HKDSE Geography/E1/Igneous Rocks
- Magma in the mantle is under great pressure.
- When a line of weakness appears in the crust and reaches the magma chamber below the magma pressure is released.
- Magma rises along the cracks to the earth's surface (extrusive/volcanic rock) or inside the earth's crust (intrusive rock)
- The magma cools and solidifies. Crystallisation occurs in the process. Interlocking crystals are formed.
||Granite, porphyry, dolerite||Gabbro|
||Rhyolite, Tuff, Obsidian, Pumice||Basalt|
More on the Specific Types
A plutonic rock, granite is made of three minerals: quartz (clear/milky), mica (black) and feldspar (pink/grey). These minerals occur as interlocking crystals with coarse or medium-sized grains. It is a well-jointed rock and light in colour.