Minerals and the Igneous, Sedimentary, Metamorphic Rock Cycle: Seventh Grade Science
The rock cycle is a continuous geological model describing how geological processes transform rocks between igneous, sedimentary, and metamorphic forms over millions of years.
The Chemistry and Criteria of Minerals
All rocks are composed of minerals. In geology, a mineral has five specific physical and chemical criteria: it must be a naturally occurring, inorganic solid, with a definite chemical composition, and an ordered internal crystalline atomic structure. If a substance fails any one of these criteria, it is not a mineral:
- Naturally Occurring: Formed by natural geological processes, not synthesized artificially in a laboratory.
- Inorganic: Not produced exclusively by living organic cellular tissue (e.g., coal is not a mineral because it forms from compacted plant debris).
- Solid: Possesses a definite shape and volume at surface temperatures (liquid mercury and water are not minerals, but solid water ice is).
- Definite Chemical Composition: Can be expressed by a chemical formula (e.g., quartz is SiO2; halite is NaCl).
- Ordered Crystalline Structure: Atoms are arranged in a repeating, three-dimensional geometric lattice.
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| MINERAL IDENTIFICATION TESTING MATRIX |
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| Physical Test | Diagnostic Method | Geological Significance |
+---------------------+-----------------------+-------------------------------+
| Hardness | Mohs Hardness Scale | Resistance to scratching; |
| | (1: Talc to 10: Diam.)| measures atomic bond strength |
+---------------------+-----------------------+-------------------------------+
| Streak | Rubbing mineral across| Powdered color; consistent |
| | unglazed porcelain | regardless of surface tarnish |
+---------------------+-----------------------+-------------------------------+
| Cleavage vs. | Breaking mineral under| Cleavage breaks along flat |
| Fracture | mechanical impact | atomic planes; fracture breaks|
| | | into irregular jagged curves |
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| Luster | Visual reflection | Metallic (shiny chrome) vs. |
| | of surface light | Non-metallic (vitreous, dull) |
+---------------------+-----------------------+-------------------------------+
The Three Great Rock Families
A rock is an aggregate of one or more minerals, mineraloids, or solid organic matter. Petrologists classify rocks into three great families based on their geological mode of formation:
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| THE THREE PRIMARY ROCK FAMILIES |
| |
| 1. IGNEOUS ROCKS: Formed by cooling & crystallization of magma/lava |
| - Intrusive (Plutonic): Cooled slowly underground; Large crystals |
| - Extrusive (Volcanic): Cooled rapidly at surface; Micro-crystals |
| |
| 2. SEDIMENTARY ROCKS: Formed by weathering, deposition, & lithification|
| - Clastic: Compacted rock fragments (Sandstone, Shale) |
| - Chemical: Precipitated minerals from solution (Limestone, Halite) |
| - Organic: Compacted biological remains (Coal, Chalk) |
| |
| 3. METAMORPHIC ROCKS: Formed by solid-state alteration via heat/press |
| - Foliated: Layered mineral banding (Slate, Schist, Gneiss) |
| - Non-foliated: Massive recrystallized grains (Marble, Quartzite) |
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Igneous Rocks: Fire-Formed Minerals
Igneous rocks crystallize from molten rock:
- Intrusive (Plutonic) Igneous Rocks: Form when magma is trapped deep beneath Earth's surface, insulated by surrounding rock. The magma cools slowly over hundreds of thousands of years, allowing atoms time to migrate and assemble into large, interlocking visible mineral crystals (phaneritic texture). Granite and gabbro are common intrusive rocks.
- Extrusive (Volcanic) Igneous Rocks: Form when lava erupts onto Earth's surface or ocean floor. Exposed to cool air or water, the lava cools rapidly within hours or days. Crystals have minimal time to grow, producing microscopic grains (aphanitic texture, such as basalt) or quenching instantly without crystals to form volcanic glass (obsidian).
Sedimentary Rocks: Layers of Geological History
Sedimentary rocks form at or near Earth's surface through the accumulation, compaction, and cementation of mineral particles and organic matter:
- Clastic Sedimentary Rocks: Composed of weathered rock fragments (clasts) transported by water, wind, or ice. Sediments are deposited in horizontal beds, compacted under overlying weight, and cemented by minerals (calcite, silica) precipitating from groundwater. Classified by grain size: conglomerate (gravel), sandstone (sand), and shale (clay).
- Chemical Sedimentary Rocks: Form when dissolved mineral ions precipitate out of water due to evaporation or chemical changes (e.g., rock gypsum and rock salt/halite).
- Organic (Biogenic) Sedimentary Rocks: Composed of the accumulated shells or remains of living organisms (e.g., limestone composed of calcium carbonate shells, and coal formed from compacted ancient swamp plants).
Metamorphic Rocks: Transformed by Heat and Pressure
Metamorphism occurs when pre-existing parent rocks (protoliths) are subjected to extreme heat (200 to 800 degrees Celsius) and confining lithostatic pressure deep within the crust, without melting into liquid magma. Metamorphism is a solid-state recrystallization:
- Foliated Metamorphic Rocks: Subjected to directed tectonic stress. Platy mineral grains (like mica) recrystallize and reorient perpendicular to the direction of maximum stress, creating parallel layered bands or foliation. As metamorphic grade increases, shale transforms through a progressive sequence: Shale --------> Slate --------> Schist --------> Gneiss.
- Non-Foliated Metamorphic Rocks: Composed of single-mineral crystals that recrystallize under uniform confining pressure, forming interlocking crystalline textures without mineral banding (e.g., limestone recrystallizes into marble; quartz sandstone recrystallizes into quartzite).
The Dynamic Rock Cycle
The rock cycle is not a linear, one-way conveyor belt; it is an interconnected web where any rock type can be transformed into any other rock type through Earth's internal and external geological engines.
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| THE DYNAMIC ROCK CYCLE WEB |
| |
| +-------------------+ |
| | IGNEOUS ROCK | |
| +---------+---------+ |
| ^ | \ |
| Melting | | \ Weathering & |
| & Cool | | \ Erosion |
| | | Heat & \ |
| | | Press. v |
| | v +-------------------+ |
| +-----------+-----+--+ | SEDIMENT | |
| | METAMORPHIC ROCK | +---------+---------+ |
| +--------------------+ | |
| ^ | Lithification |
| | Heat & Pressure v |
| +----------------+-------------------+ |
| | SEDIMENTARY ROCK | |
| +-------------------+ |
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Exercises and Solutions
Exercise 1: Tracing an Atom of Silicon Through the Rock Cycle. Trace the complete journey of a single silicon atom across all three rock families over a 300-million-year geological cycle, detailing the geological processes that transform it from an intrusive igneous rock into a sedimentary rock, then into a metamorphic rock, and back into magma.
Solution 1: The silicon atom's journey through the rock cycle proceeds through the following geological transformations: First, 300 million years ago, the silicon atom bonds with oxygen within a deep magma chamber, crystallizing slowly over millennia to form a quartz mineral crystal (SiO2) within a coarse-grained intrusive granite pluton. Second, tectonic uplift and overlying crustal erosion expose the granite batholith at Earth's surface. Third, physical freeze-thaw weathering and carbonic acid chemical weathering disintegrate the granite; rainwater flushes the resistant quartz sand grain down mountain rivers into a coastal beach. Fourth, over millions of years, accumulating sand layers bury the grain under kilometers of sediment; compaction and silica cementation lithify the loose sand into a clastic sedimentary sandstone rock. Fifth, continental collision subjects the sandstone to regional metamorphism, where temperatures reach 450 degrees Celsius and tectonic pressures recrystallize the quartz grains into interlocking crystals, transforming the sandstone into a non-foliated metamorphic quartzite. Sixth, continued subduction carries the quartzite deeper into the asthenosphere, where temperatures exceed 1,000 degrees Celsius, melting the rock back into silicate magma, completing the cycle.
Exercise 2: Differentiating Foliated and Non-Foliated Metamorphism. A geology student examines two metamorphic rock specimens under a polarizing light microscope. Specimen 1 displays alternating, zebra-like bands of light quartz and dark biotite mica minerals. Specimen 2 displays an interlocking mosaic of calcite crystals that effervesces vigorously when hydrochloric acid is applied, with zero mineral banding. (a) Classify Specimen 1 as foliated or non-foliated, identify the rock type, and explain the differential stress mechanism that created its texture; (b) Classify Specimen 2 as foliated or non-foliated, identify the rock type, and name its sedimentary protolith.
Solution 2: (a) Specimen 1 is a foliated metamorphic rock, specifically gneiss. The distinctive zebra-like banding (gneissic banding) was produced by high-grade regional metamorphism under differential directional tectonic stress. Compressional forces squeezed the rock, forcing platy and elongated minerals to segregate chemically and align perpendicular to the direction of maximum stress, separating light-colored felsic minerals (quartz and feldspar) into distinct layers from dark mafic minerals (biotite mica and amphibole). (b) Specimen 2 is a non-foliated metamorphic rock, specifically marble. Its vigorous effervescence with hydrochloric acid confirms it is composed of calcium carbonate (CaCO3). Marble forms under thermal contact metamorphism or uniform confining pressure, where the calcite grains of its sedimentary protolith—limestone—recrystallize into a dense, interlocking crystalline mosaic without directional mineral alignment or foliation bands.