📘 Chapter 4: Distribution of oceans and continents
📚 Book: Fundamentals of Physical Geography
📅 Session: 2025–26
👨🏫 Presented by: Amresh kumar
🎯 Simplifying Concepts with Amresh Academy
🌍 Formation of Continents and Oceans
🕰️ Since the origin (उत्पत्ति) of the Earth, continents and oceans were formed about 3.8 billion years ago.
🌐 But these continents and oceans were not in the form they are today.
🧠 Many scientists (वैज्ञानिकों) from time to time tried to prove (साबित करना) that all the continents were together (एक साथ) in the early stages of formation.
🌐 Continental Displacement Theory (महाद्वीपीय विस्थापन सिद्धांत)
👨🔬 German scholar (जर्मन विद्वान) Alfred Wegener introduced the Continental Drift Theory in 1912.
🕰️ Wegener assumed that during the Carboniferous era (कार्बोनिफेरस युग), all landmasses were connected as a single large unit.
🏞️ He named this vast terrestrial (स्थलीय) part as Pangaea (पैन्जिया).
🧭 Wegener believed that some parts of Pangaea began to move towards the equator (भूमध्य रेखा).
📆 This process (प्रक्रिया) began in the late Carboniferous period, around 300 million years ago.
🧊 By the Pleistocene epoch (प्लीस्टोसीन युग), around 5–6 million years ago, the continents had taken a shape almost similar to what we see today.
🌍 Pangea
🧱 All the continents we see today were once part of a single landmass (एकीकृत स्थलखंड) called Pangea (पैन्जिया).
🪓 Later, Pangea split (टुकड़े होना) into two large continental bodies (महाद्वीपीय खंड):
1️⃣ Laurasia – 🌐 Northern Lands (उत्तरी भूमि)
2️⃣ Gondwana Land – 🌍 Southern Plot (दक्षिणी क्षेत्र)
🌊 Panthalassa
🌍 The vast ocean (महासागर) that surrounded Pangea was called Panthalassa (पैन्थालासा).
🌍 Evidence Supporting Continental Drift (महाद्वीपीय विस्थापन के पक्ष में प्रमाण)
🔹 Continental Fit 🧩
Coastlines of continents like South America & Africa match (मिलते-जुलते हैं).
🔹 Similar Rock Age 🪨
200 million-year-old rocks on Brazil & Africa's coast are similar (समान उम्र की चट्टानें).
🔹 Tillite ❄️
Glacial sedimentary rocks (हिमानी निक्षेप चट्टानें) found in 6 southern continents = ancient connection.
🔹 Placer Deposits 💰
Gold in Brazil and Ghana proves both lands were once together.
🔹 Fossil Distribution 🦴
Same organism remains (जीवाश्म) found on far-apart continents.
🌐 Forces Behind Continental Displacement (Wegener’s View) (महाद्वीपीय विस्थापन)
🔹 Polar-Fleeing Force 🧲
Caused by Earth’s rotation (घूर्णन), pushing continents away from the poles.
🔹 Tidal Force 🌊
Generated by the gravitational pull (गुरुत्वाकर्षण बल) of the Sun and Moon, which could move continental blocks.
🔥 Reasons for Convection Currents in the Mantle
(मैंटल में संवहन धाराओं के कारण)
🔹 Convection Currents (संवहन धाराएँ) arise due to temperature variations (तापमान में अंतर) in the mantle.
🔹 These variations are caused by radioactive elements (रेडियोधर्मी तत्व) present deep inside the Earth.
🔹 A continuous system of such currents flows throughout the entire mantle (पूरा मैंटल).
🌊 Mid-Oceanic Ridge (मध्य-महासागरीय जलोढ़ पर्वतमाला)
🔹 A chain of interconnected mountains (आपस में जुड़ी पर्वतमालाएं) running north to south in the middle of the Atlantic Ocean.
🔹 It remains submerged (जलमग्न) under ocean water.
🌍 Plate Tectonic Theory
(प्लेट विवर्तनिक सिद्धांत)
🔹 Early 20th-century scientists had difficulty accepting continental drift due to lack of explanation on how Sial-based continents could float or move.
🔹 It was believed the oceanic crust (महासागरीय भूपर्पटी) was just an extension of the basaltic layer (बेसाल्टिक परत).
🔹 In 1928, scientist Arthur Holmes proposed that due to temperature differences inside the Earth, convection currents (संवहन धाराएँ) form.
🔹 These currents move the plates, causing the continents to drift (खिसकना) over time.
🌐 Meaning of Plate – Plate Tectonics Theory (प्लेट विवर्तनिक सिद्धांत "प्लेट" का अर्थ)
🔹 A plate is a large, solid (ठोस), and irregular-shaped (असम रूपाकार) block made up of both continental and oceanic parts.
🔹 It behaves like a rigid unit (कठोर इकाई) that moves over the Earth’s surface.
🌍 According to Plate Tectonics Theory:
There are 7 major and some minor plates on Earth.
🧭 Major Plates:
🔸 Antarctic Plate
🔸 North American Plate
🔸 South American Plate
🔸 Pacific Plate
🔸 Indo-Australian Plate
🔸 African Plate
🔸 Eurasian Plate
🌐 Plate Movements (प्लेटों की गति)
🟦 Divergent Boundary 🔀
🔸 Plates move away from each other (विपरीत दिशा में)
🔸 New crust forms (नई भूपर्पटी बनती है)
🔸 Also called Spreading Zone 🌊
🔸 Example: Mid-Atlantic Ridge 🌍
🟥 Convergent Boundary 🔽
🔸 Plates move toward each other (सामने की ओर)
🔸 One plate subducts (धंसती है) under the other
🔸 Crust gets destroyed (नष्ट होती है)
🔸 Also called Subduction Zone 🌋
🔸 Example: Pacific & American Plates
🟨 Transform Boundary 🔁
🔸 Plates slide horizontally
🔸 No crust is created or destroyed
🔸 Also called Conservative Boundary ⚠️
⚖️ Wegener's vs. Plate Tectonics Theory (वेगनर बनाम प्लेट विवर्तनिक सिद्धांत)
🟩 Wegener’s Theory 🌍
🔸 Only continents move (सिर्फ महाद्वीप गतिशील हैं)
🔸 Proposed Pangea was the original supercontinent
🔸 Movement due to polar-fleeing & tidal force (ध्रुव बल और ज्वारीय बल)
🔸 Couldn’t explain ocean floor rocks (महासागरीय चट्टानें) or mid-ocean ridges
🔸 No explanation for old continental rocks (प्राचीन महाद्वीपीय चट्टानें)
🟥 Plate Tectonics Theory 🌐
🔸 Whole lithospheric plates move (प्लेट गतिशील हैं)
🔸 Pangea was formed by plate convergence (प्लेटों के मिलन से बनी)
🔸 Movement due to convection currents (संवहन धाराएँ)
🔸 Explains sea-floor spreading, ridges, and rock age patterns
🧩 Equality in Continents (Fit Evidence) (महाद्वीपों में समानता का प्रमाण)
🔹 In 1964, scientist Bullard created a map using a computer program 🖥️
➡️ It showed a perfect fit (सटीक मेल) between the Atlantic coasts 🌍
🔹 Zig-saw Fit 🧷
➡️ The boundaries (सीमाएं) of continents match like puzzle pieces 🧩
➡️ For example:
• North & South America fit with
• Europe & Africa along the Atlantic Ocean
🔥 Convection Currents in the Mantle (मैंटल में संवहन धाराएँ)
🔹 Cause:
➡️ Due to temperature variation (तापमान में अंतर) caused by radioactive elements (रेडियोधर्मी तत्व) deep inside the Earth 🌍
🔹 Effect:
➡️ These temperature differences create convection currents (संवहन धाराएँ)
➡️ The system of these currents exists throughout the entire mantle (पूरा मैंटल)
🔄 These currents are continuous & active, making the lithospheric plates move over time.
🌊 Hypothesis of Ocean Floor Development (महासागरीय तल के विकास का सिद्धांत)
📅 Presented by: Harry Hess, 1961
🔍 Based on study of magnetic properties (चुंबकीय गुण) of rocks near mid-oceanic ridges.
🌋 Main Idea:
🔸 Continuous volcanic eruptions (ज्वालामुखीय विस्फोट) at the mid-ocean ridge create new crust (नई परत).
🔸 This new lava fills cracks and pushes the crust to both sides 🔁
🔸 Thus, the ocean floor expands (महासागरीय तल फैलता है).
⚖️ While one side forms new crust, the old crust is destroyed in ocean trenches (महासागरीय खाइयों में नष्ट होती है).
📌 So, ocean floor is not shrinking but recycling ♻️
🌋 Main Earthquake & Volcano Belts (भूकंप और ज्वालामुखी के प्रमुख क्षेत्र)
🔹 1. Mid-Atlantic Belt 🌊
• In the central Atlantic Ocean
• Chain of earthquakes & volcanoes runs parallel to coastlines (समांतर)
• Extends into the Indian Ocean 🌐
🔹 2. Alpine–Himalayan Belt 🏔️
• From Alps to Himalayas
• Similar to seismic activity along Pacific coast
• Includes collision zones (टक्कर क्षेत्र)
🔹 3. Pacific Ring of Fire 🔥
• Circular belt on the edge of Pacific Ocean
• Most active (सक्रिय) region for earthquakes and volcanoes
• Known as the Ring of Fire 🔥🌋
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