Plates

Guest Lecture by Dr. Webster

Vocabulary of the Day

• Asthenosphere• Continental crust• Convergent margin• Divergent margin• Earthquake• Lithosphere

• Oceanic crust• Plate tectonics

Subduction• Transform margin• Volcano• Volcanoes

Goals

• Distinguish among types of plate tectonic settings produced by plates diverging, converging, and sliding past each other.

• Relate modern and ancient geologic features to each kind of plate tectonic setting.

• Relate certain geologic hazards to specific plate tectonic settings.

• Universal principles are discovered through observation and experimental verification.

Prior Knowlege

• Please get out a piece of paper and write your name on it.

• What do you already know? I’m going to show you a few geographic features, I would like you to tell me

• A) What is it (name if you know)?

• B) How it was formed?

Question 1

• A) What is it? (name if you know)

• B) How did it form?

Question 2

• A) What is it? (name if you know)

• B) How did it form?

Question 3

• A) What is it? (name if you know)

• B) How did it form?

Question 4

• A) What is it? (name if you know)

• B) How did it form?

Question 5

• A) What is it? (name if you know)

• B) How did it form?

Question 6

• What ONE idea or concept do all five of those geographic features have in common?

Plate Tectonics history

• Plate Tectonics is relatively modern model to explain many disparate observations - like the 5 slides from the beginning.

• Observations supporting a ‘drift’ theory were made as far back as 1600s

• Taylor and Wegener in early 1900s published more complete drift theory

Drift

• Observations supporting drift– Fossils– Shapes of continents

• BUT - in early 1900’s lacked any way to explain drift

• What could make rock cross rock?

Post WWII

• Paleomagnetism• Seafloor spreading• + Drift evidence• Observations all need to be explained

by a coherent model• Plate Tectonics!• This was a major shift in the scientific

view of how the world works

Model of Earth Today

• Draw

• Three things can happen at the boundaries

Today - convergent

• Two types of crust

• Drawing

• So - can have 3 combos of convergent

• Oceanic crust-oceanic crust

• Continental crust-continental crust

• Continental crust-oceanic crust

Two Types of Crust

• Oceanic– Thinner (7 km)– Different material

(basalt)– More dense

(3 g/cm^3)– Younger (200 million

years old)

• Continental– Thicker (10-70 km)– Volcanic rock with

sedimentary on top– Granitic materials– Less dense

(2.7 g/cm^3)– Older - billions of

years

Model of plate tectonics

• Must explain the appearance of landscape features - observations

Continental-Oceanic

• Predict - which will go under?

• What prior knowledge use to predict?

• Draw

• Subduction

Melting Temperatures

Material Melting point (degrees C)

Water 100

Quartz, K-feldspar, Na-plagioclase, micas.

600

Amphibole, Ca/Na- plagioclase

800

Olivine, pyroxene, Ca-rich plagioclase

1000

http://hyperphysics.phy-astr.gsu.edu/HBASE/geophys/meltrock.html

Subduction predictions

• Make a list

• Where do we expect to see continental -oceanic crust collisions

SA

• http://geology.com/plate-tectonics.shtml

You try

• Oceanic crust - oceanic crust

• Label features you expect to see.

Examples on Earth

• http://geology.com/plate-tectonics.shtml

• Top down drawing of area

• Can you identify any other places on Earth?

Aleutians

Continental-Continental

• Where on the map?

Himalayas

• Folded sedimentary rock – Sedimentary -

suggests ocean bed– Folding pattern

suggests bottom to top pressure

http://library.thinkquest.org/10131/geology_visual.html

Hazards associated with convergent margins

• Volcanoes

• Earthquakes

Summary so far

• The plate tectonic model describes how plates can collide.

• Colliding plates explain volcanoes, earthquakes, oceanic trenches, island arcs, mountains, and the relative positioning of these features.

• One model - describes many observables.

Plate Tectonics Model

• Hypotheses often cause scientists to develop new experiments that produce additional data.

• Think of an experiment that might support plate tectonics model

More support for plate tectonics

• And also directly measured motions with VLBI in 1980’s!

• And ground based positions with satellites in 1990’s

Assessment

• Write a paragraph describing how the Aleutian Islands and nearby geographical features formed. Sketch the relevant plate configuration with features.