CSC 599: Computational Scientific Discovery

Lecture 5b: The Plate Tectonics Paradigm

Outline

ImportancePre-Plate TectonicsBig PictureRamificationsComplicationsEarthquake PatternsEarthquake Database

Importance

Plate tectonics gives geologists there first wholestic view of the Earth over time and space . . . Most earthquakes Volcanos Mountains Continents

but it does not explain everything:1. Earthquakes in middle of continent?2. Early, early Earth?3. Maybe ancient Mars, but today's Venus?

Pre-Plate Tectonics

Theory 1: Earth made by GodEvidence: BibleExplains: Mountains formed during flood

Theory 2: Earth was hot, cooled off, shrunkEvidence: Further go down in mine hotter it getsExplains: Mountains are shrink ripples

Theory 3: Geomorphic CyclesEvidence:

Appalachian mountains built out of mountainsThose mountains were built out of earlier mountains

Geomorphic Cycles

1. Mountain is lifted No idea what lifts rock

2. Erodes into sea Sand, mud forms plain

at sea level3. Eroded rock solidifies

underground Sand to sandstone Mud to shale

4. Solidified rock lifted Cycle starts again What lifts rock?

Rebound?

Geomorphic Cycles (2)

Explains Appalachian Mountians Taconic Orogeny (550 – 440 MYA)

Explains carbonate (ie. Marine shell) rock in NY, New England

Acadian Orogeny (360 – 320 MYA) Explains:

Angular unconformities (nonparallel strata) Igneous intrusions, Regional metamorphism Deformation of older rock

Alleghenian Orogeny (299 - 251 MYA) Also called “Appalachian” Created “piedmont” from NJ to Alabama

Continental Drift (1)

“Hey! Africa matches South American” Abraham Ortelius (1596)

Dutch/Belgian Cartographer & geographer Francis Bacon (1620)

British Philosopher, Statesman Benjamin Franklin (1700s)

British/American Publisher, Scientist and Statesman Antonio Snider-Pellegrini (1858)

French geographer

Continental Drift (2)

Franklin Coxworthy Roberto Mantovani (1889-1909)

Italian GeologistWilliam Henry Pickering (1907)

US astronomerFrank Bursley Taylor (1908)

Amateur US geologistAlfred Wegener (1912)

German meteorologist Compared

Coastlines Rock types Fossils

Continental Drift (3)

Fossil Evidence for joined continents Laurasia (N. supercontinent) broke up before

Gondwanaland (S. continent) Dinosaurs more diverse in N. hemisphere than S. Coal fields in Europe and N. America line up

Evidence stronger in S. hemisphere than N.

Continental Drift (4)

Continental Drift's Downfall Anglo-American geologists dismissed Wegener

Fuddy-duddies: “How can rocks move?”Wegener: “Continents move by centrifugal force to

equator”Fuddy-duddies: “It takes way more force to plow thru

ocean rock”

Wegener died in Greenland in Greenland, 1930s Doing experiments with balloons & jet stream

Plate Tectonics

Explains most

Earthquakes Volcanoes Magnetism Fossils among continents Rock ages among continents Over WHOLE planet for MUCH of its life!

Plate ideas of geology and plate tectonics1. Geological time2. Rocks flow!3. Continents come and go

Geological Time

Earth way oldMYA =Millions of

Years Ago

Don't know most history No fossils

before Cambrian Explosion

Old rocks cycled

Big Picture (2)

2. Rocks flow! Continents move at

a couple of cm/year Continents float on

top of and moved by mantle

Mantle gently churning

Liquid outer core churning

Causes magnetic field

Big Picture (3)

3. Life of Plates Spreading ridges

New (ocean) plate made Subduction zone

Ocean recycled in mantle Transform fault

Plates slide past each other Plates come and go

Continental plates light, old and float (N. America)

Ocean plates heavy, new and sink (Juan de Fuca)

Evidence (beyond Wegener's)

Magnetism and spreading ocean ridgesClosest to ridge = youngestFarthest from ridge = oldestSymmetric magnetism both sides

Ramifications

Subduction zones in more detail:

Ocean under continent

Ocean under ocean

Continent under continent

Complications

1. Hot spots Upwelling of mantle rock punch thru continent

Get volcanic mountain As continent moves above hot spot volcano

moves on continent Get volcanic rock mtn chain eroding away from active

volcano Hot spots may be necessary to keep plates

buoyant and moving

Complications (2)

2. Plate boundaries not always clear India and Australia?

3. Plates deform internallyChina = Australia smashing into Eurasia

Chinese rocks are a mess, very few level strata

Basin and Range in N. America

Problems

1. Earthquakes in middle of continents?New Madrid faulting

system Two estimated mag

8 quakes, 1811 & 1812

Problems (2)

2. Early-early Earth? Pangaea only 500 MY Old

Earth 4500 MY Old Much more hotter way back when

More initial heat Several times more radioactive decay When Iron melted it fell to core

Released 1022 megatons of gravitational energy What was “crust” like then? Isolated island arcs?

Problem (3)

3. If plate tectonics such a good theory why don't we see it on other Earth-like planets? Mars

Maybe at one time Some evidence of ancient ridge-like bands Mars is smaller --> cooled faster --> no tectonics now

Venus Magellan spacecraft radar-imaged whole planet “Know more about surface of Venus than of Earth” Expected “ocean crust” (spreading ridge, subduct zone) See “continental crust” (mountains, strike-slip faults) Explanation: “Venus' surface so hot that is naturally

buoyant”

Earthquakes

Earthquake patterns (1)

In space:Along faults

Transform faults

Subduction zonesWadati-Benioff zone = seismically active portion of fault

Earthquake patterns (2)

In Time:Measured in months:

Fore shocksMain shocksAfter shocks

Measured in centuries (?)“Elastic rebound theory”

BIG QUAKE (releases most energy)(many years of many small quakes)BIG QUAKE (releases most energy)(many years of many small quakes)BIG QUAKE (releases most energy)

Is it true?Not enough data to sayJapanese have longest record of (large) quakes:

several centuries

Earthquake patterns (3)

In magnitudeBy definition “main shock” bigger than:

fore shocks after shocks

Gutenberg-Richter Relationship

Lower magnitude -> more earthquakes

Log(N(M)) = a-bMM = magnitudeN(M) = # quakes of mag M & greatera,b = constants

Modeling earthquakes

Spring-slider block model 2 moving plates matrix of blocks between them w/friction blocks connected by springs when one block moves, can trigger others

Reproduces Gutenberg/Richter stats!

Earthquake Databases

USGS records (http://neic.usgs.gov/neis/epic/)Where:

longitude: -180 to 180 degreeslatitude: -90 to 90 degreesdepth: 0 to 700 (?) km

When: timesince 1973

How big: MagnitudeM

L: “Richter Scale”Just a measure of earth-shaking in California

MS: Travel along surface

Mb: Travel deep

MW: Seismic moment = (rock rigidity)(fault area)(slip dist)Has more “rational” relationship w/the energy

Problems with db

Location Missing if too small/under sea Inaccurately placed if too small/under sea

Time Missing if too small/under sea

Magnitude Inconsistent scale Missing if too small/under sea