Chapter 2: Plate Tectonics and Ocean Floor

Lecture 2+3

plate tectonics

plate tectonics

outermost portion of earth is composed of patchwork of thin, rigid plates which move horizontally like icebergs.

Alfred Wegener

german meteorologist proposed tectonics in 1912, called it continental drift

Francis Bacon

proposed in 1620 that continents looked like they fit together.

Pangaea

Wegener proposed pangaea which existed 200 mya and panthalassa (one large ocean)

continental drift evidence

fit of continental margins across oceans, matching rock sequences and mountain ranges on different continents, glacial deposits in equator areas. fossil distribution

Sir Edward Bullard

1960’s, he used computer models to fit continents 2000m below sea level which made them fit really well.

The origins of continents and oceans

Wegener published saying continents plow through ocean basins to their present day positions and that the leading edges of the continents deformed into mountain ridges because of drag imposed by ocean rocks. Not possible, disproved

sea floor studies from WWII

used sonar, enabled study of magnetic field. became important for navigators. opposite polarity causes magnetic objects to align parallel to the magnetic field produced by the bar magnet

igneous rocks

polarity can be caused by magnetite in basalt

paleomagnetism

study of earth’s ancient magnetic field, interprets where rocks first formed. magnetite Fe3O4. igneous rocks preserve direction of earth’s magnetic field when rocks cool through curie temps

curie temperature

580 C, most basalt crystallized at temps below 900 C.

magnetic dip

magnetite particles in sedimentary rocks or igneous rocks such as basalt align with earth’s magnetic field. Magnetic dip directly related to latitude

magnetic polarity reversals

184 major reversals in past 83 million years. occurs every 450,000 years, takes about 5000 years. recorded in rocks. strength of magnetic fields has also changed

polar wandering

location of N pole has changed over time. Continents are actually moving, not the magnetic field.

magnetic anomalies

regular pattern of N S magnetism stripes. symmetrical about long underwater mountain range

magnetometer

instrument towed behind ocean vessel. measures Earth’s magnetic field and how it was affected by sea floor rocks

Harry Hess

WWII submarine captain and geologist. depth recordings show sea floor features. published history of ocean basins. contained idea of sea floor spreading, and associated circular movement of rock material in mantle called convection cells

Mid ocean ridge

continuous underwater mountain range that winds through every ocean basin in world. new ocean floor forms at crest and splits in two and spreading occurs

subduction zone

sloping area from trench where old sea floor is brought into convection zones to melt

Frederick Vine and Drummond Matthews

analysis of igneous rock stripes around mid ocean ridge, stripes record earth’s magnetic polarity. alternates btwn. normal polarity and reverse polarity. new basalt added to ocean floor at mid ocean ridge.

age of ocean floor

late 1960’s deep sea drilling, radiometric dating of ocean rocks, symmetric pattern of age distribution about mid ocean ridges. youngest ocean floor along mid ocean ridge

atlantic ocean floor

simplest and most symmetric pattern of age distribution

pacific ocean floor

simplest and most symmetric pattern of age distribution.

how old is the ocean

4 billion years old, but the oldest ocean floor is only 180 million years old.

heat flow

heat from earth’s interior released to surface. high at mid ocean ridges, low at subduction zones

earthquakes

occur at subduction zones of ocean trenches. others at mid ocean ridges and major faults in sea floor and land. activity mirrors tectonic plate boundaries

lithosphere

rigid outer part of the earth, consists of crust and upper mantle.

asthenosphere

the upper layer of the earth's mantle, below the lithosphere, in which there is relatively low resistance to plastic flow and convection is thought to occur.

tectonic forces

slab pull, slab suction

types of plate boundaries

divergent, convergent, transform

divergent plate boundary

where plates are moving apart, such as the mid ocean ridge

convergent plate boundary

an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction

transform plate boundary

plates slide sideways past each other. lithosphere is neither created nor destroyed

7 major plates

pacific plate, N. American plate, Eurasian plate, african plate, antarctic plate, indo-australian plate

divergent boundary features

plates move apart, rift valley, central downdropped linear depression, new ocean floor created: rifting further splitting apart and area drops below sea level. rift valley floods with seawater

spreading rates

total widening rate of an ocean basin resulting from the motion of both plates away from spreading center. faster spreading rate, broader mountain range

types of spreading centers

oceanic rise, oceanic ridge, ultra slow

oceanic rise

fast spreading, gentle slopes, east pacific

oceanic ridge

slow spreading, steep slopes, mid atlantic

ultra slow

deep rift valley, widely scattered volcanoes, arctic and SW india

convergent boundary features

plates move towards eachother, oceanic crust destroyed, most important features: ocean trench, volcanic arc. deep focus earthquakes

oceanic continental convergence

where denser oceanic crust subducts and a continental arc is created, rising basalt magma mixes with granite of continental, produces lava in volcanic eruptions at surface that is intermediate.

oceanic oceanic convergence

where older denser sea floor subducts and oceanic arc is created. denser plate subducted, older oceanic plate denser bc has more time to cool and contract. produces deepest trenches, causes island arc

continental continental convergence

where continental crust is too low in density to subduct, instead a tall uplifted mountain range is created. no subduction, too low in density. tall mountains uplifted. himalayas from India asia collision.

transform boundary features

offsets oriented perpendicular to mid ocean ridge. called transform faults, give mid ocean ridge a zigzag appearance. Two types of transform faults: oceanic transform faults, continental transform faults

oceanic transform fault

ocean floor only zig zag

continental transform fault

cuts across continent zig zag

hotspots

areas of intense volcanic activity as well, not necessarily associated with plate boundaries

mantle plumes

vertical tube shaped areas of hot molten rock that arise deep within the mantle. origin and development of mantle plumes and hotspots according to mantle plume hypothesis.

nematath

chain of extinct volcanoes that is progressively older as one travels away from a hotspot. more than 100 volcanoes, stretch over 3000 miles. NW migration direction

seamounts

tall volcanic peaks that resemble some volcanoes on land. rounded tops, cone shaped

tablemounts or guyots

volcanoes with flattened tops. valcanoes along the crest of mid ocean ridge. some rise above sea level, become island, wave erosion flattens seamount. now a tablemount, continues to be carried away from its source.

paleogeography

study of ancient continents

continental accretion

continental material added to edges of continents through plate motion. 540 mya, many continents are unrecognizable. N America and Antarctica lies on equator 480 mya.

when did pangaea occur

240 mya

how long ago did pangaea separate

180 mya, N. and S. America rifted from Europe and Africa, forming Atlantic ocean

how long ago did S. America and Africa separate

120 mya

how long did india collide with Asia

45 mya, while Australia moves N from Antarctica

future geographical predictions

Atlantic will enlarge, pacific will shrink. New sea from E. Africa rift valleys. Further himalaya uplift. Separation of N. and S. America. Part of California in Alaska

terranes

a crust fragment formed on a tectonic plate and accreted or "sutured" to crust lying on another plate. The crustal block or fragment preserves its own distinctive geologic history, which is different from that of the surrounding areas—hence the term "exotic" terrane

Wilson Cycle

plate tectonics model shows life cycle of ocean basins. formation, growth, destruction. Stages: embryonic, juvenile, mature, declining, terminal, suturing