Lecture 4: Comprehensive Study Notes on Plate Tectonics and Planetary Evolution
The Significance of Plate Tectonic Theory
Plate tectonics is regarded as a unifying theory essential for understanding the evolution of Earth as a planet.
It provides a definitive framework for understanding major geomorphological features on the surface of the Earth.
This theory is critical for explaining geological hazards, including volcanic eruptions, earthquakes, and tsunamis.
It helps researchers understand rock distribution, such as why specific rocks dominate the continents while others are localized to the ocean floor.
Analogy: The importance of the theory of plate tectonics to physical geography is equivalent to the importance of the theory of evolution to the understanding of life on Earth.
Planetary Heat and Driving Mechanisms
Earth is currently a cooling planetary body that retains its original primordial heat.
Interior heat is sustained by radioactive decay of minerals and elements within the Earth, effectively keeping the planet "on the boil" and preventing it from cooling as rapidly as it otherwise would.
The arrangement of a hot interior and a cold exterior leads to the development of thermal convection currents.
Analogy for Convection: A boiling pot on a stove exhibits convection currents because it is hotter at the bottom where the heat source is and cooler at the top. Adding peas to such a pot demonstrates the mechanism as they travel up and down with the current.
Comparative Planetology: Earth vs. Mars
Earth and Mars are two of the terrestrial planets in the solar system, alongside Mercury and Venus.
Mars is significantly smaller than Earth and has already cooled down, losing its internal heat.
Consequences of Martian Cooling:
Plate tectonics no longer operate on Mars to any significant extent.
Mars lacks flowing water () because it lost its atmosphere.
Volcanic activity has shut down, ending the generation of gases that sustained the atmospheric volume.
The Martian atmosphere dissipated, losing the greenhouse effect previously provided by Carbon Dioxide () that once kept the planet warm.
For all practical purposes, Mars is considered a "dead planet," while Earth remains dynamic.
Earth sustains its atmosphere through geological activity (volcanic emissions) and biological contributions (photosynthesis).
The Earth's hydrosphere exists in three states: solid (ice), liquid, and gas.
Lithospheric Structure and Convection Processes
Asthenosphere: A weak part of the mantle where rocks are plastic and move under extreme temperatures and pressures.
Lithosphere: The rigid, "eggshell" outer layer of the Earth.
Tectonic Plates: Surface segments of the lithosphere.
Rock movement driven by thermal convection in the asthenosphere exerts a drag effect on the lithosphere, moving tectonic plates.
Plates diverge (pull apart) at mid-ocean ridges and converge (collide) where oceanic floor meets continental crust.
Classifications of Plate Boundaries
Divergent Plate Boundaries: Locations where plates move away from each other. A rift forms in the crust, and magma rises to create new crustal material.
Convergent Plate Boundaries: These are typically destructive boundaries where plates collide. In some cases, crust is forced back down into the mantle.
Transform Plate Boundaries: Locations where plates slide past each other horizontally. Significant examples include New Zealand and the West Coast of America.
The Process of Continental Breakup: The East African Rift System
Alfred Wegener proposed that all continents were joined in a jigsaw-like supercontinent called Pangaea during the Permian period ( years ago).
Pangaea broke into Laurasia (Northern Hemisphere) and Gondwana (Southern Hemisphere) before breaking into modern continents.
Continental lithosphere is quite thick, whereas oceanic lithosphere is thin.
The East African Rift System:
This is a modern example of continental breakup.
The system runs from Lebanon to South Africa.
The rift valleys are approximately to wide and to deep.
It is famous as the "cradle for the evolution of hominids."
Includes Saudi Arabia pulling apart from Egypt, with the sea flooding into the Red Sea and the Gulf of Aden.
Three-Branch Rifting: Rifts typically develop in three directions. One branch usually stops developing.
Example: When Australia rifted from Antarctica ( years ago), the main rift was south of Tasmania. A third branch became the Bass Strait, which stopped progressing while the main rift moved Australia north.
Stages of Continental Extension and Ocean Basin Formation
1. Rift Valley Stage: Convection currents beneath the continent heat the lithosphere, causing thermal expansion and "doming." As the crust fails, blocks of rock chunk down into the rift (faulting), creating depressions that often form lakes (e.g., the Great African Rift Valley in Kenya).
2. Narrow Sea Stage: As continents continue to pull apart and the valley depresses further, the sea floods in, creating a narrow body of water like the Red Sea.
3. Full Ocean Stage: The continents move further apart, leaving a full-blown ocean basin like the Atlantic or Indian Oceans.
Mid-Ocean Ridges and Oceanic Lithosphere
Mid-Ocean Ridges (MOR): Linear rifts along the ocean floor where magma erupts and welds onto the sides of diverging plates.
Magma Composition: These magmas are derived from the mantle and are rich in Magnesium (), Iron (), and Silica ().
Isostasy and Density: Oceanic rocks are very dense compared to the weak asthenosphere.
Analogy: Just as a lead fishing sinker sinks into plasticine, the dense oceanic crust sinks into the weak asthenosphere.
This sinking creates the depressions on the Earth's surface that contain the world's oceans.
Volcanic activity ceases on the continents as they move away from the ridge and is instead concentrated at the mid-ocean reaches.
Paleomagnetism and the Discovery of Sea Floor Spreading
Historical Context: Sea floor spreading was discovered through military necessity during World War II to improve submarine warfare mapping.
Instrument: American scientists used magnetometers towed behind ships to measure the magnetism of rocks on the ocean floor.
Mechanism: Basaltic magma contains the mineral Magnetite, which is magnetic. As the magma cools and solidifies, magnetite crystals align with the Earth's magnetic North Pole like tiny compass needles.
Magnetic Striping: Researchers found matching pairs of magnetic stripes (positive and negative anomalies) on either side of mid-ocean ridges. These indicate that the plates moved apart from a central point.
Magnetic Field Generation: The Earth's magnetic field is created by electrical currents in the liquid outer core (molten iron and nickel).
Field Reversals: The Earth's spin on its axis and orbital cycles are not perfectly consistent. These cycles periodically cause the convection currents in the outer core to flip, causing the North and South Poles to reverse position. This flipping is recorded in the magnetite minerals on the ocean floor.
Convergent Plate Boundaries and Subduction Zones
As the Earth is not expanding, lithosphere created at divergent boundaries must be destroyed elsewhere at convergent boundaries.
Subduction: The process where an oceanic plate (thin and dense) is pushed beneath another plate back into the mantle to be assimilated or melted.
Constraint: Only oceanic lithosphere can be subducted; continental lithosphere is too light and thick to be pushed into the mantle.
Ages of Crust: Due to subduction, ocean floor rocks rarely exceed years in age, while continental rocks can be billions of years old.
Ocean Trenches: Deep depressions reaching up to water depth (compared to an average depth of to ). Despite immense pressure, delicate organisms inhabit these trenches.
Benioff Zone: A line of earthquakes that defines the upper part of a subducting slab. It typically begins at the trench and dips at an angle of roughly beneath the island arc.
The Formation of Island Arcs and Continental Growth
Subduction of oceanic plates can occur beneath another oceanic plate or a continental plate.
Island Arc: Created when one oceanic plate subducts under another, causing melting that rises as volcanoes (e.g., the Indonesian Archipelago).
Partial Melting Principles:
The subducting slab is wet () and only partially melts as it descends.
Part of the chemical composition is left behind, resulting in magma with a much higher concentration of Silica () than the original basalt.
This high-silica magma is viscous and contains more gas, leading to explosive and dangerous volcanic eruptions.
The resulting rocks are less dense and "float" higher in the asthenosphere, forming the foundations of new continents.
Continental-Continental Collisions: Folded Mountain Belts
When two continental plates collide, subduction does not occur because the rocks are too light and thick.
The intense heat and pressure make the rocks plastic, causing them to fold and "concertina."
Folded Mountain Belts (Origin): Massive mountain ranges formed by this thickening and folding, such as the Himalayas and the European Alps.
Himalayas: Formed by India pushing against Eurasia; still actively rising today.
Melbourne Example: Rock layers in the Eastern Freeway (near Doncaster) exhibit up-and-down folding. These are the eroded cores of a mountain range that was once as large as the Himalayas millions of years ago.
Questions & Discussion
Q: Does sea flooding into rifts (like the Red Sea) cause global sea levels to fall?
A: Not significantly at a local level. However, the volume of ocean basins is affected by the rate of sea floor spreading and magma production at mid-ocean ridges, which is one factor influencing long-term global sea level changes.
Q: Is magma always made of mantle material?
A: No. While mantle-derived magma occurs at mid-ocean ridges, magma also forms within the lithosphere or continental crust through melting at depth.
Q: What happens to subducted material that doesn't melt?
A: Eventually, it is pushed deep enough into the asthenosphere to melt completely and be absorbed into the mantle.
Q: How do we know about partial melting?
A: By measuring the chemistry of magmas from arc volcanoes and comparing them to those at mid-ocean ridges; the chemical differences prove that only certain compositions from the descending slab are melting.
Ethical and Practical Implications:
Indonesia: Farmers inhabit the slopes of explosive volcanoes because volcanic soils are extremely fertile. However, these volcanoes are unpredictable and dangerous.
Greater Sunrise Oilfield: Located between Australia and East Timor. A dispute exists because East Timor wants a pipeline across the subduction trench for industrial development, while oil companies refuse due to the high risk of earthquakes and pipeline failure in a deep-sea trench.