The Earth’s outermost layer, the lithosphere, is fragmented into numerous large and small tectonic plates. These slabs of rock are in constant, slow motion, interacting along their boundaries. These interactions define three main types of plate boundaries: convergent, transform, and divergent, where plates separate. This examination focuses specifically on the forces at work where plates move apart.
The Primary Tectonic Force: Tensional Stress
The force present at a divergent boundary is known as tensional stress, a pulling force that acts to stretch the lithosphere apart. This stress acts perpendicular to the boundary line, trying to increase the distance between the separating plates. Tensional stress is fundamentally different from the compressional forces found where plates collide. If this stress exceeds the rock’s strength, the rigid crust will fracture, leading to the creation of normal faults where blocks of crust slip down relative to one another.
The Process of Plate Separation
The geological engine that generates tensional stress is the slow movement of the Earth’s mantle beneath the plates. Within the mantle, immense convection cells operate, where hotter material rises and cooler material sinks. At a divergent boundary, upwelling currents deliver hot, buoyant material beneath the lithosphere, causing the crust to arch slightly before fracturing. As the plates are dragged apart, pressure on the underlying hot mantle rock decreases significantly. This drop in pressure, called decompression melting, allows the rock to melt, generating magma that rises to create new lithosphere.
Distinct Geological Structures Formed
The application of tensional stress and subsequent plate separation result in two distinct types of large-scale geological features, depending on whether divergence occurs beneath a continent or an ocean. When the force acts on continental crust, it forms a continental rift valley, such as the East African Rift System. If separation continues, the rift will deepen, flood with water, and evolve into an oceanic divergent boundary. Most active divergent boundaries are found beneath the oceans, forming vast submarine mountain chains called mid-ocean ridges. The Mid-Atlantic Ridge is a structure where the North American and Eurasian plates are moving apart. Because rising magma continually adds new crust, divergent boundaries are referred to as constructive boundaries.