HKDSE Geography/E1/Igneous Rocks

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Igneous rocks - rocks formed from magma or lava


Formation[edit]

  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[edit]

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[edit]

Granite[edit]

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[edit]

Tuff[edit]

Rhyolite[edit]

Pumice[edit]