Medium Spreading-Rate Mid-Ocean Ridges
Mid-Ocean Ridges with medium spreading rates are transitional in character between slow- and fast-spreading ridges. The rift valley of a mid-ocean ridge with a medium spreading (5-10 cm/yr) is only about 50-200 m deep. The rift is only 5 km across. Cross-section of East Pacific Rise at 21 degrees N from Macdonald (1982). Vertical exaggeration = 4x.
The Juan de Fuca Ridge, only about 200 miles (300 km) west of the state of Washington, is perhaps the most studied ridge with a medium spreading rate. The Juan de Fuca Ridge creates part of the Pacific plate and all of the Juan de Fuca plate. Map of the northeast Pacific from Johnston and Embley (1990). Note the presence of linear seamount chains associated with hot spots.
One of the most interesting features of the Juan de Fuca Ridge is the Axial volcano. This active volcano is located at the intersection of the Cobb-Eikelberg hotspot and the Juan de Fuca Ridge. The hot spot has produced the Cobb-Eikelberg chain, a broad band of seamounts which extend over 900 miles (1,500 km) to the northwest. Image courtesy of U.S. Dept. of Commerce / NOAA / OAR / ERL / PMEL / VENTS Program.
Axial seamount is a broad volcano with a summit caldera and two rift zones. The large size of the volcano (roughly 12 by 18 miles; 20 x 30 km) is related to the input from the hotspot. The presence of the caldera suggests eruption from a shallow magma chamber and subsequent collapse. The summit of the volcano is less than a mile (1.4 km) below sea level. Recent lava flows and active hydrothermal systems suggest the volcano is active. This cross-section of the volcano shows pillow lavas (black ovals) which are older than 50,000 years. Sheet flows that range in age from 9,000-5,000 years drape the flanks of the volcano. Sheet flows less than 5,000 years old fill the floor of the caldera. From Zonenshain and others (1989).