Rock Classification: The basics

Overview of all 3 major types of rocks, with subcategories and textures included.

Foliated Metamorphic Rock

Formed under intense temperature and directional pressure, forming bands/layers.

Non-Foliated Metamorphic Rock

Formed under lighter or uniform pressure. No layered texture.

Gneissic Texture

Distinct mineral bands. Metamorphic rock texture.

Schistose Texture

Larger, visible minerals. Metamorphic rock texture.

Slaty Texture

Fine-grained, can be split into slabs. Metamorphic rock texture.

Extrusive Igneous Rock

Igneous rock formed outside the crust (in lava). Smaller crystals due to rapid formation.

Intrusive Igneous Rock

Igneous rock formed inside the crust (in magma). Larger crystals due to slow formation.

Silica content in increasing order.

Ultramafic (<45%), Mafic (45-52%), Intermediate (52-65%), Felsic (>65%).

Igneous Rock Texture

Crystalline, due to interlocking formations during cooling.

Igneous Rock Hardness and Density

Hard and dense.

What elements are Mafic and Ultramafic rocks rich in?

Iron and Magnesium.

Phaneritic Texture

Igneous rock texture with coarse, visible minerals. Ex: Granite.

Typically found in intrusive rocks.

Aphanitic Texture

Igneous rock texture with fine-grain minerals. Ex: Basalt.

Crystalline structure is not typically visible to the naked eye. Typically found in extrusive rocks.

Porphyritic Texture

Large and small crystals, indicating complex cooling. Igneous rock texture. Ex: Porphyritic Andesite.

Glassy Texture

Formed from very rapid cooling, atoms do not have time to form crystalline structure. Igneous rock texture. Ex: Obsidian.

Found in extrusive rocks.

What leaves voids/vesicles in rock?

Escaping gas, found in igneous rock.

What distinguishes grain texture in Metamorphic and Igneous rocks?

Igneous rocks have random grain orientation, whereas Metamorphic rocks have uniform orientation, even if the grain sizes and/or layers are not uniform.

What determines silica content in igneous rocks?

Temperature of initial cooling; Mafic rocks begin crystalization at higher temperatures (1200-1300°C) and with olivine, where Felsic rocks begin at lower temperatures and with pyroxene and feldspar.

Clastic Sedimentary Rock

Sedimentary rock formed by compaction of rock fragments (clasts). Ex: Sandstone, shale.

Chemical Sedimentary Rock

Sedimentary rock formed by precipitation of chemical minerals. Ex: Limestone, gypsum.

Organic Sedimentary Rock

Sedimentary rock formed by accumulation of organic debris. Ex: Coal, chalk.

What do rounded clasts indicate?

Significant movement via wind or water.

What do angular clasts indicate?

Little transport/movement from source area.

Sorting

Refers to uniformity of sedimentary grain textures. The more well-sorted, the more uniform in size the grains are.

Sedimentary vs Igneous/Metamorphic

Sedimentary rocks are typically softer and less dense.