Rock Identification and Classification

• Rock identification involves determining rock types based on their characteristics
• Rocks are classified into three main groups: igneous, sedimentary, and metamorphic

Igneous Rocks

• These rocks are formed from the cooling and solidification of magma or lava
• Intrusive igneous rocks cool slowly beneath the Earth's surface resulting in large crystals, known as phaneritic texture
• Extrusive igneous rocks cool quickly on the Earth's surface, resulting in small or no crystals, known as aphanitic or glassy texture
• Composition ranges from felsic (high silica content, light-colored) to mafic (low silica content, dark-colored) and ultramafic

Sedimentary Rocks

• These rocks are formed from the accumulation and cementation of sediments
• Sediments can be fragments of other rocks (clastic), chemical precipitates, or organic matter
• Clastic sedimentary rocks are classified by particle size such as gravel (conglomerate/breccia), sand (sandstone), silt (siltstone), and clay (shale)
• Chemical sedimentary rocks form from precipitation of minerals from solution, for example, limestone from calcium carbonate
• Organic sedimentary rocks form from the accumulation of organic matter, for example, coal from plant remains
• Sedimentary rocks have characteristics like bedding, layering, and the presence of fossils

Metamorphic Rocks

• These rocks are formed when existing rocks are changed by heat, pressure, or chemical reactions
• Foliated metamorphic rocks have a layered or banded appearance because of the alignment of minerals under pressure, for example, slate, schist, gneiss
• Non-foliated metamorphic rocks lack a layered appearance, for example, marble, quartzite
• Metamorphism can occur regionally over large areas or locally around igneous intrusions, known as contact metamorphism

Rock Identification Techniques

• Identify the rock's color; light-colored rocks are generally felsic, while dark-colored rocks are generally mafic
• Determine the rock's texture such as phaneritic (coarse-grained), aphanitic (fine-grained), porphyritic (mixed grain sizes), glassy, clastic, crystalline, foliated, or non-foliated
• Assess the hardness of the rock by performing a scratch test; using the Mohs Hardness Scale as a reference
• Look for features such as fossils, vesicles, layering, or mineral alignment
• Perform an acid test: hydrochloric acid (HCl) will cause effervescence (bubbling) for carbonate rocks

Mineral Composition

• Rocks are composed of one or more minerals
• Common rock-forming minerals include quartz, feldspar, mica, amphibole, pyroxene, olivine, and calcite
• Identifying the presence and abundance of specific minerals can help identify the type of rock
• Use a hand lens or microscope to identify individual minerals within the rock

Texture

• Phaneritic texture has visible crystals, which indicates slow cooling
• Aphanitic texture has microscopic crystals, which indicates rapid cooling
• Porphyritic has large crystals (phenocrysts) in a fine-grained matrix, which indicates two stages of cooling
• Glassy textures have no crystals, which indicates rapid cooling
• Vesicular textures contain gas bubbles, which indicates rapid cooling with dissolved gases present
• Fragmental textures are composed of cemented rock fragments

Structures

• Bedding refers to layering in sedimentary rocks
• Cross-bedding refers to inclined layers within bedding, indicating deposition by currents
• Ripple marks are wave-like structures on bedding surfaces, indicating deposition in water or air
• Foliation refers to the parallel alignment of minerals in metamorphic rocks
• Lineation refers to the parallel alignment of elongated minerals in metamorphic rocks

Common Igneous Rocks

• Granite is phaneritic and felsic, and contains quartz, feldspar, and mica
• Diorite is phaneritic with an intermediate composition, and contains plagioclase feldspar, amphibole, and pyroxene
• Gabbro is phaneritic and mafic, and contains pyroxene and plagioclase feldspar
• Basalt is aphanitic and mafic, and contains pyroxene and plagioclase feldspar
• Rhyolite is aphanitic and felsic, and contains quartz and feldspar
• Andesite is aphanitic with an intermediate composition, and contains plagioclase feldspar and amphibole or pyroxene
• Obsidian is glassy, usually felsic, and has no crystals
• Pumice is vesicular, felsic, and lightweight

Common Sedimentary Rocks

• Conglomerate is clastic, has gravel-sized particles, and rounded clasts
• Breccia is clastic, has gravel-sized particles, and angular clasts
• Sandstone is clastic, has sand-sized particles, usually quartz
• Siltstone is clastic, has silt-sized particles
• Shale is clastic, has clay-sized particles, and often splits into thin layers
• Limestone is chemical, is composed of calcium carbonate, and effervesces with acid
• Chert is chemical, and composed of microcrystalline quartz
• Coal is organic, and composed of plant remains

Common Metamorphic Rocks

• Slate is foliated, fine-grained, and results from low-grade metamorphism of shale
• Phyllite is foliated, fine-grained, from slightly higher grade metamorphism than slate, and has a silky sheen
• Schist is foliated, medium- to coarse-grained, and contains visible platy minerals (e.g., mica)
• Gneiss is foliated, coarse-grained, has a banded appearance, and contains alternating layers of light and dark minerals
• Marble is non-foliated, medium- to coarse-grained, is composed of recrystallized calcite or dolomite, and effervesces with acid
• Quartzite is non-foliated, medium- to coarse-grained, and composed of recrystallized quartz
• Amphibolite is foliated to non-foliated, medium- to coarse-grained, and composed of amphibole and plagioclase feldspar

Geological Context

• Understanding the geological setting in which a rock is found can provide clues to its origin
• Rocks found in volcanic areas are likely to be extrusive igneous rocks
• Rocks found in mountain ranges are likely to be metamorphic or intrusive igneous rocks
• Sedimentary rocks are commonly found in basins or areas of past marine environments

Weathering and Alteration

• Weathering and alteration can change the appearance of rocks, making identification more difficult
• Chemical weathering can dissolve minerals or alter them into clay minerals
• Physical weathering can break down rocks into smaller fragments
• Alteration can change the color and texture of rocks

Tools for Rock Identification

• Hand lens are for examining mineral grains and textures
• Geologist's hammer is for breaking rocks to expose fresh surfaces
• Acid bottles are for testing carbonate rocks
• Moh’s hardness scale is for hardness comparison
• Streak plates are for determining the streak color of minerals
• Rock and mineral identification books or field guides are available to identify samples

Documentation

• Record observable information:
• Color and luster
• Grain size and shape
• Hardness, cleavage, and fracture
• Inclusions
• Physical and chemical reaction
• Environment found

Learn about rock identification and classification. Rocks are classified into three main groups: igneous, sedimentary, and metamorphic. This includes how they are formed, their characteristics, and how to identify them based on composition and texture.