10 Questions
What is the primary reason for the difference in height of adjacent portions of oceanic lithosphere?
The difference in the ages of the oceanic lithosphere
Which type of plate boundary is exemplified by the San Andreas Fault?
Transform boundary
What is the result of the friction between the two plates at a transform boundary?
The buildup of huge stresses over time
What is the characteristic of earthquakes at a transform boundary?
They are shallow and devastating
What is the purpose of normal and/or inverse faulting at slight bends in a transform fault?
To take up the relative motions
What is the difference between a transform boundary in the ocean and on a continent?
The shape of the boundary
What is the relationship between the age of the oceanic lithosphere and its buoyancy?
Younger lithosphere is more buoyant
What is the consequence of the release of built-up stresses at a transform boundary?
Devastating seismic events
What is the characteristic pattern of deformation at the San Andreas Fault?
Complex and varied
What is the name of the famous earthquake that is an example of the consequences of built-up stresses at a transform boundary?
The 1906 San Francisco earthquake
Study Notes
Volcanism at Divergent Boundaries
- 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.
Characterization of Ridges
- 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
- 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.
Slow and Fast Divergent Boundaries
- 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.
Continental Transform Boundaries
- 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.
Learn about the volcanic activity that occurs at divergent boundaries, where magma erupts from the Earth's surface. Understand the process of isostatic equilibrium and the role of the asthenosphere.
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