HKDSE Geography/E1/Metamorphic Rocks

There are two main types of metamorphism:

Dynamic and regional metamorphism

Formation:

1. Internal stress accumulates along a fault or fold
2. High pressure forces the minerals to realign to form metamorphic rocks
3. Rocks are flatted and the minerals are aligned towards the plane of flattening
4. A foliated structure is formed

Conditions:

• Associated with large-scale mountain-building processes (the collision of two continental plates or the subduction of an oceanic plate)
• High temperature and pressure

Examples:

• Shale/Mudstone/Siltstone → Slate → Phyllite → Schist → Gneiss
• Granite → Gneiss

Contact/thermal metamorphism

1. Rising magma intrudes into the rock strata
2. Heat from magma intrusion creates high temperature, which melts and recrystallises the rock crystals into new rock

This is a localised process. The zone of contact between the existing rock and the intrusion - i.e. where the metamorphism takes place - is the aureole. The rocks do not have a foliated structure.

Conditions:

• High temperature
• Magma intrusion

Examples:

• Bituminous coal → Anthracite
• Sandstone → Quartzite
• Limestone → Marble