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Unlock This CourseImagine Earth's surface as a giant jigsaw puzzle made of massive rocky pieces that slowly move around. These pieces are called tectonic plates and they're constantly shifting, colliding and separating - creating the mountains, valleys and ocean floors we see today. This movement might be incredibly slow (just a few centimetres per year), but over millions of years, it has shaped our entire planet.
The theory of plate tectonics revolutionised our understanding of Earth's geology. It explains why earthquakes happen in certain places, how mountain ranges form and why we find similar fossils on different continents thousands of kilometres apart.
Key Definitions:
Earth consists of several layers. The outermost solid layer (lithosphere) is broken into tectonic plates. These plates "float" on the semi-liquid asthenosphere below, like pieces of ice on water. Heat from Earth's core creates convection currents that drive plate movement.
Tectonic plates interact at their boundaries in three main ways, each creating different geological features and phenomena. Understanding these interactions helps explain why certain areas experience more earthquakes and volcanic activity than others.
At divergent boundaries, plates move away from each other. As they separate, magma rises from the mantle to fill the gap, creating new oceanic crust. This process is called seafloor spreading.
Underwater mountain ranges where new oceanic crust forms. The Mid-Atlantic Ridge is a famous example, running down the centre of the Atlantic Ocean.
When divergent boundaries occur on land, they create rift valleys. The East African Rift System is slowly splitting Africa into two parts.
Gentle volcanic eruptions occur as magma rises to create new crust. These eruptions are typically less explosive than at other boundary types.
Iceland sits directly on the Mid-Atlantic Ridge, making it one of the most geologically active places on Earth. The island is literally being pulled apart by divergent plate movement, growing by about 2 centimetres per year. This creates numerous geysers, hot springs and volcanic eruptions that provide geothermal energy for the country.
Convergent boundaries occur where plates move towards each other. The outcome depends on the types of crust involved - oceanic crust is denser than continental crust, so it usually gets pushed underneath in a process called subduction.
When two continental plates collide, neither can subduct, so they push upwards creating mountain ranges. The Himalayas formed this way when India crashed into Asia.
When oceanic plates subduct, they create deep ocean trenches. The Mariana Trench, Earth's deepest point, formed where the Pacific Plate subducts beneath the Philippine Plate.
Subducting plates melt as they descend, creating chains of explosive volcanoes. The "Ring of Fire" around the Pacific Ocean is a famous example.
At transform boundaries, plates slide past each other horizontally. No new crust is created or destroyed, but the friction between plates can cause powerful earthquakes when the plates suddenly slip.
California's San Andreas Fault is a transform boundary where the Pacific Plate slides northwest past the North American Plate. This movement causes frequent earthquakes, including the devastating 1906 San Francisco earthquake. The fault is easily visible from the air as a long scar across the landscape.
Scientists have gathered compelling evidence from various sources to support the theory of plate tectonics. This evidence comes from studying rocks, fossils and the ocean floor.
Identical fossils of plants and animals have been found on continents now separated by vast oceans. For example, fossils of the reptile Mesosaurus are found in both South America and Africa, suggesting these continents were once connected.
The ocean floor provides some of the strongest evidence for plate tectonics. Scientists have discovered that oceanic crust gets progressively older as you move away from mid-ocean ridges, exactly what you'd expect if new crust was forming at these boundaries.
The ocean floor shows alternating magnetic stripes that mirror on both sides of mid-ocean ridges, recording Earth's magnetic field reversals over time.
Rock samples from the ocean floor confirm that the youngest rocks are at mid-ocean ridges, with age increasing towards the continents.
Higher heat flow measurements at mid-ocean ridges indicate rising magma, supporting the seafloor spreading theory.
What actually causes these massive plates to move? The answer lies deep within Earth's interior, where heat from radioactive decay and leftover heat from Earth's formation creates convection currents in the mantle.
Hot material in the mantle rises towards the surface, cools, then sinks back down, creating circular currents. These currents drag the overlying tectonic plates along, like conveyor belts moving the plates around Earth's surface.
When dense oceanic plates subduct, their weight helps pull the rest of the plate along. This "slab pull" is thought to be one of the strongest forces driving plate movement.
Plate tectonics doesn't just create geological features - it affects climate, ocean circulation and the distribution of life on Earth. The movement of continents has changed ocean currents, altered weather patterns and created barriers that led to the evolution of different species.
Today, we can measure plate movement using GPS satellites and other precise instruments. Most plates move at rates similar to fingernail growth - about 2-10 centimetres per year. While this seems slow, over geological time it adds up to thousands of kilometres of movement.
Throughout Earth's history, continents have repeatedly come together to form supercontinents, then broken apart again. The most recent supercontinent, Pangaea, existed about 300 million years ago. Scientists predict that in about 250 million years, the continents will come together again to form a new supercontinent called "Pangaea Ultima."
Understanding plate tectonics is crucial for human safety and development. Most of the world's earthquakes and volcanic eruptions occur at plate boundaries, so knowing where these boundaries are helps us prepare for natural disasters and make informed decisions about where to build cities and infrastructure.
Plate boundaries are zones of geological instability where earthquakes, volcanic eruptions and tsunamis are most likely to occur. The "Ring of Fire" around the Pacific Ocean is home to 75% of the world's active volcanoes.