The Earth’s outermost layer, known as the lithosphere, is divided into large, rigid sections called tectonic plates. These plates are in constant, slow motion, gliding over a weaker, partially molten layer beneath them. The regions where these immense plates meet and interact are called plate boundaries. These boundaries are areas of significant geological activity, including the formation of mountains, volcanic eruptions, and earthquakes. Can earthquakes happen at divergent boundaries?
Understanding Divergent Plate Boundaries
Divergent plate boundaries are locations where two tectonic plates are moving away from each other. This separation causes the Earth’s lithosphere to stretch and thin under tensional forces. As the plates pull apart, magma rises from the mantle below to fill the gap. This magma cools and solidifies, creating new crustal material, a process known as seafloor spreading.
These boundaries commonly form underwater mountain ranges called mid-ocean ridges. The Mid-Atlantic Ridge, which runs through the Atlantic Ocean, is a well-known example of an oceanic divergent boundary. Divergent boundaries can also occur within continents, leading to the formation of continental rift valleys. The East African Rift Valley and the Red Sea are examples where continental landmasses are being pulled apart.
Why Earthquakes Occur at Divergent Boundaries
Earthquakes occur at divergent boundaries due to tensional stress on the crust as plates move apart. This pulling-apart motion causes the rocks to stretch and thin. Eventually, the stress accumulated in the rocks exceeds their strength, leading to fracturing. When these rocks fracture, they slip along faults, releasing stored energy as seismic waves, which causes the ground to shake.
The type of faulting characteristic of divergent boundaries is known as normal faulting. In a normal fault, the block of rock above the fault plane moves downward relative to the block below due to the extensional forces. Earthquakes at these locations are shallow, occurring at depths less than 30 kilometers. This shallow depth is because the crust is relatively thin at divergent boundaries, and the presence of hot, ductile magma close to the surface prevents deeper, brittle fracturing of rocks.
Features of Earthquakes at Divergent Boundaries
Earthquakes along divergent boundaries have a lower magnitude compared to those at convergent or transform plate boundaries. While they are common, individual seismic events along the spreading segments are small and less frequent due to the elevated rock temperatures. However, earthquakes are more common along the transform faults that often offset segments of mid-ocean ridges.
The depth of these earthquakes is consistently shallow, within the upper 30 kilometers of the crust. This shallow depth, combined with their often underwater location along mid-ocean ridges, means they have a limited impact on human populations. In continental rift zones, such as the East African Rift Valley, earthquakes are moderate in magnitude, though not as powerful as those associated with other boundary types. Research indicates that earthquakes at divergent boundaries radiate energy primarily at longer periods and have lower average stress drops compared to earthquakes at convergent boundaries.