Model Answer
0 min readIntroduction
The theory of plate tectonics, a cornerstone of modern geology, revolutionized our understanding of Earth’s dynamic processes. Prior to the mid-20th century, the concept of continents being fixed in their positions was widely accepted. However, accumulating evidence, particularly from the fields of palaeomagnetism and sea floor spreading, challenged this notion. These discoveries validated the idea that continents and ocean basins are not stationary but are constantly moving and reshaping the Earth’s surface. This movement is driven by forces within the Earth and has profoundly impacted the planet’s geological history and continues to do so today.
Palaeomagnetism: A Record of Past Magnetic Fields
Palaeomagnetism, the study of the record of Earth’s magnetic field in rocks, provides compelling evidence for continental drift. As molten rock cools, magnetic minerals within it align themselves with the prevailing magnetic field. This alignment is ‘locked in’ as the rock solidifies, preserving a record of the magnetic field’s direction and intensity at the time of formation.
- Apparent Polar Wander Paths: Different continents exhibit different ‘apparent polar wander paths’ – the path traced by the magnetic pole over time as determined from rocks on that continent. If continents were fixed, they should have the same polar wander path. The differing paths indicate that continents have moved relative to each other and to the magnetic poles.
- Magnetic Reversals: Earth’s magnetic field periodically reverses its polarity (North becomes South and vice versa). These reversals are recorded in rocks as alternating bands of normal and reversed polarity. The pattern of these reversals is symmetrical on either side of mid-ocean ridges, providing further evidence for sea floor spreading (discussed below).
Sea Floor Spreading: The Birth of New Oceanic Crust
Sea floor spreading is the process by which new oceanic crust is created at mid-ocean ridges. These ridges are underwater mountain ranges where magma rises from the mantle and solidifies, forming new basaltic crust. This newly formed crust then moves away from the ridge in both directions, pushing older crust further away.
- Evidence from Mid-Ocean Ridges: Mid-ocean ridges are characterized by a central rift valley, shallow earthquakes, and volcanic activity. These features are consistent with the upwelling of magma and the creation of new crust.
- Age of Oceanic Crust: The age of oceanic crust increases with distance from the mid-ocean ridge. The youngest rocks are found at the ridge itself, while the oldest rocks are found furthest away. This supports the idea that the crust is continuously being created and moving outwards.
- Magnetic Striping: As mentioned earlier, the symmetrical pattern of magnetic reversals on either side of mid-ocean ridges provides strong evidence for sea floor spreading. The pattern is a mirror image on both sides, indicating that the crust was formed at the ridge and then spread outwards.
Continental Drift and Plate Tectonics: A Unified Theory
The evidence from palaeomagnetism and sea floor spreading, combined with other observations such as the fit of the continents (especially South America and Africa) and the distribution of fossils, led to the development of the theory of plate tectonics. This theory proposes that Earth’s lithosphere is divided into several large and small plates that are constantly moving and interacting with each other.
- Driving Forces: The movement of plates is driven by convection currents in the mantle, ridge push (gravity sliding of plates down the elevated mid-ocean ridges), and slab pull (the sinking of denser oceanic plates into the mantle).
- Plate Boundaries: Interactions between plates occur at plate boundaries, resulting in earthquakes, volcanoes, and mountain building. There are three main types of plate boundaries: divergent (where plates move apart), convergent (where plates collide), and transform (where plates slide past each other).
| Plate Boundary Type | Characteristics | Examples |
|---|---|---|
| Divergent | New crust is created; volcanic activity; shallow earthquakes | Mid-Atlantic Ridge, East African Rift Valley |
| Convergent (Oceanic-Continental) | Subduction; volcanic arcs; deep earthquakes; mountain building | Andes Mountains, Cascade Range |
| Convergent (Oceanic-Oceanic) | Subduction; volcanic island arcs; deep earthquakes | Mariana Islands, Aleutian Islands |
| Transform | Horizontal sliding; shallow earthquakes | San Andreas Fault |
Conclusion
The evidence from palaeomagnetism and sea floor spreading irrevocably demonstrated that continents and ocean basins are not static entities. These discoveries formed the foundation of the theory of plate tectonics, which provides a comprehensive framework for understanding Earth’s dynamic processes. The ongoing movement of plates continues to shape our planet, influencing everything from the distribution of continents and oceans to the occurrence of earthquakes and volcanoes. Further research continues to refine our understanding of these complex interactions and their implications for Earth’s future.
Answer Length
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