3.6 Plate Tectonics and Volcanism

GEOL1340

Decompression Melting

Occurs when pressure decreases as hot mantle rock ascends toward Earth's surface without losing heat; primary mechanism for magma generation where overlying rock thins or fractures.

Flux Melting

Occurs when water and other volatile compounds escape from a subducting plate into the overlying hot mantle wedge, lowering the melting temperature (solidus) of mantle rocks.

Temperature Increase / Crustal Heating

Mechanism where rising magma transfers heat directly to surrounding crustal rocks, melting them to alter or diversify magma composition.

Divergent Plate Boundaries

Tectonic setting accounting for ~90% of global volcanism, driven by decompression melting of ascending mantle rock as plates pull apart.

Mid-Ocean Ridges (MORs)

Underwater divergent boundaries (e.g., Mid-Atlantic Ridge) characterized by linear rift volcanism producing mafic/basaltic oceanic crust and basaltic pillows.

Continental Rift Zones

Divergent settings within continental crust (e.g., East African Rift, Northern Cordillera B.C.) that feature continental stretching and volcanic activity.

Convergent Plate Boundaries

Tectonic subduction settings (e.g., Pacific Ring of Fire) where flux melting produces intermediate to felsic magmas and explosive volcanic arcs.

Oceanic-Oceanic Subduction

Subduction of one oceanic plate beneath another, forming arcuate volcanic island arcs bounded by deep oceanic trenches.

Oceanic-Continental Subduction

Subduction of oceanic crust beneath continental crust, forming continental volcanic arcs (e.g., the Andes, Mount Garibaldi).

Intraplate Volcanism (Hotspots)

Volcanism (~5% of active volcanoes) caused by deep mantle plumes rising independent of plate boundaries.

Oceanic Hotspots

Hotspots under oceanic crust producing low-viscosity basaltic lavas that construct broad shield volcanoes (e.g., Hawaiian Island chain).

Continental Hotspots

Hotspots under thick continental crust that melt surrounding rock to generate highly viscous, explosive rhyolitic magmas (e.g., Yellowstone) or flood basalts.

Mantle Plume

An ascending column of hot solid rock (not magma) originating deep in the mantle that spreads out into a mushroom-shaped head near the lithosphere.

Magma Buoyancy

The driving force of magma ascent, occurring because liquid magma is less dense than the surrounding solid wall rock.

Magma Viscosity Controls

Governed primarily by temperature and silica ($SiO_2$) content; high silica/cooler magmas are highly viscous, while low silica/hotter magmas are fluid.

Mafic Magma Behavior

Low-silica, low-viscosity magma that allows exsolved gases to escape easily, creating gentle/effusive eruptions and broad shield volcanoes.

Felsic / Intermediate Magma Behavior

High-silica, high-viscosity magma that traps expanding gases under extreme pressure, triggering explosive eruptions and steep composite volcanoes.

Shield Volcanoes

Broad, low-profile volcanoes formed by fluid, low-viscosity basaltic lava flows at hotspots, rifts, and divergent settings.

Composite Volcanoes (Stratovolcanoes)

Steep-sided, explosive volcanoes formed at convergent subduction zones from intermediate to felsic magmas.

Garibaldi Volcanic Belt

An active/young dormant volcanic arc in British Columbia and Yukon resulting from subduction along the western Canadian margin.

Precambrian Shield Volcanism

Ancient, non-active volcanic remnants (such as metamorphosed basalts) found in central Canada (e.g., Manitoba) that are billions of years old.

Northern Cordillera Volcanic Province

A volcanic region in northwestern B.C. (e.g., Tseax River) driven by crustal fracturing and stress from the Pacific and North American plate boundary.

Oceanic Crust Formation Thickness

Calculated from partial melting of the ~60 km thick triangular mantle zone; a ~10% partial melt yields approximately 6 km of oceanic crust.