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Unlock This CourseThe Earth's crust is broken into large pieces called tectonic plates that float on the semi-molten mantle below. These plates are constantly moving, albeit very slowly (about 2-10 cm per year - roughly the same rate your fingernails grow!). Where these plates meet, we find different types of boundaries that create various geological features and natural hazards.
Key Definitions:
There are 7 major tectonic plates (Pacific, North American, Eurasian, African, Antarctic, Indo-Australian and South American) and many smaller ones. Together, they cover the entire surface of the Earth like pieces of a jigsaw puzzle!
Tectonic plates can interact in three main ways, creating different types of boundaries with unique features and hazards.
At constructive boundaries, plates move away from each other. As they separate, magma rises from the mantle to fill the gap, creating new crust. This process is called sea-floor spreading when it happens in oceans.
Features:
The Mid-Atlantic Ridge is a perfect example of a constructive boundary. It runs down the middle of the Atlantic Ocean, where the North American and Eurasian plates are moving apart. Iceland sits directly on this boundary, which is why it has so many volcanoes. In fact, Iceland is growing by about 2cm every year as new land is created!
At destructive boundaries, plates move towards each other. When an oceanic plate meets a continental plate, the denser oceanic plate is forced beneath the lighter continental plate in a process called subduction.
Features:
The boundary between the Nazca Plate and the South American Plate is a classic destructive boundary. The Nazca Plate (oceanic) is being subducted beneath the South American Plate (continental), creating the Andes Mountains and the Peru-Chile Trench. This area experiences frequent earthquakes and is home to many active volcanoes like Cotopaxi in Ecuador.
At conservative boundaries, plates slide past each other horizontally. No crust is created or destroyed, but friction between the plates causes them to stick. When they eventually slip, energy is released as earthquakes.
Features:
The San Andreas Fault in California is formed by the Pacific Plate sliding past the North American Plate. This 1,300 km fault line is responsible for many earthquakes, including the devastating 1906 San Francisco earthquake that killed over 3,000 people. Scientists predict that another major earthquake (known as "The Big One") will occur along this fault in the future.
When two continental plates collide, neither can be subducted because continental crust is too light. Instead, the crust crumples upwards to form fold mountains. This is a special type of convergent (destructive) boundary.
The Himalayas were formed when the Indo-Australian Plate collided with the Eurasian Plate about 50 million years ago. This collision is still happening today, causing the Himalayas to grow taller by about 5mm each year! This region experiences frequent earthquakes, like the devastating 2015 Nepal earthquake that killed nearly 9,000 people and injured nearly 22,000.
Earthquakes occur when energy stored in rocks is suddenly released. At plate boundaries, plates don't slide smoothly past each other - they stick due to friction. As the plates continue to move, pressure builds up until the rocks suddenly break or slip, releasing energy as seismic waves.
The focus is the point underground where the earthquake begins. The epicentre is the point on the Earth's surface directly above the focus.
The Richter scale measures magnitude (energy released). Each step up means 10 times more ground movement and 32 times more energy released!
P-waves (primary) are fast, pushing waves. S-waves (secondary) are slower, shaking waves that cause more damage.
Volcanoes form when magma from the mantle reaches the Earth's surface. The type of volcano and eruption depends on the plate boundary where it forms.
Found at constructive boundaries, shield volcanoes have gently sloping sides and produce runny lava that flows easily. Eruptions are usually gentle and predictable. Example: Mauna Loa in Hawaii.
Found at destructive boundaries, composite volcanoes are steep-sided and produce thick, sticky lava and ash. Eruptions are explosive and dangerous. Example: Mount Fuji in Japan.
About 75% of the world's active volcanoes are located in the "Ring of Fire," a horseshoe-shaped belt that follows the edges of the Pacific Plate. This area also experiences about 90% of the world's earthquakes! Countries like Japan, Indonesia and Chile are particularly vulnerable to volcanic eruptions and earthquakes because they sit on this ring.
Plate movements create both hazards and benefits for human societies:
Understanding the different types of plate movements helps us predict where earthquakes and volcanoes are likely to occur. Each boundary type creates distinctive landforms and hazards:
By studying these patterns, scientists can help communities prepare for natural hazards and reduce their impact on human lives and property.