km to the centre at 6,371 km. It comprises:
  • Outer core – liquid iron and nickel, responsible for generating the Earth’s magnetic field.
  • Inner core – solid iron and nickel, extremely dense due to immense pressure.

Seismic waves generated by earthquakes have been instrumental in deducing this layered structure, as they travel at different speeds through materials of varying densities.

Plate Tectonics and Continental Drift

The theory of plate tectonics provides a unifying framework for understanding Earth’s geology. It explains that the lithosphere, comprising the crust and uppermost mantle, is broken into rigid plates that float on the ductile asthenosphere. These plates continuously move, albeit slowly, due to convection currents in the mantle.
Types of plate boundaries include:

• Divergent boundaries, where plates move apart (e.g., Mid-Atlantic Ridge).
• Convergent boundaries, where plates collide, leading to mountain formation or subduction (e.g., Himalayas, Andes).
• Transform boundaries, where plates slide past each other (e.g., San Andreas Fault).

The continental drift theory, proposed by Alfred Wegener in 1912, was an early explanation for the movement of continents, suggesting that they were once joined in a supercontinent called Pangaea. Modern plate tectonics has expanded upon this concept with greater evidence from seafloor spreading, paleomagnetism, and fossil distribution.

Geological Processes and Rock Formation

Geological processes operate over different time scales, continuously shaping the Earth’s surface. The rock cycle illustrates how three major rock types transform under varying conditions:

• Igneous rocks form from the cooling and solidification of magma or lava (e.g., granite, basalt).
• Sedimentary rocks originate from the compaction and cementation of sediments derived from pre-existing rocks (e.g., limestone, sandstone).
• Metamorphic rocks arise from alteration of existing rocks under high temperature and pressure (e.g., marble, schist).

Erosion, weathering, and deposition are key surface processes that break down and transport materials, while internal processes such as volcanism and mountain building reshape the landscape.

Geological Time Scale and Earth’s Evolution

The geological time scale divides Earth’s history into eons, eras, periods, and epochs, based on major events such as mass extinctions and the emergence of life forms. The four main eons are:

1. Hadean (4.6–4.0 billion years ago): Formation of the Earth and early crust.
2. Archaean (4.0–2.5 billion years ago): Stabilisation of the crust and appearance of the earliest life.
3. Proterozoic (2.5 billion–541 million years ago): Development of multicellular organisms and oxygenation of the atmosphere.
4. Phanerozoic (541 million years ago–present): Proliferation of complex life, major evolutionary changes, and continental reconfigurations.

This time scale provides a chronological framework for interpreting rock formations, fossils, and tectonic events.

Geological Features and Landforms

The Earth’s geology manifests in diverse landforms shaped by internal and external forces. Mountains, volcanoes, plateaus, valleys, and plains arise from a combination of tectonic movements and erosional processes. For example:

• Fold mountains such as the Himalayas result from compressional forces at convergent boundaries.
• Volcanic landforms like Mount Etna and Mauna Loa form where magma reaches the surface.
• Rift valleys, such as the East African Rift, develop where plates diverge.

Additionally, sedimentary processes in rivers, glaciers, and deserts contribute to the formation of deltas, dunes, and moraines, reflecting ongoing surface dynamics.

Importance and Applications of Geology

Geology has profound implications for human civilisation. It aids in the exploration of natural resources such as minerals, fossil fuels, and groundwater, and helps in disaster management by predicting and mitigating the effects of earthquakes, landslides, and volcanic eruptions. Engineering geology contributes to the safe design of structures and foundations, while environmental geology assists in managing soil degradation, waste disposal, and groundwater contamination.
Modern techniques such as remote sensing, geophysical surveys, and radiometric dating have enhanced the precision of geological studies, allowing scientists to reconstruct past environments and predict future changes.

Contemporary Challenges and Research Directions

Contemporary geological research addresses issues like climate change, sea-level rise, and resource depletion. Understanding past geological records helps predict future environmental trends. Moreover, planetary geology—comparing Earth with celestial bodies such as Mars—offers insights into the planet’s uniqueness and the processes that sustain life.