Felsic rocks are a major category of igneous rocks, formed from the cooling and solidification of molten material. The name “felsic” is a portmanteau combining feldspar and silica, highlighting the two most abundant chemical components. This grouping classifies magma and rocks based on their relative concentrations of lighter elements. Felsic rocks are fundamental components of the Earth’s continental crust.
Mineral Composition and Physical Appearance
The defining characteristic of felsic rocks is their high concentration of silica, or silicon dioxide (SiO2), which typically exceeds 65% of the rock’s total weight. This high silica content determines the rock’s overall chemistry and the specific minerals that crystallize. The primary minerals forming felsic rocks are rich in lighter elements like silicon, oxygen, aluminum, sodium, and potassium. These include quartz, potassium feldspar (orthoclase), sodium-rich plagioclase feldspar, and muscovite mica.
The abundance of these light-colored minerals results in a low density for felsic rocks compared to their darker, iron and magnesium-rich counterparts. Visually, this composition causes felsic rocks to be light in color, a characteristic known as leucocratic. This lighter coloration is due to the low amount of darker mafic minerals, which contain iron and magnesium. The specific color can range from white and light gray to pink or reddish hues, depending on the variety of feldspar present. The high silica content also makes felsic magmas highly viscous, influencing how they move and erupt.
The Formation Process and Texture
Felsic rocks are igneous, originating from the cooling and crystallization of either magma beneath the surface or lava on the surface. The rate at which this molten material cools is the sole factor determining the final texture of the rock, specifically the size of the mineral grains. This distinction divides felsic rocks into two main textural categories.
When felsic magma cools very slowly deep within the Earth’s crust, the mineral crystals have time to grow large enough to be visible. This slow cooling creates a coarse-grained texture known as phaneritic. Conversely, when felsic lava erupts onto the surface, it solidifies rapidly due to the cooler environment. This rapid cooling prevents the mineral grains from growing large, resulting in a fine-grained, or aphanitic, texture. Some felsic lavas cool so quickly that no crystals form, leading to a glassy texture.
Specific Types and Geological Location
The most widely recognized felsic rock is granite, the classic example of a phaneritic, intrusive rock formed from slow-cooling magma. Granite is a major constituent of continental mountain belts and deep continental shields. A related intrusive rock is granodiorite, which is also coarse-grained but contains a higher proportion of plagioclase feldspar than true granite.
The extrusive, fine-grained equivalent of granite is rhyolite, which forms when felsic magma reaches the surface as lava and cools quickly. Rhyolite often displays flow lines and can be difficult to distinguish from other volcanic rocks without microscopic analysis. Extremely rapid cooling can produce volcanic glass like obsidian, which has a glassy texture, or the porous material called pumice. These rocks form the bulk of the continental crust. Their lower density allows the continents to sit higher on the Earth’s mantle, a fundamental concept in plate tectonics.