Igneous Rocks 1

IGNEOUS ROCKS, Part I

What Are Igneous Rocks?

Igneous rocks are defined as rocks that form from the cooling and crystallization of molten rock, which can be in the form of magma or lava.

Types of Igneous Rocks

  • Intrusive Igneous Rocks (Plutonic): These rocks are formed from the cooling and crystallization of magma beneath the Earth's surface. The most common intrusive igneous rocks are granite, diorite, and gabbro. Because they cool slowly, they feature large visible mineral crystals.

  • Extrusive Igneous Rocks (Volcanic): These rocks are formed from the cooling and crystallization of lava on the Earth's surface. Common examples include rhyolite, basalt, and andesite. Due to the rapid cooling of lava, these rocks display small, barely-visible mineral crystals.

Examples of Igneous Rocks

  • Granite: An intrusive igneous rock that is light-colored and composed mainly of quartz and feldspar.

  • Diorite: An intermediate intrusive rock that contains a mix of dark and light minerals.

  • Gabbro: A coarse-grained, dark-colored, intrusive rock made primarily of plagioclase and augite. It is the most abundant rock in the deep oceanic crust.

  • Rhyolite: An extrusive igneous rock that is light-colored and rich in silica.

  • Basalt: A dark-colored, fine-grained volcanic rock that is common on ocean floors.

  • Andesite: An intermediate volcanic rock that features a composition between basalt and rhyolite.

Cooling Process

  • Magma is generated from melting mantle rocks and can rise through the crust.

  • As magma cools, it crystallizes to form igneous rocks, with the grain size depending on the rate of cooling: slow cooling typically yields larger crystals, while rapid cooling yields smaller crystals.

The Importance of Igneous Rocks

Igneous rocks play a significant role in geology for the following reasons:

  1. Volume: They constitute more than 90% of the Earth's crust and mantle.

  2. Dating: Their absolute ages can be determined through radiometric dating techniques, anchoring the geological time scale.

  3. Economic Value: Many igneous rocks contain associated mineral ores, valuable for economic purposes, such as platinum, gold, and silver.

Geological Time Scale

Igneous rocks often provide absolute dating, which helps to establish the timing of geological events and the age of the Earth, estimated to be about 4.6 billion years old. Important extinction events can also be dated using these rocks.

Textures of Igneous Rocks

Texture in igneous rocks refers to the size, shape, and arrangement of the mineral grains that comprise the rock. Texture can provide insights into the environment and conditions under which the rock crystallized.

Common Textures in Igneous Rocks

  1. Coarse Grain (Phaneritic): Indicates slow cooling, resulting in large crystals. Examples include pegmatitic and phaneritic textures.

    • Pegmatitic: Crystal sizes can exceed 1 cm in diameter.

    • Phaneritic: Grain sizes range from 1mm to 10mm in diameter.

  2. Fine Grain (Aphanitic): Indicates rapid cooling, resulting in crystals less than 1mm in size, which can only be identified under a microscope.

  3. Porphyritic: Characterized by two distinct crystal sizes - larger crystals known as phenocrysts surrounded by a finer-grained matrix.

  4. Glassy: Formed from extremely rapid cooling; appears glass-like with no discernible crystals (e.g., obsidian).

  5. Vesicular: Contains gas bubbles creating a spongy appearance (e.g., pumice and scoria).

  6. Pyroclastic: Composed of fragments from volcanic eruptions, such as tuff and volcanic breccia.

Coarse-Grained Textures

Coarse-grained textures suggest that the rock formed from magma cooling slowly underground. Examples demonstrate distinct mineral grains visible to the naked eye, giving clues about their formation environment.

Fine-Grained Textures

Fine-grained textures suggest that the rock formed from lava cooling rapidly above ground. These textures may appear glassy or contain small vesicles.

Porphyritic Textures

Porphyritic rocks exhibit a unique texture that resulted from dual cooling processes. Initial slow cooling produced larger crystals, while rapid cooling formed the surrounding fine-grained matrix.

Examples of Textures

  • Glass: Obsidian (glassy texture).

  • Vesicular: Pumice (lightweight and porous) and scoria.

  • Pyroclastic: Tuff (an aggregate of volcanic ash).

  • Porphyritic Textures: Can feature either phaneritic or aphanitic matrices variable in size indicating differing cooling histories.

Summary of Properties

  • Phaneritic Texture: Indicates slow cooling, with crystals visibly forming underground.

  • Aphanitic Texture: Indicates rapid cooling, yielding small crystal sizes.

  • Overall Texture Implications: Textures serve as indicators of crystallization conditions and cooling rates in igneous rock formation.

Overall, igneous rocks are a fundamental component of the Earth's structure and have diverse implications in geological sciences, economic geology, and the study of Earth's history.