Volcanism at Divergent Boundaries

Study Notes

Magma erupts from divergent boundaries due to decompression melting of the asthenosphere.
As the lithospheric plate thins, the asthenosphere rises to re-establish isostatic equilibrium.
If the asthenosphere rises quickly, it doesn't have time to cool, and its volatile elements induce melting at high temperature and low pressure.
Decompression melting occurs when the asthenosphere crosses the solidus, a pressure-temperature line beyond which melting occurs.

Ridges consist of several eruption centers, each about 2-3 km long, separated by gaps of about 1 km.
Magma chambers link between and across the active segments, forming magma along the whole ridge.
Eruptions occur only at individual volcanic centers.

Convergent plate boundaries are regions where two lithospheric plates move towards each other.
The Afar region in Northern Ethiopia is a Y-shaped rift system, where the continental lithosphere stretches and splits.
The African Plate is stretching along the East African Rift Valley, forming two new plates: the Nubian and Somalian Plates.

Plates move away from divergent boundaries at different speeds, ranging from less than 10 mm/year to more than 100 mm/year.
Slow-spreading ridges, like the Mid-Atlantic Ridge, form valleys at their center and produce less magma.
Fast-spreading ridges, like the East Pacific Rise, form mountain chains at their center and produce more magma.

Conservative plate boundaries can occur in continental plates, such as the San Andreas Fault in California.
The San Andreas Fault separates the North American and Pacific Plates, producing shallow earthquakes and leaving a visible trace in the topography.
Huge stresses build up over time, releasing as devastating seismic events when they are released.

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