1.Explain the difference between porphyritic and pegmatitic textures.
Rocks with porphyritic texture display a mix of large and small crystals, resulting from
magma that cools slowly at first, then rapidly. In contrast, pegmatitic texture is marked by
large crystals, formed when magma cools at an extremely slow rate.
2. Name the following rocks:
(a) An extrusive rock with 40% Ca-rich plagioclase and 60% pyroxene
Basalt
(b) An intrusive rock with 65% plagioclase, 25% amphibole, and 10% pyroxene
Diorite
(c) An intrusive rock with 25% quartz, 20% orthoclase, 50% feldspar, and minor amounts of
biotite.
Granite
3. With respect to tabular intrusive bodies, what is the difference between a concordant body
and a discordant body?
A concordant body is an intrusive rock body that runs parallel to surrounding rock layers and
does not disrupt the layers. Whereas, a discordant body cuts across the rock layers, disrupting
their arrangement. An example is a dike.
4. Why does a dyke commonly have a fine-grained margin?
A dyke's fine-grained margin is a result of rapid cooling at its edges, where magma meets
cooler surrounding rocks. This swift cooling process produces smaller crystals near the edges,
whereas the center, cooling more gradually, forms larger crystals.
5.What is the difference between batholith and a stock?
A batholith is a large intrusive rock body over 100 square kilometers, while a stock is
smaller, less than 100 square kilometers. Both are formed from solidified magma beneath the
surface.