GEL 001: Convergent & Transform Tectonics

oceanic vs. continental crust

• oceanic: composed of silicate rock dominated by elements Si and O, along with smaller amounts of iron (Fe) and magnesium (Mg)

  • created by solidification of molten rock along mid-ocean ridges and is then transported laterally by seafloor spreading

  • rock composing the crust is full of iron and magnesium that makes the rock dense, relative to continental rock

  • rock has an average density of 2.9 g/cc

• continental: formed by a variety of different processes that tend to incorporate lighter elements into the rock such as potassium, sodium, and aluminum

  • rock has an average density of 2.7 g/cc

convergent plate boundary

plate boundary where two plates converge to produce linear mountain belts

• lithosphere is commonly destroyed along convergent margins by burial back into the mantle

• three types of convergent plate boundaries: oceanic/continental convergence, oceanic/oceanic convergence, and continent/continent convergence

compressional stress

primary force at convergent boundaries

• squeezing and deformation caused by the squeezing motion of the two plates toward each other

deformation & uplift

• deformation: bending and breaking of rock, commonly associated with tectonic compression

• uplift: when rocks create mountain ridges

continental-oceanic convergence

along convergent boundaries where a plate composed of oceanic lithosphere dives beneath a continental part of a plate, lithospheric material is returned to the mantle

subduction / subduction zone

• subduction: the process where oceanic lithosphere descends down into the mantle

• subduction zone: where water-saturated sediment of the seafloor is dragged down; contributes to the creation of magma

creation of magma

as the subducting plate reaches depths of about 100-150 km where the temperatures and pressures are just right, the water is released into the overlying rock of the asthenosphere & causes it to melt - creating magma

continental volcanic arc

when the volcanoes align roughly parallel to the convergent margin, forming a linear mountain chain called a continental volcanic arc

Cascadia subduction zone

the north-south linear trend of the Cascade Range is related to subduction along the Pacific Northwest coast where small oceanic plates (Juan de Fuca and Gorda oceanic plates) subduct beneath the less dense North American continental plate margin

• produces a linear chain of active volcanoes like Lassen Peak, Mt. Shasta, Mt. Hood, Mt. St. Helens, and Mt. Rainier

• hazards along subduction zone include large-magnitude earthquakes, active volcanism, and the potential for tsunami

• rate of subduction along zone is faster than rate of seafloor spreading along the Juan de Fuca and Gorda mid-ocean ridges—thus the ridges slowly migrate toward the subduction zone, eventually to be consumed

inclined zone of seismicity

earthquakes ranging in depth from near-surface to ~670 km are common along the dense subducting plate as it grinds downward against the over-riding plate

Ring of Fire

the alignment of earthquake epicenters and volcanoes circumscribing the Pacific Ocean; entirely related to subduction zones

oceanic-oceanic convergence

two plates composed of oceanic lithosphere may converge with the older ‘colder’ (and thus denser) plate subducting beneath the younger, ‘warmer’ less dense plate

volcanic island arc

rising magma supplies the raw material for the creation of linear chains of volcanic islands called volcanic island arcs

continental collision zones

a third type of convergent margin where continental plates converge with other continental plates (e.g., Himalayas/Tibetan Plateau, European Alps)

modern examples of convergent plate boundaries in the world

The Himalayas (India-Asia)—example of continent-continent collision zone

• ancient: Appalachians represent old cont-cont collision

transform plate boundaries

type of plate boundary where two plates slide horizontally past each other along major fault surfaces cutting across continents

continental transform faults

• lithosphere is neither created nor destroyed along these plate boundaries

• connect other types of plate boundaries at their ends, ‘transforming’ the divergent or convergent motion along the length of the fault

• primary force at transform boundaries is shear - blocks of rock on either side of the fault move in opposite directions, sliding laterally past one another

shear stress

when blocks of rock on either side of the fault move in opposite directions, sliding laterally past one another

fault

a planar fracture along which movement has occurred, offsetting massive blocks of rock and surface features on opposite sides of the fault

oceanic transform faults & fracture zones

• mid-ocean ridges are segmented by fracture zones extending perpendicular to the ridge axis

• transform faults: actively slipping part of a fracture zone between two ridge segments and are capable of generating earthquakes

• most oceanic transform faults connect two segments of a mid-ocean ridge

• fracture zones and associated transform faults form at the same time as ridges and accomodate differential spreading rates between mid-ocean ridge segments

• think of fracture zones and transform faults as the way the surface of a sphere would tear apart as seafloor spreading occurs at different rates along mid-ocean ridges