GEOL 106 Lecture Notes - Lecture 14: Igneous Rock, Oceanic Crust, Asthenosphere
25 May 2018
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Week 5 Textbook Notes
- Iceland volcanoes in the middle of Atlantic Ocean, home to 30 active volcanoes
5.1 Introduction to Volcanism
- volcanism typically related to plate tectonics with most volcanoes being located near active plate
boundaries
- Pacific Ring of Fire: 2/3 of all active volcanoes
o spreading or sinking plates at plate boundaries interact with other earth materials to produce
molten rock called magma
- volcanoes constructed aroun volcanic vent (opening) through which lava and other volcanic materials
are extruded onto surface
- vents may be roughly circular or they may be elongated cracks called fissures
- most magma comes from the asthenosphere: weak layer within mantle that allows overlying rigid
lithospheric plates to move around the globe
o asthenosphere is close to its melting temperature → more prone to melting than lithosphere
above or lower mantle below
- three principles that generate magma:
1. decompression melting occurs when overlying pressure exerted on hot rock within
asthenosphere is decreased
a. for a given increase in depth, an additional amount of heat is required to melt the
rock
b. decompression melting occurs and magma will begin to form even though
temperature stays that same
c. predominates where earths lithosphere is being stretched and extended
i. e.g. midocean ridges which are divergent plate boundaries
ii. continued stretching causes thinning of entire lithosphere and
asthenosphere must upwell toward surface to fill the space
iii. cooling of voluminous amounts of magma generated by decompression
eltig at idocea ridges has created all of Earth’s oceaic crust over past
200 million years
d. also occurs at hot spots where plume of superheated rock rises from deep within
mantle toward surface → responsible for small amount of decompression melting
2. addition of volatiles lowers melting temperature of rocks by helping to break chemical bonds
within minerals
a. volatiles: chemical compounds such as water, CO2, that evaporate easily and exist in
gaseous state at Earth’s surface
b. occurs at subduction zones and is responsible for Ring of Fire that surrounds Pacific
Ocean
i. where plates are sinking below adjacent plates
ii. volatiles trapped within sediments on seafloor and in minerals that compose
oceanic lithosphere are released as subducting oceanic plate is forced
downward into hotter mantle
3. addition of heat to rocks will induce melting if temperature exceeds melting temperature of
rocks tat that depth → when rising magma come into contact with other rocks
a. crustal assimilation: effect of crustal melting and magma mixing on chemical
composition of magma
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