Chapter 3 Flashcards
What processes must take place to transform rocks into sediment?
The rock must be exposed at surface so in many cases uplift and removal of overlying sediments is required. Then chemical and/or physical weathering can take place, which reduces the rock to smaller loose fragments. These fragments are sediments that can be eroded and then transported by a variety of maechanisms.
What processes normally take place in the transformation of sediments to sedimentary rock?
Sediments are buried beneath other sediments where, because of the increased pressure, they become compacted and dewatered. With additional burial they are warmed to the point where cementing minerals can form between the grains (less than 200˚C).
What are the processes that lead to the formation of a metamorphic rock?
Rock is buried within the crust and heated because of the geothermal gradient. At temperatures over 200˚C some of the existing minerals may become unstable and will be converted to new minerals, or recrystallized into larger crystals.
What is the significance of the term reaction in the name of the Bowen reaction series?
As the temperature decreases minerals that formed early (e.g., olivine) may react with the remaining magma to form new minerals (e.g., pyroxene).
Why is it common for plagioclase crystals to be zoned from relatively calcium-rich in the middle to more sodium-rich on the outside?
Calcium-rich plagioclase forms early on in the cooling process of a magma, but as the temperature drops, a more sodium-rich variety forms around the existing crystals.
What must happen within a magma chamber for fractional crystallization to take place?
Early-forming minerals, which are typically quite dense (e.g., olivine) may sink to the bottom of the magma chamber (if the magma is not too viscous) and thus become separated from the rest of the magma, resulting in a change to the composition of the remaining magma (it becomes more felsic).
Explain the difference between aphanitic and phaneritic textures.
If the texture is aphanitic the crystals are too small to see without a microscope. In rocks with phaneritic textures the minerals are large enough to see and distinguish from each other with the naked eye. The dividing line is somewhere between 0.1 and 1 mm, depending on the minerals.
Explain the difference between porphyritic and pegmatitic textures.
In porphyritic rocks there are two distinct crystal sizes that are indicative of two stages of cooling (slow then fast). The fine material can range from glass to several mm, as long as the coarse crystals are distinctively larger. In pegmatitic rocks the crystals are consistently coarser than 1 cm, and can be much larger. Pegmatites form the slow cooling of water-rich magmas.
Name the following rocks:
An extrusive rock with 40% Ca-rich plagioclase and 60% pyroxene
An intrusive rock with 65% plagioclase, 25% amphibole, and 10% pyroxene
An intrusive rock with 25% quartz, 20% orthoclase, 50% feldspar, and minor amounts of biotit
n extrusive rock with 40% Ca-rich plagioclase and 60% pyroxene is basalt.
An intrusive rock with 65% plagioclase, 25% amphibole, and 10% pyroxene is diorite.
An intrusive rock with 25% quartz, 20% orthoclase, 50% plagioclase, and minor amounts of biotite is granite.
With respect to tabular intrusive bodies, what is the difference between a concordant body and a discordant body?
A concordant body (a sill) is parallel to any pre-existing layering (bedding or foliation) in the country rock is. A discordant body (a dyke) cuts across any pre-existing layering or is situated at any angle in country rock that has no layering (e.g., granite).
Why does a dyke commonly have a fine-grained margin?
A rock has to crack in order for a dyke to intrude into it, and it has to be cool to crack. When the hot magma intrudes into the cold country rock its margins cool quickly (forming small crystals), while its centre cools more slowly (forming larger crystals).
What is the difference between a batholith and a stock?
A batholith has an exposed area of greater than 100 km2; a stock has an exposed area less than that.
Describe two ways in which batholiths intrude into existing rock.
Batholiths (or stocks) intrude into existing rock by (a) melting through the country rock, or (b) causing the country rock to break and fall into the magma (stoping), or (c) pushing the country rock aside.
Why is compositional layering a common feature of mafic plutons but not of felsic plutons?
Compositional layering forms when early-crystallizing mineral sink toward the bottom of a magma chamber. This can only happen in non-viscous magma, and mafic magma is typically much less viscous than felsic magma.
