Plate Tectonics describes how the Earth’s rigid outer layer, the lithosphere, is divided into large, moving slabs called tectonic plates. These plates are in constant, slow motion, typically moving a few centimeters each year. The interaction between these segments generates the planet’s major geological features and events. Boundaries are formed where the edges of these plates meet, defined by the relative direction of plate movement, such as sliding past one another, colliding, or pulling apart.
The Mechanics of Divergent Plate Boundaries
A divergent plate boundary is a linear feature where two tectonic plates actively move away from each other. This separation is ultimately driven by the slow, immense circulation of material within the Earth’s mantle, known as convection currents. Hot, buoyant material rises toward the surface, spreads laterally, and drags the overlying plates apart, initiating the splitting of the lithosphere.
The action of pulling the crust apart subjects the lithosphere to tensional stress. This stress acts to stretch the rock body horizontally in opposite directions, causing the Earth’s crust to thin and weaken.
As tensional stress continues, the brittle upper crust fractures. This fracturing is a mechanical response to the pulling action, allowing the crust to extend and beginning the formation of a rift.
Identifying the Primary Fault Type
The direct result of tensional stress and the subsequent stretching and fracturing of the lithosphere is the formation of a Normal Fault. A Normal Fault is a type of dip-slip fault, meaning the movement is primarily vertical along the inclined fault plane. This type of faulting is characterized by the blocks of rock sliding down the fault surface due to the pull of gravity.
To understand the movement, geologists use the terms hanging wall and footwall to describe the two blocks of rock separated by the fault plane. The footwall is the block of rock that lies beneath the fault plane, while the hanging wall is the block situated above the fault plane.
In a Normal Fault, the hanging wall block moves downward relative to the footwall block. This downward slip effectively lengthens the crust horizontally, which is the necessary result of the tensional stress acting on the region. The fault plane itself is an inclined surface, and the movement follows this angle, or dip, which is often steeply angled at 60 degrees or more, though shallower angles are also common.
The formation of Normal Faults serves as the mechanism by which the crust accommodates the extension at a divergent boundary. As the plates continue to separate, new faults form, and existing ones accumulate more displacement. The collective movement along these numerous Normal Faults is what accounts for the measurable separation and thinning of the Earth’s lithosphere at the boundary.
Large-Scale Geological Structures
The repeated action of Normal Faults across a wide zone at a divergent boundary creates distinctive large-scale geological structures. The most characteristic features are alternating blocks of uplifted and down-dropped crust, referred to collectively as horst and graben topography. These structures are created by the geometry of parallel Normal Faults.
A graben is a block of crust that has subsided, or dropped down, between two parallel Normal Faults whose fault planes dip inward toward each other. These down-dropped blocks form the low-lying areas, which are often observed as rift valleys. The East African Rift Valley is a prominent example of a continental rift, where the African plate is actively being pulled apart, forming an extensive system of grabens.
Conversely, a horst is an elongate block of crust that remains elevated relative to the blocks on either side. A horst is bounded by two Normal Faults whose fault planes dip away from each other, resulting in a relatively uplifted ridge or plateau. Horsts and grabens typically occur adjacently, creating a landscape of parallel ridges and valleys that can span tens of kilometers.
In oceanic settings, the continuous process of extension, Normal Faulting, and the upwelling of magma results in the formation of mid-ocean ridges. The Mid-Atlantic Ridge exemplifies this type of divergent boundary, where the crust is separating and new oceanic lithosphere is being generated. These underwater mountain ranges are themselves characterized by a central rift valley—a massive graben—where the most active extension and faulting occurs.