Tectonic Processes and Hazards

Overview

This study guide covers the fundamental processes that drive tectonic activity and the resulting hazards that impact human populations. You will explore the internal structure of the Earth, the mechanics of plate movement, and the specific features of different plate boundaries. Examiners expect candidates to clearly distinguish between processes like ridge push and slab pull, and to accurately link specific physical processes to the resulting hazard types. You must also understand the causes of intra-plate events, such as hot spot volcanoes, and be able to describe the characteristics of earthquake waves and secondary hazards like tsunamis, liquefaction, and landslides.

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The Theory of Plate Tectonics

Earth's Internal Structure and Convection

The Earth consists of the inner core, outer core, mantle, and crust. The lithosphere (crust and uppermost solid mantle) is broken into tectonic plates. Heat from the core generates convection currents in the semi-molten mantle, which slowly drag the overlying plates.

Ridge Push and Slab Pull

Two other key forces drive plate movement:

• Ridge Push: At constructive boundaries, rising magma forms new, elevated oceanic crust. Gravity causes this newer, higher crust to push the older crust away from the ridge.
• Slab Pull: At destructive boundaries, dense oceanic crust sinks into the mantle under its own weight, pulling the rest of the plate behind it.

Plate Boundaries and Processes

Destructive (Convergent) Boundaries

Process: Oceanic crust collides with continental crust and subducts (sinks) beneath it because it is denser.
Features: Ocean trenches, fold mountains, stratovolcanoes.
Hazards: Highly explosive volcanic eruptions (due to viscous, gas-rich magma) and powerful earthquakes. The Benioff zone is the area of earthquake activity along the subducting plate; focal depth increases with distance from the trench.

Constructive (Divergent) Boundaries

Process: Two plates move apart, and magma rises to fill the gap, creating new crust.
Features: Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and rift valleys.
Hazards: Less explosive shield volcanoes with fluid basaltic lava, and relatively shallow, less severe earthquakes.

Collision Boundaries

Process: Two continental plates collide. Because both have low density, neither subducts. Instead, the crust crumples upwards.
Features: High fold mountains (e.g., the Himalayas).
Hazards: Major earthquakes, but no volcanoes as there is no subduction and melting.

Conservative (Transform) Boundaries

Process: Two plates slide past each other horizontally (e.g., San Andreas Fault).
Features: Major fault lines.
Hazards: Powerful, shallow-focus earthquakes caused by the sudden release of built-up friction. No volcanic activity.

Earthquakes and Seismic Waves

Earthquakes originate at the focus underground. The point directly above on the surface is the epicentre. Energy travels as seismic waves:

• P (Primary) Waves: Fastest, compressional waves travelling through solids and liquids.
• S (Secondary) Waves: Slower, shear waves travelling only through solids.
• L (Love/Surface) Waves: Slowest, but cause the most surface damage.

Secondary Hazards

• Liquefaction: Shaking causes saturated soil to lose strength and behave like a liquid, undermining foundations.
• Landslides: Shaking destabilizes slopes.
• Tsunamis: Massive ocean waves triggered by the displacement of the seabed during submarine earthquakes at subduction zones.

Volcanic Hazards

Volcanic eruptions produce multiple hazards:

• Lava Flows: Streams of molten rock. Basaltic lava (constructive margins/hot spots) is fast and fluid; andesitic/rhyolitic lava (destructive margins) is slow and viscous.
• Pyroclastic Flows: Deadly, fast-moving clouds of hot gas, ash, and rock fragments.
• Ash Falls: Tephra ejected into the atmosphere, causing respiratory issues and roof collapses.
• Volcanic Gases: Emissions like sulfur dioxide (SO_2) and carbon dioxide (CO_2).
• Lahars: Destructive volcanic mudflows formed when ash mixes with water (rain or melted snow).
• Jökulhlaups: Glacial outburst floods caused by volcanic heat melting ice beneath a glacier.

Intra-Plate Earthquakes and Hot Spots

Not all activity occurs at plate boundaries. Hot spots are areas where isolated mantle plumes (columns of hot magma) burn through the lithosphere, creating shield volcanoes (e.g., Hawaii) in the middle of a plate. Intra-plate earthquakes can occur along ancient, reactivated fault lines within plates.