Model Answer
0 min readIntroduction
Seafloor spreading is a crucial process in plate tectonics, responsible for the formation of new oceanic crust. Proposed initially as part of the broader theory of continental drift by Alfred Wegener in the early 20th century, it gained substantial support with the discovery of mid-ocean ridges and subsequent research. This process, driven by mantle convection, fundamentally reshapes the Earth’s surface and explains many geological phenomena. Understanding the mechanism and the evidence supporting seafloor spreading is vital for comprehending the dynamic nature of our planet. This answer will detail the process and provide four key pieces of evidence demonstrating the expansion of oceanic crust.
Mechanism of Seafloor Spreading
Seafloor spreading occurs at divergent plate boundaries, primarily along mid-ocean ridges. The process is driven by convection currents within the Earth’s mantle. Hotter, less dense material rises from deep within the mantle, creating upwelling currents. These currents exert a force on the overlying lithospheric plates, causing them to move apart. As the plates separate, magma from the asthenosphere rises to fill the gap, solidifying and forming new oceanic crust. This newly formed crust is basaltic in composition and adds to the edges of the plates, effectively pushing older crust further away from the ridge.
Evidences for Expansion of Oceanic Crust
1. Paleomagnetic Striping
One of the most compelling pieces of evidence is the discovery of paleomagnetic striping. As basaltic magma cools and solidifies at mid-ocean ridges, iron-rich minerals within the rock align themselves with the Earth’s magnetic field. The Earth’s magnetic field periodically reverses its polarity (North becomes South and vice versa). This results in alternating bands of normally and reversely magnetized rocks on either side of the mid-ocean ridge. These bands are symmetrical and parallel to the ridge, providing a clear record of past magnetic reversals and demonstrating that new crust is continuously being added. The age of the rocks increases with distance from the ridge.
2. Age of Oceanic Crust
Studies of the age of oceanic crust, determined through radiometric dating of rock samples, reveal a clear pattern. The youngest rocks are found at the mid-ocean ridges, while the oldest rocks are located furthest away from the ridges, near the continental margins. This age gradient supports the idea that new crust is created at the ridges and moves outwards as spreading continues. For example, the oldest oceanic crust, found in the western Pacific Ocean, is approximately 200 million years old, while rocks near the mid-Atlantic Ridge are much younger, some being less than a few million years old.
3. Heat Flow Measurements
Heat flow measurements across the ocean floor also provide evidence for seafloor spreading. Higher heat flow is observed near mid-ocean ridges, indicating the presence of hot, rising magma. As you move away from the ridge, the heat flow decreases, reflecting the cooling of the oceanic crust as it moves further from the heat source. This gradient in heat flow is consistent with the model of seafloor spreading, where new, hot crust is continuously being created at the ridges.
4. Sediment Thickness
The thickness of sediment layers on the oceanic crust also supports the theory. Sediment accumulates over time on the ocean floor. The thickness of sediment increases with distance from the mid-ocean ridge. This is because the crust further from the ridge has had more time to accumulate sediment. Analysis of sediment cores reveals that the oldest sediments are found furthest from the ridge, while the youngest sediments are found closest to the ridge, further corroborating the idea of continuous crustal expansion. The maximum sediment thickness is generally found near the continental margins.
| Evidence | Description |
|---|---|
| Paleomagnetic Striping | Symmetrical bands of normally and reversely magnetized rocks parallel to mid-ocean ridges. |
| Age of Oceanic Crust | Youngest rocks at ridges, oldest rocks furthest away, demonstrating continuous creation. |
| Heat Flow Measurements | High heat flow near ridges, decreasing with distance, indicating cooling crust. |
| Sediment Thickness | Increasing sediment thickness with distance from ridges, reflecting accumulation over time. |
Conclusion
In conclusion, seafloor spreading is a fundamental process driving plate tectonics, evidenced by paleomagnetic striping, the age of oceanic crust, heat flow measurements, and sediment thickness distribution. These observations collectively demonstrate that new oceanic crust is continuously being created at mid-ocean ridges, pushing older crust away and contributing to the dynamic reshaping of the Earth’s surface. Continued research and monitoring of these processes are crucial for understanding the long-term evolution of our planet and mitigating associated geological hazards.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.