Comprehensive Notes on Plate Tectonics

Plate Tectonics: Margins and Landforms

To understand the theory of plate tectonics, we must have a clear understanding of Earth's structure.

Inner Core: Solid; composed predominantly of iron and nickel; approximately five times denser than surface rocks.
Outer Core: Liquid state; primarily made of iron; movements in this layer are considered responsible for Earth's magnetic field.
Mantle: Mostly solid but behaves as a viscous fluid over geologic time scales; sometimes likened to caramel in consistency.
Crust: Solid layer divided into two main types:
- Oceanic Crust: Thickness varies from 5 to 7 km; younger (under 200 million years); generally thinner, denser and heavier, capable of sinking.
- Continental Crust: Thickness varies between 10 to 70 km; older and less dense, not subject to sinking. (e.g., granite).

Lithosphere: Rigid layer made up of the crust and the upper mantle; inflexible and buoyant; responsible for carrying tectonic plates, thus causing continental drift.
Asthenosphere: Located just beneath the lithosphere; capable of flow or deformation (viscoelasticity); involved in convection currents driving tectonic movements.
Mohorovićic Discontinuity (Moho): Boundary separating the crust from underlying mantle marked by a change in density and composition.

Historical Context: Prior to the 20th century, many geologists believed Earth's features were static. Notably, in 1620, Francis Bacon suggested that continents could fit together like a jigsaw.
Continental Drift: Proposed by Alfred Wegener in 1912; he suggested all continents were once part of a single landmass (Pangaea) that drifted apart.
The hypothesis was initially rejected until the 1950s, when evidence from various fields, such as paleomagnetism, emerged supporting it.

Glacial Deposits:
- Distribution of late Paleozoic glacial deposits and striations across continents in current tropical locations suggests past glaciation when these areas were located near polar regions.
Orogeny (Mountain Building):
- Similar rock formations, structures, and ages found across continents (e.g., Appalachian Mountains in North America and Caledonian Mountains in Scotland).
Fossil Distribution:
- Identical fossils found in similar aged rocks on different continents indicate previous connections;
- Mesosaurus fossils found only in Africa and South America.
- Lystrosaurus fossils found in Africa, Antarctica, and South America reveal adjacency of continents.

Major earthquake and volcano patterns coincide with tectonic plate boundaries, revealing how lithosphere sections move independently.
Distribution analysis shows alignment of earthquakes primarily along convergent and diverging plate boundaries.

Paleomagnetism:
- Magnetic orientation of iron particles in magma solidifies according to Earth's magnetic field direction; records shifts in magnetic pole positions confirm continents' movements.
Age and Pattern of Ocean Basins:
- Oceanic crust < 260 million years old; younger near mid-ocean ridges. Radiometric dating confirms age discrepancies.

Proposed by Harry Hess in the 1960s;
- Describes how mid-ocean ridges act as zones where tectonic plates are pulled apart, creating new oceanic crust from magma, a theory supported by:
1. Existence of submarine mountain chains.
2. Alternating magnetic polarities indicating periodic reversals of the Earth's magnetic field.
3. Active volcanic processes along ridges.

Satellite Geodesy: Measures surface movements with high precision (cm/year), providing evidence of tectonic movements beyond known fault lines.
Seismic Tomography: Uses seismic waves to visualize Earth's internal structure, revealing motion and dynamics of tectonic plates.

Map of Earth's Tectonic Plates: Shows various plates including North American, Eurasian, African, and Pacific Plates, with their movement rates.

Constructive Margins: Plates move apart, leading to rift valleys and mid-ocean ridges.
Destructive Margins: Plates collide, resulting in subduction zones, trenches, and mountain building.
Conservative Margins: Plates slide past one another, leading to transform faults, such as the San Andreas Fault in California.

Volcano definition: A vent or crack in Earth's surface allowing lava and gases to escape.
Types of eruptions: Range from gentle lava flows to violent pyroclastic events.
Lava Types:
- Basaltic Lava: Low viscosity; flows easily. Eg. Pahoehoe (smooth); Aa (rough surfaces).
- Andesitic Lava: Moderate viscosity; cubic in shape.
- Rhyolitic Lava: High viscosity, forming steep domes.

Lahars (mudflows), lava flows, pyroclastic flows, and tephra (volcanic ash) can devastate areas around volcanoes.
Lahars and ash can cause widespread disruptions, including in air traffic and infrastructure.
Notable eruptions include Mount Pinatubo, responsible for massive destruction and climate impact due to ash ejection.

Mid-Atlantic Ridge: A prominent example of a constructive margin where new oceanic crust is formed and features volcanic activity.
Island Arcs: Formed at destructive margins where oceanic plates subduct, leading to volcanic activity, e.g. Aleutian Islands from the Pacific Plate subducting beneath the North American Plate.