A divergent plate boundary is where two tectonic plates actively move away from each other. This separation leads to the formation of new crustal material as molten rock rises from beneath the Earth’s surface. The primary type of fault associated with these areas of crustal extension is the normal fault. This geological activity often results in significant seismic events and the creation of distinct landforms.
Understanding Divergent Boundaries
At divergent boundaries, Earth’s lithospheric plates are subjected to tensional stress, meaning they are pulled apart. This pulling motion causes the crust to thin and stretch. As the plates separate, hot magma from the underlying mantle rises to the surface. This magma then cools and solidifies, forming new crustal rock.
This continuous process of extension and new crust formation is known as seafloor spreading in oceanic environments. The stretching of the crust creates cracks and fractures. These fractures accommodate the tensional forces, allowing the plates to move further apart and facilitating the ascent of molten material.
Normal Faults
A normal fault is a geological fault where the Earth’s crust undergoes extension. It is characterized by the downward movement of the hanging wall relative to the footwall. To visualize this, imagine a miner standing in a tunnel along the fault; the hanging wall is where their lamp would hang, and the footwall is under their feet.
The inclined fault plane allows the hanging wall to slide down under the influence of gravity, accommodating the stretching of the crust. Normal faults typically have steep dip angles, often ranging between 45 and 90 degrees. They can occur as individual fractures or in groups, leading to landscapes where blocks of crust are uplifted (horsts) or down-dropped (grabens).
Where Divergent Boundaries and Their Faults Occur
Divergent boundaries and their associated normal faults are prominent in two main geological settings. Mid-ocean ridges, such as the Mid-Atlantic Ridge, are prime examples where new oceanic crust is generated. Along the Mid-Atlantic Ridge, plates spread apart at approximately 2 to 5 centimeters per year, resulting in a deep rift valley comparable in depth and width to the Grand Canyon.
Another significant example is found in continental rift zones, like the East African Rift Valley. Here, continental landmasses are pulling apart, leading to the formation of extensive rift valleys bounded by numerous normal faults. The East African Rift, where the Somalian and Nubian plates are separating at a rate of about 6 to 7 millimeters per year, showcases a classic horst and graben topography, with crustal thinning from a typical 36 kilometers to about 20 kilometers in some areas.