Plate Tectonics and Plate Margins

Physical Processes at Constructive Plate Margins

• When plates pull away from each other, the earth's crust stretches and thins. At a constructive plate margin, plates move apart at an average rate of about 2 cm per year.
• As the overlying lithosphere thins, pressure on the mantle below is reduced. This triggers decompression melting, turning solid mantle rock into basaltic magma without needing an increase in temperature.
• The new magma rises through fissures to fill the expanding gap and erupts as low-viscosity (runny) lava. Successive eruptions over time build broad, gently sloping shield volcanoes.
• As the crust separates, it fractures and creates tension. This sudden snapping and faulting causes shallow-focus earthquakes of low magnitude.

Step-by-Step Mechanism: Formation of a Shield Volcano

1. Tectonic plates move apart due to convection currents, ridge push, or slab pull.
2. The overlying lithosphere thins, reducing pressure on the mantle below and causing decompression melting.
3. The new magma rises through fissures to fill the expanding gap and erupts as low-viscosity lava.
4. The lava flows over long distances before cooling to form new oceanic crust.
5. Successive eruptions build broad, gently sloping shield volcanoes.

Physical Processes at Destructive Plate Margins

• What happens when two colossal plates crash into each other? At a destructive plate margin, two plates converge at 2–5 cm per year, and crust is destroyed. The resulting features depend on the types of plates colliding:
• Oceanic and Continental Plates: The thinner oceanic plate undergoes subduction (sinks beneath the thicker continental plate) because it is denser and heavier. The exact point where the plate bends downward forms a deep-sea ocean trench. As the dense plate descends, immense heat and friction cause it to melt. Water dragged down creates highly viscous (thick) andesitic magma. This erupts explosively to form steep-sided composite volcanoes, which are made of alternating layers of ash and lava. Deep-focus earthquakes occur violently along the sinking plate. The immense pressure also crumples the overriding continental plate upwards to form fold mountains (e.g., the Andes).
• Oceanic and Oceanic Plates: When two oceanic plates collide, the older and denser of the two subducts beneath the other, forming deep ocean trenches and island arcs (e.g., Japan or the Caribbean).
• Continental and Continental Plates (Collision Zones): When two continental plates meet, neither is dense enough to subduct. Instead, the collision crumples and forces the crust upwards to form massive fold mountains (e.g., the Himalayas). This causes shallow earthquakes, but because there is no subduction and no melting crust, there are no volcanoes.

Physical Processes at Conservative Plate Margins

• Try rubbing two rough pieces of sandpaper past each other—they catch and stick. At a conservative plate margin, tectonic plates slide past each other along a massive fracture called a fault line.
• Because plate edges are jagged, friction causes them to repeatedly snag and lock together. While the plates are stuck, convection currents and slab pull continue to drive movement, causing a massive build-up of pressure and stress over time.
• When the built-up tension finally overcomes the friction, the plates suddenly jerk forward. This sudden slip releases stored energy as extremely destructive shallow-focus earthquakes.
• Crucially, the plates can move in opposite directions, or they can move in the same direction at different speeds. There are zero volcanoes at conservative margins because no crust is melted into magma, and no gaps are created for magma to rise.

Step-by-Step Mechanism: Earthquake Formation at a Conservative Margin

1. Tectonic plates slide past each other along a fault line.
2. Because plate edges are jagged, friction causes them to repeatedly snag and lock together.
3. While the plates are stuck, convection currents and slab pull continue to drive movement, causing a massive build-up of pressure and stress over time.
4. When the built-up tension finally overcomes the friction, the plates suddenly jerk forward. This sudden slip releases stored energy as extremely destructive shallow-focus earthquakes.