Intrusive and Extrusive Igneous Rock Structures

Intrusive and Extrusive Igneous Rock Structures Basalt dikes hosted in a granitoidpluton, with metasediment roof pendant; Wallowa Mts, Oregon

Intrusive (Plutonic) Magma cools slowly at depth Characteristic rock texture Characteristic structures Igneous Structures • Extrusive (Volcanic) • Magma cools quickly at surface • Characteristic rock textures • Characteristic structures

Intrusive Batholith Stock Lopolith Laccolith Volcanic neck Sill Dike Extrusive Lava flow or plateau Volcano (many types) Crater Caldera Fissure Igneous Structures

Contacts (boundary between two rock bodies) can be: Concordant Does not cross cut country rock (surrounding rock) structure, bedding, or metamorphic fabric Ex: laccolith, sill Discordant Cross cuts country rock structure Ex: dike, batholith, stock Intrusive Igneous Structures

Intrusive Igneous Structures • Categorized by depth of emplacement

Major scale intrusive bodies: Plutons Batholith: >100 km2 in map area (usually discordant) Stock: <100 km2 in map area Lopolith: dish-shaped layered intrusive rocks (concordant) Intrusive Igneous Structures: Large Scale

Concordant intrusives Sill: tabular shape Laccolith: mushroom-shaped Roof pendant (remaining country rock) Discordant intrusives Dike: tabular shape Volcanic neck: cylindrical Intrusive Igneous Structures:Intermediate Scale

Apophyses: Irregular dikes extending from pluton Veins: Tabular body filling a fracture (filled with 1-2 minerals) Xenoliths: Unrelated material in an igneous body Autoliths: Genetically related inclusions (related igneous material) Intrusive Igneous Structures: Small Scale

Extrusive Igneous Structures • Volcanism • Directly observable petrologic process • Redistributes heat and matter (rocks) from the interior to the exterior of the earth’s surface • Occurs in oceanic & continental settings • Volcano: • Anywhere material reaches earth’s surface

Large scale structures Lava plateau (LIP; flood basalt) Ignimbrite (ash flow tuff; pyroclastic sheet) Intermediate scale structures Shield volcano Composite volcano (stratovolcano) Caldera, crater Lava flow or dome Small scale structures Tephra (pyroclastic material) Lava flow features Cinder cone Extrusive Igneous Structures: Scale

Effusive Eruptions Lava flows and domes Erupted from localized fissures or vents Generally low silica content (basalt, “primitive” magma) Explosive Eruptions Tephra (fragmental material) Pyroclastic falls or flows Erupted from vents Generally high silica content (felsic, “recycled” magma) Photo glossary of volcano terms Extrusive Igneous Structures: Eruption Styles

Extrusive Igneous Structures: Eruption Controls • Two main controls on eruption style: • VISCOSITY • A fluid’s resistance to flow • Determined largely by fluid composition • DISSOLVED GAS CONTENT • Main magmatic gasses: H2O, CO2, SO2 (or H2S) • At high pressure, gasses are dissolved in the magma • At low pressure (near surface), gasses form a vapor, expand, and rise = “boiling” • Interaction controls eruption style: • Gas bubbles rise and escape from low viscosity magma = EFFUSIVE ERUPTION • Gas bubbles are trapped in high viscosity magma; increase of pressure = EXPLOSIVE ERUPTION

Extrusive Igneous Structures: Eruption Controls • Two main controls on eruption style: • VISCOSITY and DISSOLVED GAS CONTENT • In general, both viscosity and gas content are related to magma composition • High silica content –> higher viscosity, more dissolved gas • Low silica content –> lower viscosity, less dissolved gas

Lava Flow Dominantly basalt (low viscosity and gas) Thin and laterally extensive sheets Pahoehoe flows: smooth, ropey flows Aa or block flows: rough and irregular flows Baked zones: oxidized zones due to contact with high temperature lava flow Types of Volcanic Products: Effusive • Lava Dome • Dacite or rhyolite (high viscosity, low gas content) • Thick,steep-sidedflows

Bombs Tephra Ash Types of Volcanic Products: Explosive • Pyroclastic particles • Fragmental volcanic material (TEPHRA) • Vitric (glass shards) • Crystals • Lithic (volcanic rock fragments) • Broken during eruption of magma • Typically higher silica, high gas content • Categorized by size: • Ash (< 2.0 mm) • Lapilli (2-64 mm) • Blocks and bombs (>64 mm)

Types of Volcanic Products: Explosive • Pyroclastic fall (mainly Ash fall) • Material ejected directly from volcano (fallout, “air fall”) • Ash, lapilli (pumice, scoria), blocks, and bombs • Sorted (small particles carried further) • Laterally extensive, mantles topography • Pyroclastic flow (nueé ardante or ignimbrite) • Fast moving, high density flow of hot ash, crystals, blocks, and/or pumice • Follow topographic lows • Can be hot enough after deposition to weld, fuse vitric fragments

Types of Volcanic Products: Explosive • Hydroclastic Products • Water-magma interaction (phreatomagmatic) causes explosive fragmentation • Typically basaltic lavas • Any water-magma interaction (sea floor, caldera lake, groundwater) • Great volumes of hydroclastics on the sea floor and in the edifice of submarine volcanoes • Highly subject to alteration –> clay minerals, microcrystalline silica, and zeolite

Lava Plateaus and Flood Basalts (LIPs) Generally low viscosity, low gas content effusive lava flows (basalt) Hot spot and continental rift settings Great areal extent and enormous individual flows Erupted from fissures Examples (no modern): Columbia River Basalt Group Deccan Traps Styles of Volcanic Eruption: Effusive

Mauna Loa, Hawaii Styles of Volcanic Eruption: Effusive • Shield volcanoes • Generally low viscosity, low gas content effusive lava flows (basalt) • Hot spot and continental rift settings • Central vent and surrounding broad, gentle sloping volcanic edifice • Repeated eruption of mainly thin, laterally extensive lava flows • Modern examples: • Mauna Loa, Kiluaea (Hawaii) • Krafla (Iceland) • Erta Ale (Ethiopia)

Styles of Volcanic Eruption: Effusive • Submarine eruptions and pillow lava • Generally low viscosity, low gas content effusive lava flows (basalt) • Divergent margin (mid-ocean ridge) settings • Produces rounded “pillows” of lava with glassy outer rind • Can produce abundant hydroclastic material (shallow) • Modern examples: • Loihi, Hawaii

Cinder cone Generally low viscosity, high gas content (basalt) Subduction zone settings (also continental rifts and continental hot spots) SP Crater, Arizona Styles of Volcanic Eruption: Explosive • Small, steep sided pile of loose tephra (mainly lapilli, blocks, and bombs) • Scoria or cinder • Often form on larger volcanoes (shield or stratovolcano) • Modern example: • Parícutin, Mexico

Composite cones and Stratovolcanoes Generally higher viscosity, high gas content (andesites) Dominantly subduction zone settings Styles of Volcanic Eruption: Explosive Mayon Volcano Philippines • Composed of layers of loose pyroclastic material (fallout and flows) and minor lava flows, some shallow intrusions • Form from multiple eruptions over hundreds to thousands of years • Examples: • Mt. St. Helens, Mt. Rainier (USA) • Pinatubo (Indonesia)

Calderas and pyroclastic sheet (ignimbrite) deposits Generally high viscosity, high gas content (rhyolite) Subduction zone and continental hot spots Styles of Volcanic Eruption: Explosive Crater Lake, Oregon • Form by collapse of volcano following evacuation of the magma chamber • Often produce widespread ash, ignimbrite (pyroclastic flow) • Examples: • Krakatoa, Indonesia (modern example) • Crater Lake, Yellowstone (USA)

~500 million people live in high hazard regions Eruptions and hazards are largely predictable Main hazards: Tephra (mainly ash) Lava flows Pyroclastic flows Lahar Avalanche/landslide Volcanic gas Tsunami Climate change Volcanic Hazards

Intrusive and Extrusive Igneous Rock Structures