Plate Tectonics: Earth's Structure and Plate Boundaries

Ocean Floor Discoveries and Structure

A little over 50 years ago, it was widely thought that the ocean floors were vast plains covered entirely with sediment. However, studies, particularly those focusing on the Atlantic Ocean, led to revolutionary conclusions regarding the structure of the deep ocean basin:

• In the middle of the ocean basin, a mountainous elevation was discovered: the oceanic ridge, featuring a central groove known as the rift.
• The layer of sediment was found to be much thinner than previously assumed.
• Deep ocean rocks are relatively young: the oldest rocks found were no more than 180 million years old, and their age increased systematically from the axis of the ridge towards the continents.

Plate Tectonics: Explaining Earth's Dynamics

The theory of plate tectonics is fundamental, helping to explain many of the geological processes that occur on our planet. This theory provides explanations for:

• The formation and location of continental and oceanic crust.
• The present arrangement of continents and their movement throughout geologic time.
• The formation and expansion of the oceans.

Defining Lithospheric Plates

According to the theory of plate tectonics, the Earth's surface is divided into rigid segments, known as lithospheric or tectonic plates. These plates, formed by the lithosphere resting on the mantle, move at speeds of several centimeters per year. Based on the crust they contain, three types are distinguished:

1. Oceanic plates
2. Continental plates
3. Mixed plates (containing both oceanic and continental crust)

The boundaries between these plates are typically areas of high seismic and volcanic activity.

Mechanisms Driving Plate Movement

The movement of tectonic plates is driven primarily by two forces:

1. Mantle Convective Cells: These cells are created by the heat generated in the Earth's interior, which tends to be removed toward the planet's surface. Hot material rises from the base of the mantle and sinks again when it cools, creating a continuous circulation.
2. The Force of Gravity (Ridge Push and Slab Pull): In areas of mid-ocean ridges (dorsals), the newly formed plate material is elevated, creating a greater height. This elevation allows the plate to be pushed away from the ridge axis toward a subduction zone, where the plate sinks into the mantle (slab pull).

Transform Faults: Neutral Plate Boundaries

Transform faults are locations where lithosphere is neither formed nor destroyed; they are considered neutral limits. In transform faults, movement involves a lateral shift between two plates. The total displacement along these faults can span hundreds or even thousands of kilometers.

These fractures commonly cut and offset mid-ocean ridges, though transform faults may sometimes connect ridges and subduction zones. The movement along transform faults involves the tearing of the lithosphere. Stress builds up over time, and during an earthquake, this accumulated energy is released as the plates slide horizontally past each other. Earthquakes that occur at transform faults tend to be shallow, with hypocenters typically located at depths of up to 25 km.