Introduction
- The Earth's surface is composed of continents (29%) and oceans (71%).
- The positions of the continents and oceans have changed over time and will continue to change in the future.
Continental Drift
Early Observations:
- Abraham Ortelius (1596), a Dutch map maker, was the first to propose the possibility of continents being joined together.
- Antonio Pellegrini drew a map showing the three continents (Europe, Africa, and the Americas) together.
Alfred Wegener's Continental Drift Theory (1912):
- Wegener, a German meteorologist, proposed a comprehensive theory of continental drift.
- He suggested that all continents were once joined together in a supercontinent called Pangaea, surrounded by a mega-ocean called Panthalassa.
- Pangaea split into two large continental masses: Laurasia (north) and Gondwanaland (south).
- These masses continued to break apart, forming the continents we see today.
Evidence for Continental Drift
Matching of Continents (Jig-Saw-Fit):
- The coastlines of Africa and South America fit together like a puzzle.
- Bullard's 1964 map using a computer program to fit the continents at the 1,000-fathom line provided a near-perfect match.
- Rocks of Same Age Across the Oceans:
- Radiometric dating shows that rocks of the same age are found on continents now separated by oceans.
- Example: A 2,000-million-year-old rock belt from the Brazil coast matches with those from western Africa.
Tillite:
- Tillite deposits, formed by glaciers, are found in India, Africa, Falkland Island, Madagascar, Antarctica, and Australia, suggesting these landmasses were once connected.
Placer Deposits:
- Gold deposits in Ghana are believed to have originated from Brazil when the continents were joined.
- Distribution of Fossils:
- Identical fossil species are found on continents now separated by oceans, suggesting they were once connected.
- Examples: Lemurs in India, Madagascar, and Africa; Mesosaurus fossils in South Africa and Brazil.
Forces for Drifting (According to Wegener)
Pole-fleeing force:
- Due to the Earth's rotation and bulge at the equator, continents would drift a,vay from the poles.
Tidal force:
- Gravitational pull of the Moon and the Sun creates tides in oceanic waters, which could contribute to continental drift.
- Inadequacy of\Vegener's Proposed Forces:
- Most scientists considered these forces to be too weak to move continents.
Post-drift Studies
Ocean Floor Mapping:
- Revealed that the ocean floor is not a flat plain but has features like submerged mountain ranges (mid- oceanic ridges) and deep trenches.
- Mid-oceanic ridges are sites of active volcanic eruptions.
- Rocks on either side of the mid-oceanic ridges are similar in age and composition.
- Ocean crust rocks are much younger than continental rocks (less than 200 million years old compared to 3,200 million years old).
- Sediments on the ocean floor are unexpectedly thin, suggesting a younger age.
Convectional Current Theory (Arthur Holmes, 1930s):
- Proposed that convection currents in the mantle, caused by radioactive elements, could be the driving force behind continental drift.
Ocean Floor Configuration
Continental Margins:
- Transition zone between continents and deep-sea basins.
- Includes continental shelf, continental slope, continental rise, and deep-oceanic trenches.
Abyssal Plains:
- Flat plains between continental margins and mid-oceanic ridges.
- Where continental sediments accumulate.
Mid-Oceanic Ridges:
- Underwater mountain ranges formed by volcanic activity.
- The longest mountain chain on Earth.
- Characterized by a central rift system, a fractionated plateau, and flank zones.
- The rift system is the zone of intense volcanic activity.
Distribution of Earthquakes and Volcanoes
- Earthquakes and volcanoes are concentrated along:
- Mid-oceanic ridges.
- The rim of the Pacific Ocean (the "Ring of Fire").
- Earthquakes along mid-oceanic ridges are shallow, while those along the Ring of Fire are deep-seated.
Sea Floor Spreading (Hess, 1961)
Key Observations:
- Volcanic eruptions are common along mid-oceanic ridges.
- Rocks on either side of the mid-oceanic ridges are similar in age, composition, and magnetic properties.
- Rocks closer to the mid-oceanic ridges are younger and have normal magnetic polarity.
- Ocean crust rocks are much younger than continental rocks.
- Sediments on the ocean floor are thin.
- Deep trenches have deep-seated earthquakes, while mid-oceanic ridges have shallow earthquakes.
Hess's Hypothesis:
- Volcanic eruptions at mid-oceanic ridges cause the ocean floor to spread apart.
- New lava fills the gap, pushingolder crust to the sides.
- The ocean floor eventually sinks and is consumed at deep-ocean trenches.
Rates of Plate Movement
- Determined by studying magnetic stripes on the ocean floor.
- Rates vary from less than 2.5 cm/year (Arctic Ridge) to over 15 cm/year (East Pacific Rise).
Force
for Plate Movement
- Convection currents in the mantle, driven by radioactive decay and residual heat, are the main driving force.
Movement of the Indian Plate
- The Indian plate includes Peninsular India and the Australian continental portions.
- It has been moving northward for about 200 million years.
- It collided ,vith Asia about 40-50 million years ago, forming the Himalayas.
- The Himalayas are still rising due to the ongoing collision.
- The Deccan Traps were formed during this northward movement, around 60 million years ago.
Major Points to Remember
- The Earth's surface is dynamic and constantly changing.
- Continents were once joined together in a supercontinent called Pangaea.
- Plate tectonics is the theory that explains the movement of continents and the formation of various geological features.
- There are three types of plate boundaries: divergent, convergent, and transform.
- Convection currents in the mantle are the driving force behind plate movement.
- The Indian plate has been moving northward for millions of years and collided with Asia to form the Himalayas.
