HKDSE Geography/E1/Igneous Rocks

Igneous rocks rocks formed from magma or lava Formation 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. 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. Porphyry Tuff Rhyolite Pumice