The key physical properties of igneous rocks are primarily determined by their composition and cooling history, significantly influencing characteristics like melting temperature, viscosity, texture, density, color, and hardness.
Melting Temperature and Viscosity
Two of the most important physical properties influencing igneous rock behavior are melting temperature and viscosity (resistance to flow).
- Melting Temperature: Mafic magmas, which are relatively rich in iron and magnesium, typically have higher melting temperatures compared to silicic magmas, which are richer in silica.
- Viscosity: Mafic magmas exhibit lower viscosity, meaning they flow more easily, while silicic magmas are more viscous. This difference in viscosity is mainly due to the higher silica content in silicic magmas, which forms complex structures that impede flow.
Other Important Physical Properties
Besides melting temperature and viscosity, other key physical properties of igneous rocks include:
- Texture: Texture refers to the size, shape, and arrangement of the mineral grains within the rock. Igneous rock textures can range from coarse-grained (phaneritic), where individual minerals are visible, to fine-grained (aphanitic), where they are not, to glassy (obsidian), depending on the cooling rate of the magma. Porphyritic textures, with large crystals (phenocrysts) in a fine-grained matrix, indicate two stages of cooling.
- Density: Density is influenced by the mineral composition. Mafic rocks, containing heavier elements like iron and magnesium, are generally denser than silicic rocks.
- Color: The color of an igneous rock can provide clues about its composition. Felsic rocks (rich in feldspar and silica) tend to be light-colored (e.g., granite, rhyolite), while mafic rocks (rich in magnesium and iron) are typically dark-colored (e.g., basalt, gabbro).
- Hardness: Hardness refers to a mineral's resistance to scratching. The hardness of an igneous rock is determined by the hardness of its constituent minerals, which is assessed by the Mohs Hardness Scale. Minerals like quartz (found in many felsic igneous rocks) are relatively hard, whereas others like feldspar are moderately hard.
- Porosity and Permeability: Some igneous rocks, particularly those with vesicular textures (containing gas bubbles), can exhibit porosity (the proportion of void space) and permeability (the ability to transmit fluids). However, many igneous rocks are relatively impermeable due to the interlocking nature of their crystals.
Composition and Mineralogy
The chemical composition of the magma significantly influences which minerals crystallize and, consequently, the physical properties of the resulting igneous rock. Bowen's Reaction Series describes the order in which minerals crystallize from a cooling magma. This series shows that minerals like olivine and pyroxene crystallize at higher temperatures, while minerals like quartz and muscovite form at lower temperatures.
Examples
| Igneous Rock | Composition | Texture | Color | Density | Viscosity (of Magma) |
|---|---|---|---|---|---|
| Granite | Felsic | Coarse | Light | Lower | High |
| Basalt | Mafic | Fine | Dark | Higher | Low |
| Obsidian | Felsic | Glassy | Dark | Lower | High |
In summary, the physical properties of igneous rocks are interconnected and primarily determined by the magma's composition, the rate at which it cools, and the pressures involved during its formation. Understanding these properties helps geologists identify, classify, and interpret the origin and history of these rocks.
