Study Notes on Sea-Floor Spreading and Plate Tectonics

Sea-Floor Spreading and Plate Tectonics

Various geophysical features such as mountains, earthquakes, and volcanoes are not randomly distributed on the Earth's surface.
In 1912, German meteorologist Alfred Wegener suggested the concept of continental drift, positing that if continents are capable of vertical movement, they could also move horizontally.
The theory of sea-floor spreading was proposed in 1960, stating that new sea floor is formed as adjacent crust moves apart.
Both theories were unified into the model of plate tectonics, supported by extensive geophysical evidence over the last three decades.

The Earth's outer shell, known as the lithosphere, is approximately 100 kilometers thick and composed of rigid plates that move in different directions.
Plate boundaries are sites of tectonic activity, including earthquakes, volcanism, and mountain building.
There are seven large plates and several smaller ones, with their movements driven by the Earth’s internal heat and gravitational forces.

The Earth's interior is hot and supplies a constant flow of heat to the surface, resulting in thermal gradients.
Convection occurs as hot, low-density materials rise and cooler, denser materials sink.
This convective motion facilitates the movement of lithospheric plates on the Earth's surface.

Divergent Boundaries (Spreading Centers)
- Plates separate, leading to the rise of magma that cools to form new oceanic crust.
- Characterized topographically by mid-ocean ridges (e.g., Mid-Atlantic Ridge, East Pacific Rise).
Convergent Boundaries (Subduction Zones)
- Plates collide, with one plate being subducted beneath another, often resulting in oceanic trenches.
- Subducted crust is heated and melts, contributing to volcanism on the overriding plate near the trench (e.g., Aleutian Islands, Peru-Chile Trench).
Transform Faults (Lateral Slip Faults)
- Plates slide past one another with no creation or destruction of crust, characterized by fracture zones (e.g., San Andreas Fault, Mendocino Fault Zone).

Plate interactions are rarely smooth; they may "catch" and deform elastically until failure occurs, resulting in earthquakes.
Most significant seismic events occur in subduction zones, but transform faults can also produce large earthquakes.
Collision of continental plates can lead to mountain formation (e.g., Himalayas, Alps).

Paleomagnetism provides critical evidence for sea-floor spreading.
The Earth's magnetic field has reversed polarity approximately every half million years, creating polarity epochs.
Lava that cools at mid-ocean ridges preserves the magnetic polarity at the time of cooling, allowing for the study of past magnetic orientations.
As new ocean crust forms, it carries the recorded magnetic signatures outward from the ridge.

Magnetic anomalies in sea floor allow for the assessment of spreading rates. Positive anomalies correspond to rocks with the same magnetic polarity as present, while negative ones correspond to rocks with reversed polarity.
The rate of new sea floor formation can be calculated by measuring the distance to a magnetic anomaly of known age: ext{Rate} = rac{ ext{Distance}}{ ext{Age}}
Recorded velocities range from 2-3 cm/year for the Atlantic Ocean up to 16 cm/year for the Pacific Ocean.

Chains of volcanoes, such as the Hawaiian Islands, form over hot spots which are plumes of magma rising from deep within the mantle.
As tectonic plates move over these stationary hot spots, new volcanoes form while older ones become dormant, creating linear island chains.
The island of Hawaii is the most active, with a new volcano currently forming southeast of it.

Over 122 hot spots have been identified on both oceanic and continental plates, showing a nonuniform distribution.
Many hot spots align with mid-ocean ridges, and a notable concentration occurs on the African Plate, indicating it may be stationary.

Exotic Terrane: A fault-bounded block of rock with a geological history distinct from adjacent terranes.
- Continental crust pieces can detach and migrate long distances, becoming part of different continents.
Miniplates (Microplates) can drift on oceanic plates, contributing to continental growth and complexity.

The Indian subcontinent is an example of a large exotic terrane that collided with Asia.
Western North America's terranes feature elongated rock formations that have travelled significant distances, involving complex geological histories.

Exotic terranes exhibit paleomagnetic signatures indicative of their distant origins, aiding in reconstructing previous continental configurations.
- Notably, the Wrangellia terrane provides evidence of significant geological migration and integration into the North American continent over time.

Exotic Terrane: A rock body bounded by faults, unrelated to adjacent rock bodies.
Hot Spots: Fixed rising lava plumes originating from the mantle, often leading to volcanic activity far from plate boundaries.
Lithosphere: The rigid outer shell of the Earth, encompassing the crust and upper mantle, typically 100-150 km thick.
Lithospheric Plate: A portion of the lithosphere defined by various plate boundaries (transform faults, mid-ocean ridges, and trenches).
Magma: Molten rock within the Earth; once surface-bound, it is called lava.
Miniplates (Microplates): Small continental plates moving on oceanic crust due to tectonic drift.
Spreading Center: Mid-ocean ridges or rises where magma creates new sea floor.
Subduction Zone: Areas where one plate descends beneath another, associated with volcanic activity.
Transform Fault: A lateral slip fault along which displacement is abrupt and does not create or destroy crust.