⛏️Intro to Geology Unit 3 – Igneous Rocks: Formation and Intrusion

What Are Igneous Rocks?

• Form from the cooling and solidification of magma (molten rock beneath Earth's surface) or lava (molten rock above Earth's surface)
• Comprise one of the three main rock types along with sedimentary and metamorphic rocks
• Classified based on their mineral composition and texture
• Lack fossils due to their formation process involving extreme heat
• Can form intrusive (plutonic) rocks below Earth's surface or extrusive (volcanic) rocks above Earth's surface
  • Intrusive rocks cool slowly, allowing larger mineral crystals to form (granite)
  • Extrusive rocks cool rapidly, resulting in smaller mineral crystals or glassy texture (basalt)
• Play a crucial role in the formation of Earth's crust and volcanic landforms
• Provide valuable resources such as building materials (granite countertops) and precious metals (gold deposits)

The Rock Cycle: Where Igneous Rocks Fit In

• Igneous rocks form a fundamental part of the rock cycle, a continuous process of rock formation, transformation, and destruction
• Begin as magma or lava, which cools and solidifies into igneous rocks
• Can be exposed to weathering and erosion, breaking down into sediments
  • Sediments can accumulate and undergo compaction and cementation to form sedimentary rocks
• May be subjected to high temperatures and pressures deep within the Earth, transforming into metamorphic rocks
• Metamorphic rocks can melt, forming magma and restarting the igneous rock formation process
• Plate tectonic processes, such as subduction and uplift, play a crucial role in the rock cycle by recycling rocks and driving the formation of new igneous rocks
• The rock cycle operates on geologic timescales, typically millions to billions of years

Magma: The Source of Igneous Rocks

• Magma consists of molten rock, dissolved gases, and suspended crystals
• Forms primarily in the upper mantle or lower crust due to an increase in temperature, a decrease in pressure, or a change in composition
• Generates through various processes, including partial melting of mantle rocks, decompression melting during uplift, and flux melting by the addition of volatiles (water)
• Rises through the crust due to its lower density compared to surrounding solid rocks
• Composition varies depending on the source rock and the extent of melting
  • Mafic magma: Rich in magnesium and iron, forms from partial melting of the mantle (basaltic)
  • Felsic magma: Rich in silica, potassium, and sodium, forms from partial melting of the crust (granitic)
• Can differentiate and evolve as it rises and cools, resulting in a range of igneous rock compositions
• May be stored in magma chambers, where it can cool slowly and undergo further differentiation before solidifying or erupting

Types of Igneous Rocks

• Classified based on their mineral composition and texture
• Felsic rocks: Light-colored, rich in silica, potassium, and sodium minerals (quartz, feldspars)
  • Examples include granite, rhyolite, and obsidian
• Mafic rocks: Dark-colored, rich in magnesium and iron minerals (olivine, pyroxene)
  • Examples include basalt, gabbro, and peridotite
• Intermediate rocks: Contain a mix of felsic and mafic minerals
  • Examples include andesite and diorite
• Ultramafic rocks: Very dark-colored, extremely rich in magnesium and iron minerals
  • Examples include komatiite and dunite
• Pyroclastic rocks: Form from the consolidation of volcanic ash, pumice, and other ejected materials
  • Examples include tuff and volcanic breccia

Intrusive vs. Extrusive Formations

• Igneous rocks form either beneath Earth's surface (intrusive) or above it (extrusive)
• Intrusive (plutonic) formations:
  • Magma cools and solidifies slowly beneath the surface, allowing large mineral crystals to grow
  • Typically form large, coarse-grained rocks (granite, gabbro)
  • Associated with the formation of batholiths, stocks, and dikes
• Extrusive (volcanic) formations:
  • Magma reaches the surface as lava and cools rapidly, resulting in small crystals or glassy texture
  • Typically form fine-grained rocks (basalt, rhyolite) or glassy rocks (obsidian)
  • Associated with the formation of lava flows, volcanic cones, and pyroclastic deposits
• The cooling rate and pressure conditions determine the final texture and appearance of the igneous rock
• Intrusive and extrusive formations can occur in various tectonic settings, such as divergent boundaries (mid-ocean ridges), convergent boundaries (subduction zones), and hotspots (mantle plumes)

Textures and Structures in Igneous Rocks

• Texture refers to the size, shape, and arrangement of mineral grains or crystals within an igneous rock
• Phaneritic texture: Coarse-grained, with mineral crystals large enough to be visible with the naked eye (granite)
• Aphanitic texture: Fine-grained, with mineral crystals too small to be seen without magnification (basalt)
• Porphyritic texture: Large, well-formed crystals (phenocrysts) embedded in a fine-grained groundmass (porphyritic basalt)
• Glassy texture: Lacks visible crystals due to rapid cooling, resulting in a smooth, glass-like appearance (obsidian)
• Vesicular texture: Contains numerous small cavities (vesicles) formed by trapped gas bubbles in rapidly cooling lava (pumice)
• Structures in igneous rocks can provide insights into their formation and emplacement
  • Flow banding: Alternating light and dark bands indicating the flow of partially molten lava or magma (rhyolite)
  • Columnar jointing: Polygonal columns formed by the contraction of cooling lava or magma (Giant's Causeway basalt)
  • Xenoliths: Foreign rock fragments embedded in the igneous rock, representing pieces of the surrounding rock incorporated during magma ascent or emplacement

Key Minerals in Igneous Rocks

• Igneous rocks are composed of various minerals, depending on the composition of the magma and the cooling conditions
• Felsic rocks:
  • Quartz: Silicon dioxide ($SiO_2$), clear to white, hard, and resistant to weathering
  • Feldspars: Potassium and sodium-rich minerals, including orthoclase and plagioclase
  • Muscovite: A mica mineral, characterized by its thin, sheet-like appearance and light color
• Mafic rocks:
  • Olivine: Magnesium and iron-rich silicate mineral, typically green in color
  • Pyroxenes: Single-chain silicate minerals, including augite and enstatite
  • Amphiboles: Double-chain silicate minerals, such as hornblende
  • Biotite: A dark-colored mica mineral, rich in iron and magnesium
• Accessory minerals:
  • Magnetite: Iron oxide mineral, strongly magnetic
  • Ilmenite: Titanium-iron oxide mineral
  • Zircon: Zirconium silicate mineral, often used for radiometric dating
• The relative abundance and composition of these minerals can be used to classify igneous rocks and infer the conditions of their formation

Real-World Examples and Formations

• Igneous rocks and formations can be found worldwide, each with unique characteristics and geologic significance
• Batholiths: Large, intrusive igneous bodies formed by the slow cooling of magma deep within the Earth's crust (Sierra Nevada Batholith, California)
• Lava flows: Extrusive igneous formations resulting from the flow and cooling of lava on Earth's surface (Columbia River Basalt Group, Washington)
• Volcanic cones: Landforms created by the accumulation of lava and pyroclastic materials around a volcanic vent (Mount Fuji, Japan)
• Dikes and sills: Sheet-like intrusions of magma that cut across or parallel to pre-existing rock layers (Shiprock dike, New Mexico)
• Columnar jointing: Spectacular formations resulting from the contraction of cooling lava or magma (Devil's Tower, Wyoming)
• Geodes: Hollow, spherical rocks lined with mineral crystals, often formed in volcanic rocks (Keokuk geodes, Iowa)
• Precious metal deposits: Igneous intrusions can be associated with the formation of valuable mineral deposits, such as gold, silver, and copper (Grasberg Mine, Indonesia)
• Igneous rocks and formations not only shape Earth's landscape but also provide valuable resources and insights into our planet's geologic history