Geology Lecture Notes: Convergent and Transform Plate Boundaries
Introduction to Convergent and Transform Plate Boundaries
Convergent and transform boundaries represent specific types of lithospheric plate interactions defined by their relative motions and resulting geological features.
Convergent boundaries are particularly notable for geological hazards, including:
Dangerous tsunamis.
Very large-magnitude earthquakes.
Volcanism, including some of the most violent volcanoes on the planet.
Transform plate boundaries are characterized by lateral movement, with a prominent local example being the San Andreas Fault system in the Bay Area.
Characteristics of Convergent Plate Boundaries
There are three primary types of convergent plate boundaries, classified by the types of lithosphere involved: ocean-continent, ocean-ocean, and continent-continent.
Subduction Zones: Subduction is a process where one lithospheric plate sinks beneath another into the asthenosphere. Both ocean-continent and ocean-ocean boundaries involve active subduction.
Non-Subduction (Collision) Zones: Continent-continent convergence involves the collision of two continental masses. While subduction of an intervening ocean basin precedes this, active subduction halts once the continents meet because continental lithosphere is too buoyant to subduct.
Ocean-Continent Convergence
Mechanics of Subduction: Subduction occurs due to density differences between the meeting plates.
Continental Lithosphere: Composed of felsic crust which is the least dense type of material.
Oceanic Lithosphere: Composed of mafic crust which is significantly more dense.
Consequently, the more dense oceanic lithosphere subducts beneath the more buoyant continental lithosphere.
The Trench: A deep depression called a trench forms exactly at the point where the two plates meet.
Although a collision might intuitively suggest the formation of mountains only, the subducting oceanic plate physically drags the edge of the continental plate downward, creating this deep feature.
Depth: Trenches can be up to deep. For context, Mount Everest () could fit entirely within such a trench with room to spare.
Comparison: While mid-ocean ridges rise to above the seafloor, trenches represent equivalent significant negative relief.
Global Inference: Mapping trenches across the world allows geologists to infer the locations of active subduction zones.
Volcanic Arc Development: These boundaries produce a "Continental Volcanic Arc."
Process: As the oceanic plate subducts, it carries water (trapped in cracks and sediment) down into the mantle.
Flux Melting: Under extreme pressure, this water is released into the asthenosphere. The asthenosphere is already near its melting point; the addition of water lowers the melting point of the rock, causing it to melt.
Rise of Magma: The resulting melt is less dense than the surrounding rock and rises to the surface to form volcanoes.
Spatial Distribution: Volcanoes always form on the continent, never on the plate boundary itself. They are located inland because the subducting plate must reach a specific depth in the asthenosphere to initiate melting.
Example: The Cascade Subduction Zone (Northern California, Oregon, and Washington). Mount Lassen and Mount Shasta are part of this arc and are located well inland, not on the coastline.
Earthquake Patterns:
Earthquakes generally occur in the lithosphere at depths between and .
Subduction zones are unique because they are the only locations where the lithosphere is shoved deeper than .
This allows for the occurrence of intermediate and deep-focus earthquakes along the subducting slab.
Ocean-Ocean Convergence
Mechanics of Subduction: Because both plates are composed of mafic/ultramafic oceanic crust, composition is not the primary driver of density differences. Instead, the driver is temperature.
Newer Lithosphere: Warmer and less dense because it was recently created at high heat-flow centers.
Older Lithosphere: Colder, thicker, and more dense.
Result: The older, colder, more dense oceanic plate will always subduct beneath the newer, warmer, less dense overriding plate.
Features and Differences:
Trench: Similar to ocean-continent boundaries, but often deeper. This is because they lack proximity to continents that shed sediment into the ocean.
Sediment Infilling: The trench off the coast of Northern California and Oregon is currently filled with sediment from the Columbia River, making it less visible on seafloor maps. Conversely, the Mariana Trench in the Western Pacific is an ocean-ocean boundary and is the deepest in the world at over .
Volcanic Island Arc: Because the overriding plate is oceanic, the rising magma forms a chain of volcanic islands rather than mountains on a continent.
Earthquake Patterns: These follow the same depth patterns as ocean-continent boundaries, with shallow earthquakes near the trench and deeper earthquakes occurring further in the direction of subduction.
Seismic Mapping and the Wadati-Benioff Zone
Epicenters vs. Focal Depths: Earthquake epicenters are map projections of earthquakes that happen at depth.
Mapping Subduction: By plotting the depth of earthquake epicenters, geologists can determine the angle and direction of subduction.
Shallow earthquakes (blue on map) align with the trench.
Intermediate and deep earthquakes (green and red) plot inland in the direction the plate is sinking.
Example: In Japan, the transition from shallow to deep earthquakes indicates subduction occurring in a south-westerly direction.
Global Distribution: Most subduction zones and trenches are located around the "Pacific Rim."
Continent-Continent Convergence
Mechanics: This occurs when the subduction of an intervening ocean basin brings two continental masses into contact.
Buoyancy and Collision: Because continental material is highly felsic and buoyant, it resists subduction. Instead of one plate sinking, the plates collide and compress, forcing the material upward to create extreme topography.
Example: The Himalayas:
Result of the collision between the Indian and Asian plates.
Mount Everest is the highest point at .
The mountains are currently growing at a rate of several centimeters per year.
Geological Evidence: Early explorers found seashells in the high Himalayas. This is explained by plate tectonics: the intervening ocean basin and its coastal sediments were scraped up and uplifted during the collision.
Features:
Topography: Very tall mountain ranges.
Earthquakes: Predominantly shallow, though some deep earthquakes can occur in stagnant subducted slabs left over from the pre-collision subduction.
Volcanism: Generally absent. Although some melting occurs deep at the "suture zone," the continental crust is too thick and strong for magma to easily reach the surface.
Transform Plate Boundaries
Mechanics: Plates move parallel to one another in a lateral motion. There is no subduction or creation of new crust.
Features:
Earthquakes: Shallow earthquakes only ( to ) because there is no subducting lithosphere present at depth.
Topography: Often subtle. Features can be "healed" or covered by precipitation, erosion, and vegetation over time.
The San Andreas Fault:
A major transform boundary extending through much of California.
Discovery: Its existence was not fully understood until the $1906$ earthquake, and it wasn't designated as a plate boundary until the development of plate tectonic theory in the $1960$s.
Geography: It ends near Cape Mendocino in the north, where the Cascade subduction zone begins. The fault is slowly growing in length over millions of years.
Historical Ruptures:
$1906$ San Francisco Earthquake: Epicenter off the coast of Daly City; ruptured the fault from Central California to its northern terminus.
$1857$ Earthquake: A major rupture of the Southern California segment.
Right-Lateral Motion: The San Andreas is a right-lateral transform fault. If an observer stands on one side of the fault and looks across at the other side, the opposing block moves to the right.
Regional Risks: Other related faults, such as the Hayward Fault in the East Bay (which runs through the city of Hayward), are considered "sinking dragons." The Hayward Fault is currently overdue for a significant earthquake of approximately magnitude .