IGCSE Marine Science Unit 1 Lesson 3: Plate Tectonics Continued - Earthquakes, Volcanoes, and Tsunamis
Overview of Tectonic Plate Boundaries and Lithospheric Movement
The Nature of the Earth's Crust: The crust is not a solid, continuous shell; rather, it is composed of several massive slabs of rock known as tectonic plates. These plates effectively float on the mantle, which is the layer situated directly beneath the crust.
Mechanisms of Plate Movement: The tectonic plates are in constant, albeit slow, motion. This movement is driven by the slow flow of magma within the mantle below the crust.
Plate Boundaries: A plate boundary is defined as the specific location where two or more tectonic plates meet. There are primary classifications for these boundaries:
Convergent Plate Boundary: This occurs when two tectonic plates collide or crash together. These boundaries are frequently associated with the formation of deep ocean trenches, such as the Mariana Trench.
Divergent Plate Boundary: This occurs when two tectonic plates pull apart or spread away from each other, creating a gap. As the plates separate, magma rises from the mantle to the surface to fill the void, often resulting in the formation of mid-ocean ridges.
Transform Plate Boundary: This occurs when two tectonic plates slide past each other horizontally. This movement is often characterized by high levels of friction.
The Genesis and Characteristics of Earthquakes
Location of Seismic Activity: Earthquakes are significantly more likely to occur on or near established plate boundaries.
Definition of an Earthquake: An earthquake is characterized by the shaking or vibration of the Earth's crust.
The Mechanics of Earthquake Formation:
The primary cause is the movement of tectonic plates against one another.
Earthquakes specifically occur when plates do not move smoothly. Instead, they become "stuck" due to friction and jagged edges.
As the plates remain stuck while the mantle continues to push them, immense pressure builds up at the point of contact.
When this pressure eventually overcomes the friction, it is released suddenly, and the plates snap into a new position, initiating the earthquake.
Essential Earthquake Terminology:
Focus: The specific point within the Earth's crust where the built-up pressure is first released.
Epicentre: The point on the Earth's surface situated directly above the focus.
Release of Energy: Earthquake energy is discharged in the form of seismic waves that radiate outward from the focus. These waves are at their highest intensity and strength at the epicentre.
Boundary Types: Unlike other geological events, earthquakes can occur along any of the types of plate boundaries (convergent, divergent, and transform).
Mechanisms of Volcanic Formation
Predictability of Volcanism: Similar to earthquakes, volcanoes are most frequently found near or on plate boundaries.
The Process of Volcanic Formation:
Volcanoes often form when one tectonic plate moves under another tectonic plate.
The plate that is forced underneath sinks deep into the mantle.
This subduction process causes magma to rise from the mantle toward the crustal surface.
As magma accumulates, pressure builds within the Earth. When the pressure reaches a critical threshold, the volcano erupts.
Associated Plate Boundaries: Volcanic activity is primarily associated with two types of boundaries:
Divergent boundaries (where rising magma fills the gap between separating plates).
Convergent boundaries (where one plate is forced under another).
Notable Exception: Volcanoes do not typically occur at transform plate boundaries.
Tsunami Origins and Physical Dynamics
Definition: A tsunami is a giant ocean wave, or series of waves, generated by large-scale disturbances beneath the sea, such as underwater earthquakes or volcanic eruptions.
Scale and Magnitude: Tsunami waves are exceptionally powerful and can reach heights of up to when they strike land.
Geographic Concentration - The Ring of Fire:
Approximately of all tsunamis occur in the Pacific Ocean.
This is due to a region known as the Ring of Fire, which is densely populated with volcanoes, ocean trenches, and frequent earthquake activity.
The Ring of Fire exists because of the high concentration of convergent plate boundaries (where plates are constantly smashing together) in that region.
The Science of Wave Transformation:
Tsunami waves exhibit different behaviors depending on their proximity to land.
In the deep ocean, the waves move very fast but may not appear very tall.
As the waves approach the coastline or shore, they begin to slow down.
As they slow down, the waves become more compressed and their wavelength shortens.
This compression causes the wave energy to be directed upwards, causing the wave height to increase dramatically, often reaching several meters high upon impact with the shore.
Impacts of Tsunamis on Marine Ecosystems and Human Communities
General Detrimental Effects: Tsunamis cause massive flooding and physical destruction that affects both natural habitats and man-made structures.
Impacts on Specific Marine Ecosystems:
Coral Reefs: Powerful waves can physically break and flatten various coral species, particularly weaker or more brittle varieties.
Post-Tsunami Reef Stress: Rubble and debris may be deposited over the reef, effectively smothering the living coral. Additionally, slower-growing coral species may struggle to recover and can be overgrown by algae before they can rebuild.
Mangrove Habitats: These coastal forests are often devastated by the force of the water, which can uproot entire trees and destroy coastal vegetation.
Impacts on Marine Life:
The waves scrape seafloor sediments, which can kill various invertebrates.
The force of the water causes significant seafloor erosion.
Fish and other marine organisms are frequently swept inland and left stranded on land as the water recedes.
Impacts on Human Communities:
Destruction of vital infrastructure (buildings, roads, etc.).
Total loss of agricultural crops due to flooding and salt contamination.
General devastation and loss of livelihood for local coastal populations.
Questions & Discussion
Discussion Prompt: What impact would tsunamis have on marine life and local communities?
Summary of considerations: Participants should consider the immediate physical destruction of habitats versus long-term ecological shifts (like algae overgrowth on reefs) and the economic impact on humans regarding food security and infrastructure repair.
Review Quiz: Boundary Identification
Question: Which of these sentences best describes a "divergent plate boundary"?
A) Two tectonic plates collide, one plate slides under the other, which can cause deep trenches to form, such as the "Mariana Trench"
B) Two tectonic plates spread apart, sliding away from each other, a gap forms and magma rises to the surface; a mid-ocean ridge forms
C) Two tectonic plates slide past each other, sometimes friction can cause earthquakes
Correct Answer: B
Interative Quiz: Wave Dynamics
Question: When do tsunami waves get bigger / taller?
A) They get bigger when they are far out in the deep ocean
B) They get bigger as they get closer to the shore
C) Their location does not impact wave height
Correct Answer: B
Plenary Questions and Example Answers:
What is an earthquake? It is the shaking / vibration of the Earth’s crust due to tectonic plate boundary movement.
When do tsunami waves get taller? As they get closer to the coast/shore/land.
What damage can tsunamis do to marine ecosystems? Damage coral, cause fish or other marine life to become stranded on land, scrape the ocean bottom and damage or kill small marine animals and invertebrates, uproot or damage trees and coastal vegetation.
What normally causes a volcano to form? When two tectonic plates collide and one slides underneath the other. Magma starts to rise up and can eventually build up pressure until it erupts.
What normally causes an earthquake to form? It can happen at any of the plate boundaries. When plates don’t move smoothly past each other and get stuck. When the pressure eventually releases and they become “unstuck” again, it can cause an earthquake.