Rock And Minerals Quiz For Earth Science

Rocks can be monomineralic or polymineralic. Quartzite and marble are essentially single-mineral rocks, while granite commonly contains quartz, feldspar, and mica. Because a rock is an aggregate, the count can be one or more. This definition aligns with petrography, where thin-section analysis confirms mineral counts through optical properties. The key is that minerals have fixed compositions and structures, while rocks are physical mixtures of those minerals in varying proportions.

Igneous rocks form when molten rock cools and crystallizes. Cooling beneath the surface involves magma insulated by country rock, allowing larger crystals to grow as atoms diffuse to crystal lattices over time. This slow cooling yields phaneritic textures, such as granite. Rapid cooling at or near the surface produces fine crystals or glass. The question targets the core process: magma cooling and solidifying, which is the defining criterion for the igneous rock class.

Minerals are naturally occurring, inorganic solids with a definite chemical composition and an ordered atomic structure, such as calcite (CaCO₃) or quartz (SiO₂). Rocks are aggregates of one or more minerals, often with variable proportions and textures. This categorical difference is confirmed by tests like X-ray diffraction for minerals and modal analysis for rocks. The distinction guides classification: mineral identification precedes rock naming, since rock names depend on mineral makeup.

Compaction decreases pore space as overburden pressure increases, while cementation fills remaining pores with precipitated minerals like calcite or silica from groundwater. Together, these steps achieve lithification. Grain packing becomes tighter, porosity drops, and contacts change from point to long contacts. The mechanical energy from burial and the chemistry of percolating fluids are both required. The process converts loose sand to sandstone or mud to shale through progressive densification and cement growth.

Calcite reacts vigorously with dilute hydrochloric acid, releasing CO₂: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O. Quartz does not effervesce. While hardness separates them too (calcite Mohs 3, quartz 7), the acid test is a fast, definitive field method even on fine-grained samples. Color is unreliable because both minerals occur in many hues. Magnetism does not apply. The fizz test isolates carbonate composition irrespective of texture or crystal size.

Mafic magmas are lower in silica and richer in Fe-Mg, forming denser, darker rocks like basalt and gabbro. Lower viscosity promotes easier degassing and higher eruption temperatures, typically 1100–1200°C. Mineralogy favors pyroxene, olivine, and Ca-rich plagioclase, which absorb more light and increase density relative to felsic assemblages. Felsic magmas are silica-rich, lighter in color, and less dense. Ultramafic compositions are even more Mg-rich but are less common at the surface.

Color is influenced by trace impurities, weathering, inclusions, and defects, making it an unreliable diagnostic. Quartz can be clear, smoky, purple, or pink; calcite ranges widely as well. Streak, measured on unglazed porcelain, reveals the powdered color and is more consistent. Luster and hardness tie to structure and bonding. Hardness comparisons on the Mohs scale (talcs to diamond) and streak results reduce ambiguity that color alone would introduce.

Marble forms when limestone recrystallizes under heat and pressure, converting calcite grains into a mosaic of interlocking crystals. Fossils and sedimentary textures are obliterated as grain boundaries realign and crystal size increases. The reaction is largely polymorphic, keeping CaCO₃ but changing texture. Impurities can yield bands of color. In contrast, basalt and obsidian are igneous, and sandstone is sedimentary. Marble’s crystalline texture and effervescence in acid verify its identity.

Slow cooling allows ions to diffuse and attach to growing crystal faces over longer timescales, producing coarse grains visible to the naked eye. Phaneritic textures, typical of plutonic rocks like granite and diorite, reflect deep emplacement and extended thermal histories. Coarse grains indicate fewer nucleation sites but prolonged growth. Glassy or vesicular textures indicate rapid quenching, while clay-sized particles are sedimentary and signal mechanical or chemical weathering products, not magmatic crystallization.

Shale forms from compacted and cemented clay-sized particles less than 0.004 mm, yielding fissility and fine lamination. Conglomerate contains rounded gravel clasts, sandstone has sand-sized grains, and limestone is carbonate. Shale’s small grain size reflects low-energy depositional settings like lakes, floodplains, and deep marine basins. Burial increases effective stress and drives water out, while clay minerals align and compact, creating the characteristic thin bedding and relatively low permeability.

Rapid quenching at the surface traps dissolved volatiles as pressure drops. Gas bubbles expand faster than ions can migrate to crystal sites, so cavities freeze into the solid as vesicles. Basaltic lavas with higher volatile content commonly show vesicular textures, and extreme cases become scoria or pumice. The physics combines decompression, supersaturation of gases, nucleation of bubbles, and insufficient time for degassing before the melt crosses the glass transition and locks in the cavities.

Any rock type can become metamorphic if subjected to sufficient heat, pressure, and chemically active fluids. Igneous granite can transform to gneiss, sedimentary shale to slate and then schist, and even preexisting metamorphic rocks can re-metamorphose to higher grades. The rock cycle is dynamic: mineral assemblages reequilibrate according to pressure-temperature conditions described by phase diagrams, producing new textures like foliation, lineation, and recrystallized grains.