1. Intrusive Igneous Rocks (Plutonic)
* Formation: These rocks form when magma (molten rock) cools and solidifies *inside* the Earth's crust. This happens slowly, allowing crystals to grow large and develop a coarse-grained texture.
* Locations:
* Batholiths: Massive bodies of intrusive igneous rock, often forming the cores of mountain ranges.
* Stocks: Smaller versions of batholiths.
* Dikes: Vertical, tabular bodies of igneous rock that cut across existing rock layers.
* Sills: Horizontal, tabular bodies of igneous rock that intrude between existing rock layers.
2. Extrusive Igneous Rocks (Volcanic)
* Formation: These rocks form when lava (molten rock that has erupted onto the Earth's surface) cools and solidifies. The rapid cooling results in small crystals and a fine-grained texture.
* Locations:
* Lava flows: Streams of molten rock that flow across the Earth's surface.
* Volcanic cones: Cone-shaped mountains built up from successive eruptions.
* Pyroclastic flows: Rapidly moving mixtures of hot gas, ash, and rock fragments.
Factors Influencing Igneous Rock Formation:
* Magma Composition: The chemical makeup of the magma determines the type of minerals that crystallize and, therefore, the rock type.
* Cooling Rate: The faster the magma/lava cools, the smaller the crystals that form.
* Pressure: High pressure within the Earth's crust can affect the crystallization process.
* Volcanic Activity: The type of eruption (explosive or effusive) influences the texture and composition of the resulting rock.
Examples:
* Intrusive: Granite, Gabbro, Diorite
* Extrusive: Basalt, Rhyolite, Andesite
Key Takeaway: The location where igneous rocks form dictates the conditions under which they cool and crystallize, leading to a wide variety of rock types with unique characteristics.
