EARTH’S INTERNAL STRUCTURE AND MAGMA FORMATION PROCESSES

Continental crust Oceanic crust Crust-mantle boundary (MOHO) Upper mantle (down to km)

Mantle is subdivided mainly based on seismic wave velocities

Nature 451, (17 January 2008)

Information from seismic waves “CAT-scan” of the mantle, using SEISMIC TOMOGRAPHYCAT-scanSEISMIC TOMOGRAPHY Faster seismic velocity Colder and stronger rocks Slower seismic velocity Warmer and weaker rocks

Nature 451, (17 January 2008) Red = Slow seismic velocity Blue = Fast seismic velocity

Seismic tomography shows that some parts of the mantle are hotter than others Would result in heat transfer Conduction in lithosphere Convection in asthenosphere

4D Lithosphere Mapping: a methodology and philosophy for tracing the architecture and composition of the lithosphere through time Fig. 2.13, page 31

CONVECTION Heat transfer that results when warmer, less dense material rises, and cooler, denser material sinks

CONVECTION Convection within the mantle used to be considered by some to be a major driving mechanism of plate tectonics (“ridge push” mechanism, page 30)

Melting by lowering pressure  Melting temperature of a solid goes UP under high pressure (becomes difficult to melt)  Melting temperature of a solid goes DOWN under low pressure (becomes easier to melt) Is the major process of magma generation at divergent boundaries and hot spots (pages 30-31)

At divergent boundaries and hot spots, lithosphere becomes thinner due to pulling from subducting slabs Pressure is reduced and hot rocks from asthenosphere move upward (convection) Rocks start to melt

Melting caused by decreasing pressure is called DECOMPRESSION MELTING Example: Hawaii (hot spot) Mid-Atlantic Ridge (divergent boundary)

A little note on hot spots … Hot spot volcanoes are NOT associated with any plate boundary (example: Hawaii, Yellowstone, Galapagos island etc.) They develop over columns of very hot, plastic rock called MANTLE PLUMES (pages 39-44)

LINK TO: Dr. Hana Cizkova (Kyvalova)

Adding water under pressure Melting temperature of a solid goes DOWN when water is added (wet rocks melt more easily than dry rocks) Is the major process of magma generation at subduction zones (page 37, figure 2.18)

Increasing temperature: When magma passes through and melts other solid rocks Not a major process of magma formation