Make sure you read or watch some of the background videos/sites on continental drift before reading this chapter.
- Active continental margin and island arcs; where one plate often slides under another
- Volcanoes in the middle of the ocean; often due to rising hotspots in the mantle (there are other possible causes)
- Continental rift valley; where plates are moving apart and tearing apart the crust of a continent
- Volcanoes in the middle of nowhere; but not in Va….yet…..
Please take time to look at at least a few of these volcanoes using Google Earth.
Convection in the mantle. We need to be very clear about a common misconception. Apart from very limited partial melting in the upper mantle, the mantle of the Earth is solid. Rock hard. Over time at the high temperatures and pressure of the mantle it can still flow as if it were liquid,, but all the time it remains essentially solid. The only part of the deep earth that is molten is the outer core.
In figure 2.13 you will see that the temperature of the earth rises much faster in the crust than in the mantle, this is because radiogenic isotopes that decay and release heat are concentrated in the crust rather that the mantle.
Page 35-35. MORB stands for Mid Ocean Ridge Basalt, the typical lava erupted at….mid ocean ridges where tectonic plates are moving apart! 🙂 Apparently you were just supposed to know this? Go figure? So much for careful editing!
Summary of main kinds of basalt, each chemically distinctive).
- MORB (mid ocean ridge basalt)
- Tholeiite
- Alkali
- IAB (island arc basalt)
- Ocean island basalt (OIB)
Page 38; Fractional crystallization.
These early-formed minerals are heavy and sink through the magma to accumulate on the floor and sides of the magma chamber. Hence “cumulates”. As they selectively remove certain elements from the magma, the magma that remains progressively changes in composition. Think pingpong balls. if I start with 50 green and 50 red pingpong balls in a bag (magma chamber) and selectively remove the green ones (as in the formation of cumulates) the bag is then left with an increasing proportion of red balls. In a magma chamber, this results in an increasing concentration of sodium Na, Potassium K, Silicon, Si, Aluminum Al, sometimes iron Fe, and always, water H20)