Definition

1. Adjective:
   • Relating to igneous rock formed from magma that has erupted onto the Earth's surface: Describes rock material that has been forced out, while molten, through cracks or volcanic vents onto the Earth's surface, where it cools and solidifies rapidly.

Usage

• The term "extrusive" is primarily used in geology to classify igneous rocks based on where they solidify. It contrasts with "intrusive" rock, which cools and solidifies below the surface.
• It is typically used attributively (before a noun) to describe rocks or geological features.

Examples

• Adjective:
  • Basalt is a common extrusive rock found in volcanic regions.
  • The geologist studied the extrusive lava flows from the ancient volcano.
  • The rapid cooling of extrusive magma results in fine-grained textures.

Advanced Usage

• "Extrusive activity": Volcanic activity involving the eruption of lava onto the surface.
  • The island was formed by millions of years of extrusive activity.
• "Extrusive equivalent": Refers to the extrusive rock type that corresponds to a specific intrusive rock (e.g., rhyolite is the extrusive equivalent of granite).
  • Rhyolite is considered the extrusive equivalent of the intrusive granite.

Variants and Related Words

• Extrude (verb): To force or push something out.
  • The volcano extrudes lava onto the surface.
• Extrusion (noun): The process of forcing something out or the rock mass formed by this process.
  • The extrusion of magma created a new lava dome.

Synonyms

• Volcanic: Pertaining to volcanoes or igneous rocks formed on the surface. (Note: While closely related, "volcanic" is a broader term that can also refer to the phenomena and structures of volcanoes, not just the rock formation process.)
• Effusive: In geology, specifically refers to lava flows that are fluid and spread out, which is a type of extrusive activity.

Antonyms

• Intrusive: (Adjective) Describing igneous rock that solidifies from magma below the Earth's surface.

Adjective

1. of rock material; forced out while molten through cracks in the earth's surface