VOLCANOES

Intusive Igneous Formations

Structures resulting from the intrusion of magma into the bedrock are called plutons. There are many structural types of plutons which can be classified by their shape, tabular or massive; and by their orientation, discordant or concordant. Discordant plutons cut across existing structures while concordant plutons form parallel to existing structures or strata.

• Dikes
  • tabular discordant igneous bodies formed when magma is injected into rock fractures
• Sills
  • tabular concordant plutons of shallow depth formed when magma is injected between sedimentary bedding planes
• Laccoliths
  • similar to sills, but laccoliths cause the overlying rock to bulge and form a dome
• Batholiths
  • a plutonic body with a surface exposure greater than 100 square kilometers (smaller structures of this type are called stocks)

What forms magma?

1. heat
2. decrease in pressure
3. introduction of volatiles