4.1 Mid-Ocean Ridges

The global mid-ocean ridge system is the single largest geological feature on planet Earth, stretching approximately 65,000 km across every ocean basin. These submarine mountain chains mark divergent plate boundaries where new oceanic lithosphere is continuously created through the process of seafloor spreading. Ridges account for roughly 75% of Earth's volcanic output, yet most of this volcanism occurs unseen beneath kilometers of seawater.

Mid-ocean ridges rise 2–3 km above the surrounding abyssal plains, with their crests typically lying at depths of 2,000–3,000 m. The morphology, magmatic character, and tectonic structure of a ridge segment are fundamentally controlled by the spreading rate—the velocity at which plates diverge. This single parameter governs whether a ridge displays a broad axial high or a deep rift valley, continuous or intermittent volcanism, and robust or starved magma supply.

Spreading Rate Classification

Spreading rates are quoted as full rates (the total separation velocity between two plates) and classified into several regimes, each with distinctive morphological and magmatic signatures:

Ridge Segmentation

Mid-ocean ridges are not continuous features but are broken into discrete segments at multiple scales, reflecting variations in magma supply, mantle temperature, and tectonic architecture. The hierarchy of ridge discontinuities was formalized by Macdonald et al. (1988):

Along fast-spreading ridges like the EPR, segments between second-order discontinuities typically span 50–200 km. Mantle Bouguer gravity anomalies (MBA) reveal that melt supply is focused at segment centers, producing bull's-eye negative anomalies, while segment ends are magmatically starved and more tectonically extended.

Hydrothermal Circulation

Seawater penetrates through fractures in young oceanic crust, is heated by proximity to magma chambers, and returns to the ocean floor as buoyant hydrothermal fluid. This circulation is one of the most important heat-transfer mechanisms on Earth, responsible for extracting roughly 25–30% of global oceanic heat flux.

For a typical black smoker with ΔT ≈ 350 K and a flow velocity of ~1 m/s through a vent orifice of ~0.1 m diameter, the heat output from a single chimney can reach 10–60 MW—comparable to a small power plant.

Ridge Push Force

Ridge push is a body force arising from the elevated position and thermal buoyancy of young, hot lithosphere at the ridge axis relative to older, cooler, and denser lithosphere away from the ridge. It is one of the principal driving forces of plate tectonics, though typically smaller in magnitude than slab pull at subduction zones.

For 80 Myr old lithosphere, this yields F_rp ≈ 2–3 × 10&sup{12} N/m, a significant but secondary contribution to the plate driving force budget compared to slab pull (~3–5 × 10&sup{12} N/m).

Depth–Age Relationship

One of the most elegant predictions of plate tectonic theory is that the depth of the ocean floor increases systematically with the age of the underlying lithosphere. As newly formed crust moves away from the ridge axis, the lithosphere cools conductively, contracts thermally, and subsides. The half-space cooling model predicts:

This √t relationship holds well for lithosphere younger than ~80 Ma. Older lithosphere subsides more slowly than predicted by the half-space model, a “flattening” better explained by the plate model (Parsons & Sclater, 1977), which imposes a constant-temperature boundary at the base of a finite-thickness plate (~125 km).