Geology: Mid-Oceanic Ridges

Introduction to Mid Oceanic Ridges

• The lithosphere, which includes the Earth’s crust and upper mantle, is approximately 70-100 km thick and divided into tectonic plates.
• There are around eight primary tectonic plates, along with seven secondary plates and several tertiary plates.
• Tectonic plate boundaries include mid oceanic ridges, subduction zones, and transform faults, all associated with seismic activity.
• Plates move at speeds of a few centimeters per year, creating new crust at mid oceanic ridges and recycling old lithosphere at subduction zones.

Morphology of Mid Oceanic Ridges

• Mid oceanic ridges are underwater linear features that demarcate divergent tectonic boundaries between lithospheric plates.
• These ridges extend approximately 65,000 km globally, creating significant seafloor spreading through volcanic activity.
• Seafloor spreading rates range from 6 to 200 mm/year, categorizing ridges into slow, intermediate, and fast spreading types.
• The Gakkel Ridge (Arctic) is the slowest, spreading at about 6 mm/year, while the East Pacific Rise is the fastest, with rates of up to 200 mm/year.
• Fast spreading ridges exhibit smooth seafloor surfaces, while slow spreading ridges characterize rough topography and are marked by axial valleys or rifts.

Internal Structure of Mid Oceanic Ridges

• Beneath a mid oceanic ridge, mantle material partially melts due to decreased pressure, forming magma that pools below the seafloor before erupting.
• The average thickness of oceanic crust is around 7 km; however, only 1 to 3 km forms through eruptions.
• Eruptions occur when magma pressure forces it to the surface, creating "dikes"—magma-filled cracks facilitating the flow to the seafloor.
• Magma eruption rates can vary, with typical new lava coating the seafloor at rates of about 60 mm/year for "fast" and less than 60 mm/year for "slow" spreading.

Comparison of Ridge Types

• Fast-spreading ridges often have a narrow axial rise and smoother features dominated by volcanic activity.
• Slow-spreading ridges are characterized by broad axial valleys, rugged topography, and tectonic features evident in their formation.
• The Mid-Atlantic Ridge (2-5 cm/year) forms a noticeable rift valley, while the East Pacific Rise (6-16 cm/year) features a smooth summit without a prominent rift.

Ultraslow-Spreading Ridges

• Ultraslow-spreading ridges (<20 mm/year) exhibit unique characteristics, such as thinner seismic crust (~1-4 km) and amagmatic rifts exposing mantle material at the seafloor.
• These ridges differ from standard ridges by featuring mantle peridotite exposed due to minimal basaltic activity.

Key Locations

• Mid-Atlantic Ridge: Central in the Atlantic Ocean, forming a significant rift valley.
• East Pacific Rise: Fast spreading with a smooth summit and no prominent rift valley.
• Gakkel Ridge: The slowest spreading ridge, located between Greenland and Siberia.