Oceanic Crust Formation and Thermal Subsidence
Oceanic crust in the deep ocean consists of several layers of sediment. The most superficial layers are recent, and their age increases with depth. The age of the oldest sediment in each area is similar to the age of the basalts beneath them, representing the age of the oceanic crust at that location.
Three key observations are consistent across all oceanic basins:
- Basalts are present at the ridges, indicating formation within the last million years.
- The age of ancient basalts beneath the sediments increases with distance from the ridges.
- The age of the oceanic crust is never more than 180 million years.
Seafloor Spreading Theory
The theory of seafloor spreading can be summarized in the following stages:
- Period 1: Magma from the mantle reaches the surface at a mid-ocean ridge. The basalt cools and forms crystals containing magnetite with normal polarity.
- Period 2: The newly created lithosphere moves away from the ridge on either side. The space left is occupied by new magma, and magnetite crystals record the existing polarity (which may be inverted).
- Period 3: The process of creating new oceanic lithosphere continues, and each time, the basalts record the polarity of the magnetic fields.
Why the Ocean Floor Sinks
At the ridge, the lithosphere is extremely thin, and temperatures can reach 1000 ยบ C just 2 km below the surface. The ridge rises approximately 2 km above the surrounding seabed because the high temperatures cause the materials to be less dense and dilated.
As the newly formed crust comes into contact with ocean water, it cools, reducing its volume. As the lithosphere moves away from the ridge, it becomes thicker and more dense, causing it to subside. This progressive subsidence due to cooling is called lithospheric thermal subsidence and explains why ocean floor areas situated far from the ridges are deeper.
Thermal Subsidence Mechanism
Near the ridges, new oceanic crust is formed. As it ages, it cools, and the uppermost layer of the mantle attaches to the base of the crust. The materials comprising the oceanic crust are less dense than the mantle. However, the lithosphere and sublithospheric mantle materials have the same composition. The lithospheric mantle is denser because it is colder than the sublithospheric mantle, which is dilated by heat.
After about 100 million years, the lithosphere’s density begins to exceed that of the mantle beneath it, causing it to sink and disappear into the mantle.
Crystals and Crystallization
A crystal is a mineral with a geometric shape, characterized by flat faces, edges, and vertices.
Crystallization can occur through several processes:
- Solidification of melts: This process forms crystals in plutonic rocks as magma cools.
- Sublimation of dissolved gas substances: This process forms crystals in the sulfur ducts where volcanic gas escapes.
- Chemical precipitation from an aqueous solution.
Conditions for Crystal Development
For a crystal to develop, the following conditions are necessary:
- Time: If magma cools rapidly, atoms and ions in the melt may not arrange in an orderly fashion, resulting in glass formation or poorly developed crystals. Similarly, if an aqueous solution evaporates too quickly, numerous crystallization nuclei form simultaneously, resulting in numerous microscopic crystals.
- Space: A crystal needs sufficient space to develop. If space is limited, interference occurs with the growth of neighboring crystals, preventing any of them from acquiring geometric shapes.
- Dormancy: An agitated environment hinders crystal development.