Divergent boundary definition series#

Divergent boundary definition series#

When a series of mantle plumes exists beneath a large continent, the resulting rifts may align and lead to the formation of a rift valley (such as the present-day Great Rift Valley in eastern Africa). The buoyancy of the mantle plume material creates a dome within the crust, causing it to fracture. Spreading is hypothesized to start within a continental area with up-warping or doming of crust related to an underlying mantle plume or series of mantle plumes. The region in the outlined rectangle represent the mid-ocean ridge (Steven Earle, “Physical Geology”). Figure 4.5.3 Mechanism for divergent plate boundaries. Over time, the igneous rock of the oceanic crust gets covered with layers of sediment, which eventually become sedimentary rock. This magma oozes out onto the seafloor to form pillow basalts, breccias (fragmented basaltic rock), and flows, interbedded in some cases with limestone or chert.

The triangular zone of partial melting near the ridge crest is approximately 60 km thick and the proportion of magma is about 10% of the rock volume, thus producing crust that is about 6 km thick. The crustal material created at a spreading boundary is always oceanic in character in other words, it is igneous rock (e.g., basalt or gabbro, rich in ferromagnesian minerals), forming from magma derived from partial melting of the mantle caused by decompression as hot mantle rock from depth is moved toward the surface (Figure 4.5.3). Arrows indicate the direction of plate motion on either side of the fault (USGS, Public domain, via Wikimedia Commons). Figure 4.5.2 Closeup of the mid-Atlantic ridge system, showing transform faults perpendicular to the ridge axis. The mid-ocean ridge system can be seen as the light blue chain of mountains running throughout the oceans (). As we will see in section 4.7, movements along transform faults between two adjacent ridge segments are responsible for many earthquakes.įigure 4.5.1 Ocean floor topography. These transform faults make the mid-ocean ridge system look like a giant zipper on the seafloor (Figure 4.5.2). Although oceanic spreading ridges appear to be curved features on Earth’s surface, in fact the ridges are composed of a series of straight-line segments, offset at intervals by faults perpendicular to the ridge, called transform faults.

Running down the middle of the mid-ocean ridge is a rift valley 25-50 km wide and 1 km deep. Because the new crust formed at the plate boundary is warmer than the surrounding crust, it has a lower density so it sits higher on the mantle, creating the mountain chain. The mid-ocean ridge system is a giant undersea mountain range, and is the largest geological feature on Earth at 65,000 km long and about 1000 km wide, it covers 23% of Earth’s surface (Figure 4.5.1). Most divergent boundaries are located along mid-ocean oceanic ridges (although some are on land). Modified from 'Physical Geology' by Steven Earle*ĭivergent boundaries are spreading boundaries, where new oceanic crust is created to fill in the space as the plates move apart.