Chapter 2 of G.C. Leong’s geography, titled “The Earth’s Crust,” focuses on the internal structure of the Earth and the various types of rocks that form its exterior.
1. Structure of the Earth
The Earth is composed of three concentric layers, often compared to an onion:
- The Lithosphere (Crust): The outermost layer.
- Sial: The upper crust, consisting of Silica and Aluminium. It forms the continental blocks and has a density of 2.7.
- Sima: The lower crust, consisting of Silica and Magnesium. It forms the ocean floors and is denser (3.0).
- The Mantle (Mesosphere): Located beneath the crust, extending to a depth of 1,800 miles. It is composed of very dense rocks rich in olivine.
- The Core (Barysphere): The innermost layer.
- Nife: Composed of Nickel and Iron. It has a radius of about 2,100 miles and extremely high temperatures and pressures.
2. Classification of Rocks
Rocks are categorized into three main types based on their origin and formation:
A. Igneous Rocks
Formed by the cooling and solidification of molten rock (magma or lava).
- They are crystalline in structure and contain no fossils.
- Plutonic (Intrusive): Formed deep in the crust (e.g., Granite).
- Volcanic (Extrusive): Formed on the surface (e.g., Basalt).
B. Sedimentary Rocks
Formed from the accumulation of “sediments” over long periods.
- They are stratified (layered) and often contain fossils.
- Mechanically Formed: Accumulated from other rocks (e.g., Sandstone, Shale).
- Organically Formed: From remains of living organisms (e.g., Limestone, Coal).
- Chemically Formed: Precipitated from solutions (e.g., Rock Salt, Gypsum).
C. Metamorphic Rocks
Formed when original rocks (igneous or sedimentary) are subjected to great heat and pressure.
- Clay changes into Slate.
- Limestone changes into Marble.
- Sandstone changes into Quartzite.
- Granite changes into Gneiss.
- Coal changes into Graphite.
3. Earth Movements and Landforms
The Earth’s crust is constantly being shaped by internal forces:
- Endogenic Forces: Internal forces that cause folding (mountains) and faulting (rift valleys).
- Orogenic Processes: Mountain-building movements that act tangentially.
- Epeirogenic Processes: Continental-building movements that act vertically, causing uplift or subsidence.
[!IMPORTANT]
Key Summary: The Earth’s crust is a thin, dynamic “skin” where Sial sits atop Sima. The rock cycle constantly transitions materials between Igneous, Sedimentary, and Metamorphic states through heat, pressure, and erosion.
