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Understanding Various Rock Formations and Textures

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Learn about the formation processes of igneous, sedimentary, and metamorphic rocks, along with their unique textures, through detailed explanations and illustrations.

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Understanding Various Rock Formations and Textures

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  1. Rock Formations Notes

  2. Rocks • Three types of rocks: • Igneous - formed from molten rock (i.e.,magmaor lava) • Sedimentary - formed from the debris of other rocks • Metamorphic - rocks that have been subjected to great stresses and have changed their texture or chemical state from their original form

  3. Igneous Rock Formation • Magma (molten rock) originates deep within the interior of the Earth • Rises to surface because it is less dense than surrounding rock • As the magma rises, it cools and begins to crystallize • Similar to water freezing into ice

  4. Igneous Rock Formation • Magma – molten rock below Earth’s surface • Lava - Molten rock at Earth’s surface • Both cool to form crystals called igneous rock.

  5. Igneous Textures • The rate at which the magma (or lava) cools determines the texture, or crystal sizes, of the igneous rock • The texture also yields clues to the type of geologic setting in which the igneous rocks formed

  6. Igneous Textures • Fine-grained • Grains (crystals)usually too small to be seen with naked eye • Indicative of a fast cooling lava or magma.

  7. Igneous Textures • Coarse-grained • Grains (crystals) can be seen with the naked eye • Indicative of slow cooling history

  8. Igneous Textures • Porphyritic (Fine and Coarse grain together) • Rock composed of both large and small crystals • Indicative of both slow and fast cooling history • Shows large and small crystals

  9. Igneous Textures • Glass • No identifiable crystals • Indicative of instantaneous cooling • Example: Obsidian

  10. Textures of Igneous Rocks

  11. Sedimentary Rocks • Two types of sedimentary rocks • Detrital- formed from the debris of other rocks • Chemical - formed through chemical processes

  12. Processes involved in the formation ofsedimentary rocks • Weathering - The breaking down of rocks to form sediment • Erosion - The transport of sediment (rock debris) • via wind, rivers, animals, etc. • Deposition - The settling of rock debris in some depositional basin (low-lying area where sedimenttransportation stops • Generally a lake, ocean, or swamp.

  13. Processes involved in the formation ofsedimentary rocks • Compaction - as sediment accumulates on ocean floor, the gradually increasing weight of overlying sediment reducing the amount of pore space between grains • Cementation - liquids can flow through the pore spaces between loose sediment. As this occurs, minerals may precipitate(form) from the liquid and “cement” sediments together. • Common cements include silica and calcite.

  14. Fossils only found in sedimentary rock.

  15. Metamorphic Rocks • Occurs when rocks are subjected to conditions unlike those when the rock first formed • Causes a change in the texture and/or composition of the original rock

  16. Agents of Metamorphism • Heat • Very important because heat drives chemical reactions that result in recrystallization • Increased heat can be caused by • The contact of a magma • The subduction(lowering) of rocks to greater depths • Pressure • Increased pressure caused by • Subductionof rock to greater depths • Forces of mountain building • Hydrothermal Fluids

  17. Types of Metamorphism • Contact • Native rock comes into contact with magma • Heat is the primary metamorphic agent • Regional • Associated with large-scale mountain building • Heat and pressure are primary metamorphic agents

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