HKDSE Geography/E1/Igneous Rocks

Igneous rocks - rocks formed from magma or lava

Formation

1. Magma in the mantle is under great pressure.
2. When a line of weakness appears in the crust and reaches the magma chamber below the magma pressure is released.
3. Magma rises along the cracks to the earth's surface (extrusive/volcanic rock) or inside the earth's crust (intrusive rock)
4. The magma cools and solidifies. Crystallisation occurs in the process. Interlocking crystals are formed.

Classification

	Acidic High silica content Lighter colour	Basic Low silica content Darker colour
Intrusive Formed by intrusive vulcanicity Formed deep underground - Plutonic rocks Formed at a shallower depth = Hypabyssal rocks The deeper underground, the slower the cooling rate and coarser the crystals (i.e. larger the grains)	Granite, porphyry, dolerite	Gabbro
Extrusive Formed by extrusive vulcanicity Higher cooling rate and finer crystals (smaller grains)	Rhyolite, Tuff, Obsidian, Pumice	Basalt

More on the Specific Types

Granite

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.