Chapter 20 - Earth Through Geologic Time Flashcards
Pangaea
Wegener’s name for most recent (Permian) super-continent; northern half called Laurasia and southern half called Gondwanaland; existence of continental ice sheet covering much of Gondwanaland supports theory
Apparent polar wandering
Paleomagnetic studies can determine pole locations, which appeared to have changed over time and varied on different continents
Seafloor spreading
Harry Hess (1962) postulated seafloor moves away from mid-ocean ridges, where magma rising from earth’s interior forms new oceanic crust
Purpose of Magnetic records and plate velocities
Can only determine relative velocities
Variations in plate velocities
Oceanic are fast, continental are slow; rotate about spreading axis; point farther from spreading pole moves faster
Terranes
Small fragments of continental crust that drifted as a single unit
Mid-ocean ridge basalt (MORB)
Has distinctive chemistry; mineral assemblage dominated by serpentine, a distinctive green fibrous mineral
Ophiolites
Serpentine-dominated fragments of oceanic crust on continents.
Composition: A thin layer of sediment overlies basalt that was extruded on the seafloor, and then intruded by a thick pile of gabbro sills. Both the basalt and gabbro sills are intruded by gabbro dikes. Made of MORB (MidOcean Ridge Basalt)
Melange
Chaotic mixture of broken, jumbled & thrust-faulted rock; formed by deformation of sediment accumulated in trench along convergent margins
Back-arc basins
Form when subducting plate sinks faster than overriding plate moves forward; crust thins and basin opens behind magmatic arc
Cratons
Core of very ancient rock; has attained tectonic and isostatic stability; rocks may be deformed, but deformation invariably ancient
Orogens
Elongate regions of crust that have been intensely folded & faulted during continental collisions; all were once mountains, but only youngest are mountains today
Continental shields
Assemblage of cratons and ancient orogens that has reached isostatic equilibrium
Stable platform
Portion of continental shield covered by thin layer of little-deformed sediments
Types of Continental Margins
1) Passive continental margins
2) Continental convergent margins
3) Continental collision margins
4) Transform fault margins
5) Accreted terrane margins
Passive continental margins
Initial uplift as magma heats & expands crust; rift valley forms; rift widens & seawater enters; narrow, shallow sea forms; sea widens & deepens into large ocean
Plate triple junction
Meeting point formed by three spreading edges; three-armed rifts with one failed arm common; some large rivers flow down failed rifts associated w/Atlantic Ocean
Continental convergent margins
Oceanic lithosphere subducted beneath continental; consist of continental volcanic arc, melange and regionally metamorphosed belt adjacent and parallel to melange; where volcanoes are eroded, magma chambers are exposed as granitic batholiths
Continental collision margins
Collision forms fold-and-thrust mountains developed from thick marine sediments accumulated along passive continental margin that became continental convergent margin; always includes metamorphism and igneous activity; mountains lie in continental interiors
Transform fault margins
Continental margin & transform fault plate boundary coincide
Accreted terrane margins
Former convergent/transform fault margin to which rafted-in, exotic crustal fragments (terranes) have been added (accreted); most complex margin; all terranes are fault-bounded & differ markedly from adjacent terranes; often referred to as suspect terranes
