Cross cracks how long has plate tectonics been active

 Cross cracks
- How long has plate tectonics been active on the Earth?
o What about earthquakes that are not at plate boundaries?

 Eg. Mississippi river valley
 Intraplate earthquakes
 Earthquakes that occur away from plate margins
 Represent only about 1% of all earthquakes
 And most are along former plate margins
 Where plate margins failed to open up and create oceans (creates rivers)
o 20,000 years ago Laurentide ice sheet caused crust to subside and once
melted the crust is still gradually rebounding (causes earthquakes)
Volcanic risk
- Igneous rock
o Start out as molten rock (magma) underground
o Forms when melted rock (magma) cools down and crystallizes
- Where does magma originate?
o Almost all igneous activity happens along constructive and destructive plate margins
o Most (but not all) magma is generated at plate boundaries/margins
o Mainly forms in the upper mantle (down to about 200 km)
o Sometimes in the crust
o But some igneous activity happens away from plate margins (eg. Hawaii)
 Hotter zone beneath the lithosphere creates a string of volcanic islands
- Magmas contain dissolved gases
o Water, carbon dioxide, sulphur dioxide, fluorine, chlorine, etc.
o We can see this in rocks from trapped gas bubbles (eg. Pumice)
- Igneous rocks can form (crystallize) either:
o A) inside the Earth (intrusive)
 Batholith – big pool of igneous rock that crystallizes inside the earth (huge)
o B) on the surface of the Earth (extrusive)
 Dyke – crack that cuts up (vertical) across layers of rock on the surface to release
magma
 Sill – crack that cuts horizontal with the layers of rock to release magma
- The shape and features of an igneous rock body tell us something about the environment of formation
- The size of crystals in any igneous rock depends mainly on the rate of cooling of the magma
o Rapid – tiny crystals
 Eg. extrusive
o Slow – medium to big crystals
 Eg. intrusive
- Note: there is a range of chemical composition of magmas (igneous rocks)
o Seen most readily by their different colours
o Magmas differ in the proportion of elements
o Results in rocks of different colours and magmas with different properties
o Rocks rich in iron and magnesium with be black (mafic)
o Rocks rich in silicon and feldspar will be pinkish whitish (felsic)
- Basalt – an extrusive mafic rock (black)
- Rhyolite – an extrusive felsic rock (light)
- Andesite – extrusive igneous rock that is intermediate between felsic and mafic (grey)

Volcanic Risk Analysis and Risk Management
- A: Risk analysis
o 1. Understand the hazard in general