Midoceanic Ridges

Oceanic ridges are some of the longest and steepest features in the world. They form an almost continuous mountain chain that is about 75,000 kilometers (45,000 miles) long and 3 kilometers (nearly 2 miles) high. The ridges form along deep crustal faults that separate the ocean floors into approximately two equal halves. The outlines of the midoceanic ridges generally parallel those of the continental coastlines on either side of the ridge (Figure 1).

Figure 1

A Midoceanic Ridge and Parallel Coastlines

A tensional graben (rift valley) runs down nearly all the midoceanic ridge crests. Shallow earthquakes and abnormally high heat flows are associated with ridges. New oceanic crust is produced in the divergent plate‐boundary environment of the ridge. Mostly basaltic, these rocks are pushed away to the sides as more magma erupts and cools at the crest (Figure 2).

Figure 2

Crust Formation at a Midoceanic Ridge

The midoceanic ridges are cut by approximately perpendicular fracture zones that offset segments of the ridge (Figure 3).

Figure 3

Midoceanic Ridge Fracture Zones

A transform fault is that portion of a fracture between two offset portions of the ridge (Figure 4). Transform faults also host shallow‐focus earthquakes. The faults are equilibrium features that result from a curved midoceanic rift that is “trying” to diverge evenly on both sides. At transform faults, the segments of crust are moving in opposite directions.

Figure 4

Midoceanic Ridge Transform Faults

Seamounts are conical volcanic peaks on the ocean floor. Usually basaltic in composition, they are at least 1,000 meters high and typically form near midoceanic ridges. They can sometimes rise above sea level to form islands. Guyots are submerged, flat‐topped seamounts that were once at sea level and eroded by continual wave action. Chains of seamounts and guymots are called aseismic ridges because they are not associated with earthquakes.