Chapter 4 – Distribution of Oceans and Continents
What is the theory of continental drift?
Continental drift theory was proposed by Alfred Wegener in 1912. It explains that continents were once part of a single landmass called Pangaea, which gradually broke apart and drifted to their present positions.
What is Pangaea and how did it split?
Pangaea was a supercontinent. It broke into two parts: Laurasia in the north and Gondwanaland in the south. These further split to form today’s continents, as explained in the continental drift theory.
What is the evidence for continental drift?
Evidence includes the jigsaw-fit of coastlines (like South America and Africa), similarity of fossils, rock types, and glacial deposits across continents, which suggest they were once connected.
Who proposed sea-floor spreading theory and what does it say?
Harry Hess proposed the sea-floor spreading theory. It states that new oceanic crust forms at mid-ocean ridges and spreads outward, pushing older crust away, supporting continental drift.
What are mid-oceanic ridges?
These are underwater mountain chains formed at divergent boundaries where tectonic plates move apart and magma rises to create new crust. They are major sites for sea-floor spreading.
What is paleomagnetism?
It is the study of Earth’s past magnetic field as recorded in rocks. Alternating magnetic stripes on ocean floors support sea-floor spreading by showing symmetrical patterns on both sides of mid-ocean ridges.
How does sea-floor spreading support plate tectonics?
It shows that plates are moving apart at ridges, creating new crust. This movement of plates confirms the dynamic nature of Earth’s lithosphere and the concept of tectonic plates.
What is the theory of plate tectonics?
It combines continental drift and sea-floor spreading theories. It states that Earth’s lithosphere is divided into plates that move over the asthenosphere due to mantle convection currents.
What are tectonic plates?
They are large slabs of the Earth’s lithosphere that include both continental and oceanic crust. These plates float on the semi-fluid asthenosphere and move due to internal Earth processes.
What causes plate movement?
Plate movement is caused by convection currents in the mantle, slab pull, and ridge push forces. These processes drive the movement of the lithospheric plates on the asthenosphere.
What is a divergent boundary?
It is a plate boundary where two plates move apart. New crust is formed by magma rising from the mantle. Mid-oceanic ridges are examples of divergent boundaries.
What is a convergent boundary?
At convergent boundaries, two plates collide. One may subduct beneath the other, causing trenches, mountains, and volcanoes. Examples include the Himalayas and the Andes.
What is a transform boundary?
Here, two tectonic plates slide past each other horizontally. Crust is neither created nor destroyed. These boundaries often cause earthquakes, like at the San Andreas Fault in California.
What is the lithosphere?
The lithosphere is the rigid, outermost layer of the Earth. It includes the crust and upper mantle. It is broken into plates that move due to underlying mantle dynamics.
What is the asthenosphere?
It lies below the lithosphere and is partially molten. It allows lithospheric plates to move over it. It plays a crucial role in tectonic activities.
What is Gondwanaland?
It was the southern part of Pangaea and included present-day Africa, South America, Antarctica, Australia, and India. It split around 180 million years ago.
What is Laurasia?
Laurasia was the northern part of Pangaea. It consisted of today’s North America, Europe, and parts of Asia.
What are convection currents?
Hot material in the mantle rises, cools, and sinks back, creating a circular motion. These convection currents are the driving force behind plate movements.
What is slab pull?
It is a force where a subducting plate pulls the rest of the plate along as it sinks into the mantle due to its density.
What is ridge push?
At mid-ocean ridges, newly formed crust is elevated. Gravity pushes the older, denser crust away from the ridge, contributing to plate movement.
What is the Ring of Fire?
It’s a zone around the Pacific Ocean known for frequent earthquakes and volcanoes. It’s caused by the movement and subduction of tectonic plates.
Why are the Himalayas still rising?
The Indian Plate is still colliding with the Eurasian Plate. This ongoing convergence continues to push up the Himalayan mountains.
How are ocean trenches formed?
They form at convergent boundaries where an oceanic plate subducts beneath a continental or another oceanic plate, creating a deep depression.
What is a rift valley?
It forms at divergent boundaries on continents, where land sinks between two diverging plates. The East African Rift is a major example.
What is a hotspot?
A hotspot is a volcanic area formed by magma rising from deep within the mantle. It remains stationary while plates move above it (e.g., Hawaii).
What is the difference between continental and oceanic plates?
Continental plates are thicker, less dense, and granitic. Oceanic plates are thinner, denser, and basaltic in composition.
How do earthquakes relate to plate boundaries?
Most earthquakes occur along plate boundaries due to the stress and movement between tectonic plates—especially transform and convergent types.
What is subduction?
It’s the process where a denser oceanic plate slides beneath a lighter continental plate, melting and triggering earthquakes or volcanic eruptions.
What are tectonic forces?
Forces originating inside the Earth that cause movements of plates, formation of landforms, and deformation of Earth’s crust.
Why is the theory of plate tectonics important?
It explains the movement of continents, causes of earthquakes, volcanoes, mountain formation, and provides a unifying explanation for Earth’s geological features.