What is the primary difference in density between oceanic and continental crust?

Oceanic crust is significantly denser than continental crust. This density difference is the reason why oceanic crust subducts into the mantle when the two types of crust collide.

How does a collision boundary differ from a destructive boundary?

At a destructive boundary, a dense oceanic plate subducts under a continental plate, creating volcanoes. At a collision boundary, two continental plates of similar density meet and push upwards to form mountains, with no subduction or volcanoes.

What is the difference between 'Ridge Push' and 'Slab Pull'?

Ridge push occurs at constructive boundaries where gravity pushes new, elevated crust away from the ridge. Slab pull occurs at destructive boundaries where the weight of a subducting plate drags the rest of the plate down into the mantle.

Why is it a mistake to assume volcanoes occur at all plate boundaries?

Volcanoes require a source of magma to reach the surface. They do not occur at conservative boundaries (where plates slide past) or collision boundaries (where crust thickens upward) because there is no mechanism for magma to break through.

What is the common misconception regarding the age of the Earth's crust?

Many assume all crust is the same age, but oceanic crust is much younger (usually under 200 million years) because it is constantly recycled through subduction, while continental crust can be billions of years old.

Why do earthquakes occur at conservative boundaries if no crust is being created or destroyed?

Earthquakes are caused by the buildup of friction and pressure as plates attempt to slide past each other. When the plates finally overcome the friction and 'slip,' the stored energy is released as seismic waves.

Define the 'Lithosphere'.

The lithosphere is the rigid, outermost shell of the Earth, consisting of the crust and the brittle uppermost portion of the mantle. It is broken into the tectonic plates that move across the Earth's surface.

What is the 'Asthenosphere'?

The asthenosphere is the highly viscous, mechanically weak, and ductile region of the upper mantle. It lies just below the lithosphere and allows the tectonic plates to move due to its semi-molten state.

What is a 'Subduction Zone'?

A subduction zone is a region of the Earth's crust where one tectonic plate moves under another and sinks into the mantle. This typically occurs at destructive boundaries where oceanic crust meets continental crust.

Why are volcanoes found at constructive boundaries?

As plates move apart, the crust thins and the pressure on the underlying mantle decreases. This allows magma to rise through the gaps to the surface, creating volcanic activity and new crust.

Library Podcasts

Courses

Referral & Rewards

Plate Tectonics

Summary

Plate tectonics is the scientific theory describing the large-scale motion of the Earth's lithosphere, which is divided into several rigid plates. These plates move over the semi-molten asthenosphere, driven by internal heat and gravitational forces, leading to the formation of major geological features and the occurrence of seismic and volcanic activity at their boundaries.

1. Earth's Internal Structure

The Core: The Earth's center consists of a solid inner core of iron and nickel at temperatures reaching $5500^{\circ}C$ , surrounded by a liquid outer core that generates the planet's magnetic field.
The Mantle: This is a thick layer of semi-molten rock that behaves plastically over geological time, allowing for the slow movement of the overlying tectonic plates.
The Crust: The outermost shell is divided into the lithosphere (the rigid crust and upper mantle) and the asthenosphere (the ductile layer below), providing the physical framework for plate movement.

Diagram of a subduction zone showing the denser oceanic plate sinking beneath the continental plate, illustrating slab pull and mantle convection.

2. Compositional Differences in Crust

Oceanic Crust: This type is relatively thin ( $5-10$ km) but highly dense, primarily composed of basaltic rock. Because it is constantly recycled at subduction zones, it is geologically much younger than continental crust.
Continental Crust: This layer is significantly thicker ( $25-90$ km) and less dense, composed largely of granitic rocks. Its lower density prevents it from subducting, making it much older as it remains on the Earth's surface for billions of years.

3. Mechanisms of Plate Movement

4. Plate Boundary Dynamics

5. Key Distinctions

6. Exam Strategy & Tips