Ocean Ridges and Oceanic Lithosphere

Oceanic ridges indicate the loci of divergent plate margins where new oceanic lithosphere is created, comprising crust plus the uppermost mantle.
Considered as the longest, continuous mountain belt on Earth, the oceanic ridges show significant variability along their lengths.
Current Statistics:
- Total ridge length: ~60,000 km
- Topographic footprint: 1000–4000 km wide
- Crest elevation: ~2–3 km above neighboring ocean basins.

Global variegation in oceanic ridge full-spreading rates can be categorized as follows:
- Slow: 2.0 – 5.5 cm/yr
- Intermediate: 5.5 – 10.0 cm/yr
- Fast: > 10 cm/yr
- Ultraslow: newly defined as < 2.0 cm/yr (discussion deferred).

Deep median rift valleys (graben) are normally bounded by back-tilted, inward-facing normal faults, with fault scarps presenting many hundreds of meters of relief.
An axial topographic high (~1–5 km wide) appears due to multiple small shield volcanoes aligned parallel to the rift valley.

Mid-Atlantic Ridge:
- Rough topography with a median rift valley.
East Pacific Rise:
- Smoother slopes, indicating differences in volcanic activity across regions.

The sea floor's depth increases with distance from the central spreading ridge, showcasing a systematic relationship across the Pacific, Indian, and Atlantic Oceans irrespective of spreading rates.

Newly formed oceanic lithosphere expands and exhibits thermal buoyancy, giving rise to uplift at the central ridge. This lithosphere cools as it ages, resulting in:
- Increased plate density.
- Increased lithospheric thickness, where the lithosphere's base lies at the 1300°C isotherm.

Thickness increment from a few kilometers at the ridge crest to:
- 30 km at 5 million years (Ma)
- 100 km at 50 Ma.
Thermo-Mechanical Models:
- Half-Space (HS) Cooling Model: relates age (t; Myr) to depth (d) with the formula:
  d = 2500 + 350 imes t^{1/2} (applicable for oceanic lithosphere younger than 70 Ma).
Two-Layer Model (PSM):
- Defines a rigid upper layer and a viscous lower layer that convects and supports heat transfer to the upper layer, applicable for lithosphere older than 70 Ma.
- Coincides with the formula:
  d = 6400 + 3200 e^{-t/62.8} .

Stein and Stein (1992) proposed using a large dataset of depth and heat flow measurements:
- Statistically fitted two separate depth-age equations (GDH1) with:
- d = 2600 + 365 t^{1/2} for t < 20 Myr .
These equations follow a general format similar to HS and PSM but are considered more rigorously fitted.

Thermal contraction generates cracking and permeability:
- As oceanic crust moves away from the ridge crest, it subsides and builds up impermeable sediments.
- Pores and cracks become filled with minerals deposited by circulating seawater.
Sealing Age: appears to be ~60 Myr.