Distribution of Oceans and Continents 11 Geography Chapter 4 CBSE Notes 2025-26

In this chapter, students explore how the distribution of oceans and continents on Earth has not always been the same and has changed over millions of years. Today, continents make up about 29% of the Earth's surface, while oceans cover the rest. Scientists have studied the past positions and movements of these landmasses to understand the current arrangement and the processes behind their changes. One of the fundamental concepts is that continents and oceans continuously shift their locations due to internal Earth processes, and this movement will persist in the future as well.

Continental Drift Theory The first comprehensive explanation for shifting continents was given by Alfred Wegener, a German meteorologist, in 1912. He suggested that all continents were once unified as a supercontinent named Pangaea, which was surrounded by a vast ocean called Panthalassa. Around 200 million years ago, Pangaea began to split into two large landmasses—Laurasia in the north and Gondwanaland in the south. These major parts later broke down into the continents we see today.

Evidence Supporting Continental Drift Wegener put forward several evidences to support his theory:

• The Matching of Continents (Jig-saw Fit): The coastlines of Africa and South America have a remarkable fit. Bullard, in 1964, created a computer-generated map showing that these continents match perfectly along the 1,000-fathom line under the sea.
• Rocks of the Same Age Across Oceans: Radiometric dating reveals similar ancient rock belts on the coasts of Brazil and western Africa, indicating they were once joined.
• Tillite Deposits: Thick sedimentary rocks formed by glaciers (tillite) found in India match with similar deposits in many southern hemisphere landmasses, supporting the idea that these areas shared similar climates and histories.
• Placer Deposits: Gold deposits along the Ghana coast originated from source rocks in Brazil, suggesting the continents were connected.
• Distribution of Fossils: Identical fossil species are found in India, Madagascar, Africa, and South America, indicating connectivity between these landmasses at some point.

Wegener also proposed that the movement of continents was caused by pole-fleeing force (due to Earth's rotation and resulting equatorial bulge) and tidal force (gravitational pull from the Moon and Sun). However, scientists later found these forces to be too weak to explain continental drift.

Post-Drift Studies and New Discoveries After World War II, new evidence was collected from ocean floor mapping. Researchers found that the ocean floor was not flat but had features like ridges, trenches, and mountain ranges. Arthur Holmes, in the 1930s, introduced the idea of convection currents in the mantle, which produce the force responsible for moving the plates.

Ocean Floor Configuration The ocean floor can be divided into three main parts:

• Continental Margins: These are the transition zones from the shore to deeper ocean, including continental shelves, slopes, rises, and deep-ocean trenches.
• Abyssal Plains: These vast plains lie between the continental margins and mid-oceanic ridges, collecting sediments from the continents.
• Mid-Oceanic Ridges: The world’s longest mountain chains, submerged under the ocean, have a central rift zone with frequent volcanic activity.

Earthquakes and Volcanoes Distribution By studying maps, scientists noticed that many earthquakes and volcanoes occur along mid-oceanic ridges. Earthquake foci at these ridges are usually shallow, while deeper earthquakes happen along the Pacific and Alpine-Himalayan regions. The Pacific Rim is especially known as the “ring of fire” due to frequent volcanic activity.

Sea Floor Spreading Further studies led to the theory of sea floor spreading by Harry Hess in 1961. Key findings include:

• Volcanic eruptions at the crests of mid-oceanic ridges bring new lava to the surface.
• Rocks equidistant from the ridge’s crest have similar composition, age, and magnetic properties; youngest rocks are closest to ridges.
• Oceanic crust is young (below 200 million years), but some continental rocks are as old as 3,200 million years.
• Marine sediments on the ocean floor are much thinner than expected if oceans were as old as continents.
• Deep trenches have deeper earthquakes; ridges have shallow ones.

Hess proposed that new crust forms at ridges and spreads outward, while the older crust sinks at trenches and is consumed.

Plate Tectonics Building on these ideas, the plate tectonics theory explained that the Earth's lithosphere is divided into several major and minor plates, which float and move over the semi-fluid asthenosphere. Plates can be either mostly continental or oceanic, and they move in different directions at various speeds. Some primary plates include the Pacific, Eurasian, North American, South American, African, Antarctic, and Indian-Australian plates. Minor plates like the Cocos, Nazca, Arabian, Philippine, and Caroline plates also exist.

Movement is not limited to continents; it is the large plates that move, carrying continents with them. Pangaea formed due to converging plates and later drifted apart. Palaeomagnetic records help retrace the positions of continents across different ages.

Types of Plate Boundaries

• Divergent Boundaries: Plates move away from each other, forming new crust. Example: The Mid-Atlantic Ridge.
• Convergent Boundaries: One plate dives beneath another, destroying crust. Subduction can occur ocean-continent, ocean-ocean, or continent-continent.
• Transform Boundaries: Plates slide horizontally past each other, such as along transform faults perpendicular to mid-oceanic ridges.

The rate of plate movement varies, with the Arctic Ridge moving as slow as 2.5 cm/year and the East Pacific Rise moving as fast as over 15 cm/year. These rates are determined by studying magnetic patterns parallel to ridges in oceanic rocks.

Driving Forces for Plate Movement The Earth's plates move due to convection currents in the semi-fluid asthenosphere below the lithosphere. These currents are driven by heat from radioactive decay and residual heat within the Earth. The process involves heated materials rising, spreading out, cooling, sinking, and then repeating the cycle, which pushes and pulls the plates in different directions.

Movement of the Indian Plate The Indian Plate today includes peninsular India and the Australian landmass. Its boundaries are complex, with continent-continent convergence along the Himalayas, oceanic spreading zones east and south, and connections to various ridges and trenches. Historically, India was an island off the Australian coast, separated from Asia by the Tethys Sea. After starting its northward drift about 200 million years ago when Pangaea broke up, India eventually collided with Asia roughly 40–50 million years ago. This collision caused the Himalayan mountain uplift. During its movement, the Deccan Traps volcanic activity started around 60 million years ago. Even now, the Himalayas continue to rise due to ongoing plate collision.

CBSE Class 11 Geography Chapter 4 Notes – Distribution of Oceans and Continents: Key Points for Revision

These Class 11 Geography Chapter 4 notes provide clear explanations of the main concepts like continental drift, sea floor spreading, and plate tectonics. With simple language, well-organized points, and updated examples, students can use these notes to strengthen their understanding for quick revision before exams.

All the important facts and theories about the distribution of oceans and continents are compiled in one place, saving study time. These CBSE revision notes highlight evidence, processes, and key terms, helping students prepare efficiently and build a strong conceptual foundation for future Geography topics.