Thursday, January 21st, 1999
Announcements: First writing assignment is due Tuesday, January 26^th; turn in two copies of writing assignment

Lecture notes:

Tectonic process = continental rifting

• Oceanic rifting
• Formation of ocean floor
• Lithosphere plates
• Continental vs. oceanic rocks
  Continental rocks have a lower density and are a white, light color like granite; oceanic rocks have a higher density and are a darker color
• Formation of earth (accretion, heat)
• Convective cooling – density driven, thermally controlled process of moving rock materials; it’s what makes lithosphere plates move (between 1 and 10 cm/yr.)
• Accretion of meteorites (coming together)
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1. Crater Lake
1. Wizard Island is in the middle of the lake; magma that is shaken up and oozes up to the surface to create an island in the center; develops a resurgent dome; island continues to get bigger and will eventually blow off
2. Water is extremely pure, completely sterile; it is pure because there are no streams leading into or out of the lake
3. When the volcano erupts, the water will evaporate; it will take about another 10 years before another lake forms after the volcano erupts

• Ryalytic rock – typical Cascade products; appears very crumbly
• Assalytic rock – darker; appears very jagged
• Cascade volcanoes have a nice, gradual slope; good soil quality; smooth shape
• Mt. Lassen looks like a pile of sand from a distance

1. Andes Mountains
1. Andesite is the dominant material in the Andes
2. Subduction volcanoes have a majority of andesite rock

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Subduction – process of going under; when it sinks you get major earthquakes and then volcanoes

Isostasy – equal weight; any slice of ocean crust weighs same as same-width section of continental crust

crust – mantle = okay
lithosphere – asthenosphere = better terms

• Refers to flow vs. fracture properties of rocks
• Asthenosphere – like tar, oozy and sticky
• Lithosphere – hard outer shell of the earth

1. ocean going under continent



*Always get a chain of coeval volcanoes; same age

 

2. ocean under ocean

Basalt – low silica; sticky material; makes up the majority of the ocean floor; always dark, black

Andesite – makes up majority of craters; found in continental and oceanic subduction; medium in color; most Cascades are andesite

Ryolite – usually very light in color

• Average continents have chemistry of andesite
• RECALL: ocean can be created or destroyed
• BUT: continents can only grow
• HOW?: by adding products of subduction; related volcanism

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• Go back 4.4 billions years; hotter, convection, more rapid; more subduction events, grew earliest continents
• Most volcanoes live for about 2-5 million years