Earth's Structure & Landforms

Learning Objective: Explain Earth's structure and how major landforms are created

The Ground Beneath Your Feet

If you experienced the devastating earthquake of April 25, 2015, you know firsthand that the Earth is not a solid, unchanging mass. The very existence of the Himalayas -- the roof of the world, visible from your own country -- is proof that the Earth's surface is constantly moving. Nepal sits on one of the most geologically active zones on the planet, making this lesson about much more than textbook theory.

Layers of the Earth

The Earth is composed of four main layers:

1. Crust -- the thin, rocky outer shell (5--70 km thick). The continental crust under Nepal is about 40--70 km thick. There are two types: oceanic crust (thinner, denser, under oceans) and continental crust (thicker, lighter, forming landmasses).

2. Mantle -- a thick layer of semi-solid rock (about 2,900 km thick) below the crust. The upper mantle flows slowly due to heat from the core, driving plate movement.

3. Outer Core -- liquid iron and nickel (about 2,200 km thick), responsible for Earth's magnetic field.

4. Inner Core -- solid iron and nickel (about 1,200 km radius), with temperatures exceeding 5,000 degrees Celsius.

Plate Tectonics: Why Nepal Has Mountains

The Earth's crust is broken into large pieces called tectonic plates that float on the mantle. There are about 15 major plates. These plates move slowly -- a few centimeters per year -- and their interactions create the world's major landforms.

The Himalayas were formed by the collision of the Indian Plate and the Eurasian Plate, which began approximately 50 million years ago. The Indian Plate, moving northward at about 5 cm per year, pushed under the Eurasian Plate, forcing the land upward. This process, called continental collision, is still ongoing -- Mount Everest (Sagarmatha) grows by about 4 mm per year.

Types of plate boundaries:

• Convergent -- plates collide (forming mountains, like the Himalayas)
• Divergent -- plates move apart (forming mid-ocean ridges)
• Transform -- plates slide past each other (causing earthquakes, like the San Andreas Fault)

Nepal's Physiographic Regions

Nepal's unique geography is a direct result of plate tectonics. From south to north, Nepal has five physiographic regions:

1. Terai -- flat plains (60--300 m elevation), extension of the Indo-Gangetic plain
2. Siwalik (Churia) Hills -- the youngest Himalayan range (300--1,500 m)
3. Middle Hills (Mahabharat Range) -- (1,500--2,700 m), where Kathmandu Valley sits
4. High Himalayas -- including 8 of the world's 14 peaks above 8,000 m
5. Inner Himalaya (Trans-Himalaya) -- rain shadow region, including Mustang and Dolpo

Rivers of Nepal

Nepal's major rivers -- Koshi, Gandaki, and Karnali -- originate in the Himalayas and flow southward into India, eventually reaching the Ganges. These rivers are fed by glacial melt and monsoon rainfall, and their enormous hydroelectric potential (estimated at 83,000 MW) represents Nepal's most significant natural resource.

Earthquakes in Nepal

Nepal lies in a seismically active zone because the Indian Plate continues to push into the Eurasian Plate. Major historical earthquakes include:

• 1934 Nepal-Bihar earthquake (8.0 magnitude) -- destroyed much of Kathmandu
• 2015 Gorkha earthquake (7.8 magnitude) -- killed nearly 9,000 people and destroyed over 600,000 structures

Understanding earthquakes is not optional knowledge in Nepal -- it is essential for survival and preparedness.

Think Critically

Nepal has an estimated hydroelectric potential of 83,000 MW, yet it has developed only a fraction of this. What geological, economic, and political factors might explain this gap?

Summary

• The Earth has four layers: crust, mantle, outer core, and inner core.
• Plate tectonics explains how landforms are created through plate collisions, separations, and sliding.
• The Himalayas formed from the collision of the Indian and Eurasian plates approximately 50 million years ago.
• Nepal has five physiographic regions from the Terai to the Trans-Himalaya.
• Nepal is earthquake-prone due to its location on an active plate boundary.