Oceanic Lithosphere and Plate Tectonics
TEMA – 8. Plate Tectonics
Oceanic Lithosphere
Study Skills.
For seabed analysis, the following methods are used:
- Surveys to identify rocks.
- Sonar to map the undersea terrain.
- Geophysical research cruises collecting seismic and magnetic data.
- Submersibles designed for direct observation and sampling.
Undersea Structures
Continental Shelf. Submerged prolongation of the continent. Maximum depth 200 meters.
Continental Slope. Steep (40%) slope to the edge of the continental shelf.
Abyssal Plain. Area of low slope. Seabed.
Oceanic Ridges. Submarine mountain ranges.
Volcanic Islands (sharp peaks = seamounts, flat top = guyots).
Oceanic Trenches. Deep depressions (up to 11,000 m).
Continental Margins
They are transition zones between continents and oceans. Are classified as:
Passive Continental Margins. Low seismic activity. Continental shelf is wide (up to 70 km) and gently sloping. Steep continental slope.
Active Continental Margins. Seismic activity. Continental slope with steep slopes that end in trenches.
Lithological Composition of the Oceanic Lithosphere
Layers:
- Marine sediments from the continent.
- Igneous basement rocks (basalts).
- Below are the gabbro (magmatic rocks that cooled slowly).
Mid-Ocean Ridges
The ridges are curved, thinned, and distended areas.
They are marine mountain ranges (over 60,000 km).
Features:
- The ridges do not have folds and consist of magmatic material.
- The center piece is a deep, flat rift valley. Relief in steps. Multiple cross faults.
- Many shallow-focus earthquakes.
Seafloor
Marine surveys reveal that: Sediment thickness varies with distance from the ridge axis (maximum along the continent). There are magnetic stripes, alternating on both sides of the ridge, with changes in polarity. Basaltic lavas are younger near the ridge and older farther away. Marine earthquakes and volcanoes often occur near the ridges. Heat flow is high at the ridges and minimal at the trenches. These data prove seafloor spreading. Magma rises from the mantle at the rift, cools, and is incorporated into the oceanic lithosphere.
Hot Spots
In the mantle, there are high-temperature heat plumes. When a plume reaches the lithosphere, it partially melts, producing a magma stream that emerges as a hot spot, creating a region of volcanic activity. Geologists have identified over 100 hot spots, with the Hawaiian Islands being the most well-known.
Continental Structures
In the continental lithosphere, two types of areas can be distinguished:
Cratons: Areas that have been stable for over 500 million years. They occupy the interior of continents and include:
- Continental Shields. Large, flat areas that form the basis of the continents.
- Stable Platforms. Plains of sediments covering continental shields.
Orogens: Mountainous areas formed by compression during plate collisions.
Continental Drift
Theory proposed by A. Wegener in the early 20th century. According to this theory, continents (consisting of sial) rest on a substrate (the sima) that emerges at the level of the oceans. There was a single continent, called Pangea, which broke apart, forming the present continents that moved to their current positions.
Arguments to support his theory: Matching shapes of continents, similar fossil flora and fauna on both sides of the Atlantic, overlapping fold axes of ancient mountain ranges on both sides of the Atlantic, and geological continuity between eastern South Africa, Australia, and Argentina. Wegener could not explain how a rigid Earth could produce such large continental shifts.
Theory of Plate Tectonics
According to this theory, the lithosphere is a rigid, non-continuous layer, fractured into blocks called lithospheric plates that move relative to each other.
According to this theory: Plates move apart, collide, or slide laterally. Plates move on the upper mantle, which has higher temperatures and more plastic materials. Plate movements cause earthquakes, volcanoes, and mountain ranges. Plate boundaries include mid-ocean ridges, trenches, and transform faults. Plate movement is due to (i) convection currents in the mantle and (ii) the suction mechanism of subduction zones.
Lithospheric Plates Today
Current plates have a thickness of 70 km under the ocean and 120 km under the continents. There are eight large plates: Pacific, Eurasian, African, South American, North American, Indo-Australian, Nazca, and Antarctic. The plates are in constant motion and change shape and size.
Wilson Cycle
It explains the redistribution of plates over Earth’s history through the opening and closing of oceans.
Stages:
- Initial phase: Heat accumulation in the mantle causes fragmentation and magma upwelling.
- Rift phase: Grabens form with a central rift valley where magma emerges.
- Opening phase: Depression is flooded with water, forming an ocean.
- Expansion phase: Deep ocean expands as continents move apart.
- Subduction phase: Oceanic lithosphere sinks beneath continental lithosphere at subduction zones, forming mountain ranges.
- Supercontinent phase: All continents collide, starting the cycle again.
Why Do Plates Move?
Plate movement is driven by Earth’s internal heat flow from the core to the crust. It is explained by two mechanisms:
- Columns of hot material rising from the core to the surface as convective cells.
- The lithosphere is pulled into subduction zones due to gravitational forces from the weight of the subducting plate.