Chapter 20 - Earth Through Geologic Time

Question 1

Pangaea

Answer 1

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

Question 2

Apparent polar wandering

Answer 2

Paleomagnetic studies can determine pole locations, which appeared to have changed over time and varied on different continents

Question 3

Seafloor spreading

Answer 3

Harry Hess (1962) postulated seafloor moves away from mid-ocean ridges, where magma rising from earth’s interior forms new oceanic crust

Question 4

Purpose of Magnetic records and plate velocities

Answer 4

Can only determine relative velocities

Question 5

Variations in plate velocities

Answer 5

Oceanic are fast, continental are slow; rotate about spreading axis; point farther from spreading pole moves faster

Question 6

Terranes

Answer 6

Small fragments of continental crust that drifted as a single unit

Question 7

Mid-ocean ridge basalt (MORB)

Answer 7

Has distinctive chemistry; mineral assemblage dominated by serpentine, a distinctive green fibrous mineral

Question 8

Ophiolites

Answer 8

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)

Question 9

Melange

Answer 9

Chaotic mixture of broken, jumbled & thrust-faulted rock; formed by deformation of sediment accumulated in trench along convergent margins

Question 10

Back-arc basins

Answer 10

Form when subducting plate sinks faster than overriding plate moves forward; crust thins and basin opens behind magmatic arc

Question 11

Cratons

Answer 11

Core of very ancient rock; has attained tectonic and isostatic stability; rocks may be deformed, but deformation invariably ancient

Question 12

Orogens

Answer 12

Elongate regions of crust that have been intensely folded & faulted during continental collisions; all were once mountains, but only youngest are mountains today

Question 13

Continental shields

Answer 13

Assemblage of cratons and ancient orogens that has reached isostatic equilibrium

Question 14

Stable platform

Answer 14

Portion of continental shield covered by thin layer of little-deformed sediments

Question 15

Types of Continental Margins

Answer 15

1) Passive continental margins
2) Continental convergent margins
3) Continental collision margins
4) Transform fault margins
5) Accreted terrane margins

Question 16

Passive continental margins

Answer 16

Initial uplift as magma heats & expands crust; rift valley forms; rift widens & seawater enters; narrow, shallow sea forms; sea widens & deepens into large ocean

Question 17

Plate triple junction

Answer 17

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

Question 18

Continental convergent margins

Answer 18

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

Question 19

Continental collision margins

Answer 19

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

Question 20

Transform fault margins

Answer 20

Continental margin & transform fault plate boundary coincide

Question 21

Accreted terrane margins

Answer 21

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