Volcanos

IGNEOUS MELT

  • Volcanic Landform:

    • Defined as a physical feature on Earth’s surface created by volcanic activity including lava, ash, or magma.

  • Three Features that Control Igneous Landforms:

    1. Magma chemistry

    2. Gas content

    3. Eruption/rupture style (size of opening).

  • Viscosity of a Melt:

    • Definition: Resistance to flow.

    • Increases with:

    • More silica

    • More crystals

    • More dissolved gas

    • Lower temperature.

  • Temperature and Viscosity:

    • Higher temperature → lower viscosity (more fluid).

    • Lower temperature → higher viscosity (thicker).

  • Igneous Chemistry and Viscosity:

    • Lowest Viscosity:

    • Mafic (basaltic)

    • Highest Viscosity:

    • Felsic (rhyolitic).

  • Trapped Gases:

    • Produce vesicles (gas bubbles/holes in the rock).

  • Largest Geologic Structures on Earth:

    • Large igneous provinces (LIPs) / basalt plateaus.

  • Composition, Gas Content, and Rupture Opening Impact:

    • Low silica + low gas + large opening → fluid lava, wide shield volcanoes.

    • High silica + high gas + small opening → explosive eruptions, cinder cones/composite volcanoes.

    • Very high silica + trapped gas → lava domes.

SHIELD VOLCANOES

  • Lava Flows Characteristics:

    • Very low-viscosity; flows easily and far.

    • Low viscosity results in broad, gently sloping volcano shape.

  • Lava Flow Quantity:

    • Created from hundreds to thousands of thin, stacked lava flows.

  • Lava Chemistry:

    • Mafic (basaltic).

  • Characteristics of Lava Flows:

    • Pahoehoe:

    • Smooth, ropy, forms from very fluid lava; flows fast.

    • Aa:

    • Rough, jagged, rubbly; forms when lava cools or flows more slowly.

  • Pillow Basalts:

    • Rounded “pillow-shaped” basalt formed when lava erupts underwater.

  • Basalt Plateau:

    • A huge, thick pile of flood basalt formed from massive, repeated fissure eruptions.

  • Coloration of Mafic Rocks:

    • Dark color primarily results from iron (Fe) and magnesium (Mg).

CINDER CONE VOLCANOES

  • Composition:

    • Made 100% out of loose pyroclastic fragments (cinders, ash, lapilli).

  • Pyroclastic Materials Classification:

    • Classified by size:

    • Ash – smallest (powder-like)

    • Lapilli – pebble-sized (2–64 mm)

    • Blocks – solid chunks > 64 mm

    • Bombs – > 64 mm but molten/soft when thrown.

  • Formation of Pyroclastic Materials:

    • Bombs: Blobs of molten lava ejected into the air that solidify while flying.

    • Blocks: Solid rock chunks blasted out without melting.

    • Spatter: Blobs of still-molten lava that land and weld together.

  • Agglomerate (Volcanic Breccia):

    • A rock made of large, angular volcanic fragments cemented together.

  • Comparison in Size:

    • Cinder cones are much smaller than shield volcanoes, typically measuring hundreds of meters tall.

  • Color of Sunset Crater:

    • Red color due to oxidation (iron-rich lava fragments rusting in air).

COMPOSITE VOLCANOES (STRATOVOLCANOES)

  • Size Comparison:

    • Larger than cinder cones, smaller than shield volcanoes.

    • Typically feature steep, tall, classic “cone-shaped” mountains.

  • Viscosity and Eruption Characteristics:

    • Higher viscosity due to higher silica (intermediate to felsic), leading to explosive eruptions.

  • Structure Composition:

    • Layers of:

    • Andesitic to rhyolitic lava flows

    • Pyroclastic layers (ash, lapilli, bombs)

    • Volcanic debris.

  • Shape Description:

    • Tall, steep-sided, symmetrical cone with alternating layers.

  • Pyroclastic Materials Classification:

    • Classified by size:

    • Ash (fine), lapilli (small pebbles), blocks/bombs (large fragments).

  • Formation of Pyroclastic Materials:

    • Bombs: Molten blobs that solidify in the air.

    • Blocks: Solid rock ejected explosively.

    • Spatter: Molten droplets that land and weld together.

  • Agglomerate (Volcanic Breccia):

    • Made of large, angular, volcanic fragments.

  • Cinder Cone Size Comparison:

    • Cinder cones are much smaller than composite volcanoes (few hundred meters tall).

  • Sunset Crater Color:

    • Iron-rich cinders oxidized (“rusted”).

PHYSICAL FEATURES OF IGNEOUS ROCKS

  • Porphyritic Texture in Andesite:

    • Characterized by large crystals (“phenocrysts”) embedded in a fine-grained groundmass.

  • Pyroclastic Flow:

    • A mixture of hot gas + ash + rock racing down a volcano.

    • Formation: Results from the collapse of eruptive columns or lava domes.

    • Speeds: Ranges from 100–450 mph.

    • Hazards: Capable of burning, burying, or obliterating everything in its path, considered extremely deadly.

    • Historical significance: Killed tens of thousands since the 1700s (over 29,000+ identified).

  • Lahar:

    • Definition: A mudflow consisting of volcanic ash mixed with water.

    • Formation: Occurs from melted snow/ice, rainfall, or crater lake water.

    • Hazards: Can bury towns, destroy bridges, and travel extremely quickly.

    • Survival Tips: Recommended to move to high ground immediately.

  • Mount Rainier:

    • Location of active volcanic hazard.

    • Eruptive cycle: Occurs every few hundred years.

    • Lahar Risk: Potential for lahars that could bury parts of Tacoma’s valley areas.

LAVA DOMES AND COLLAPSE CALDERAS

Lava Domes

  • Formation Characteristics:

    • Requires very high-viscosity, high-silica magma (felsic: rhyolite/andesite).

  • Materials Expelled:

    • Thick, pasty lava alongside gas-charged ash during small explosions.

  • Mechanics of Formation:

    • Magma oozes slowly upward, piles over the vent, solidifying near the surface and forming a steep mound.

  • Igneous Rock Formation:

    • Rocks formed in a lava dome primarily include rhyolite and obsidian, with some andesite.

  • Talus:

    • Defined as a pile of broken rock fragments at the base of a steep slope or dome.

  • Mount Lassen:

    • Description: A lava dome complex located in Northern California (Cascade Range); experienced its last eruption from 1914 to 1917.

Collapse Calderas

  • Crater Lake:

    • Description: A collapsed caldera filled with water, formed after Mount Mazama erupted.

    • Notable for its extreme depth and clear water located in the Cascade Range (Oregon).

  • Collapse Process:

    • Results from a massive eruption that empties the magma chamber, causing the ground above to collapse inward.

  • Rocks and Pyroclastics Associated:

    • Includes rhyolitic ash, tuff, welded tuff, pumice, and volcanic breccias.

  • Cascading Upward Process:

    • New magma rises, pressure builds, eruptions occur, and the chamber empties, leading to a cycle of collapse and eruption.

  • Formation of Lakes:

    • A lake forms as the large depression traps rain and snowmelt.

  • Tuff:

    • Defined as rock made from compacted volcanic ash; commonly has felsic (rhyolitic) chemistry.

YELLOWSTONE SUPERVOLCANO

  • Last Eruption: Approximately 640,000 years ago.

  • Comparison of Ash Eruptions: Considered much larger - known as a super-eruption.

  • Ash Travel Range:

    • Ash could spread across thousands of miles throughout the U.S. and beyond.

  • Kill Zone:

    • Refers to the region near the caldera where everything would be obliterated, spanning tens of miles around the eruption center.

  • Consequences of Ash Deposits:

    • Could lead to the collapse of roofs, destruction of crops, blocking sunlight, and global cooling, leading to travel shutdown.

PLUTONIC IGNEOUS LANDFORMS

  • Formation Location:

    • Develop deep underground where magma cools slowly.

  • Bowen’s Reaction Series:

    • Describes the crystallization order of minerals:

    • Mafic minerals crystallize first (e.g., olivine → pyroxene → amphibole → biotite)

    • Followed by felsic minerals (e.g., K-feldspar, muscovite, quartz).

  • Columnar Jointing:

    • Forms when thick lava or magma cools and contracts, resulting in cracking into polygonal columns.

  • Breccia Composition:

    • Made up of angular rock fragments.

  • Formation of Dikes, Sills, and Columns:

    • Dikes: Vertical or steeply angled intrusions that cut across geological layers.

    • Sills: Horizontal intrusions that are parallel to rock layers.

    • Columns: Vertical pillars formed due to cooling contraction.

  • Deep Plutonic Structures:

    • Include batholiths, stocks, laccoliths, lopoliths, and plutons.

  • Cooling Rate of Deeper Plutons:

    • Cool slower than those nearer the surface due to insulation by surrounding rock.

  • Common Minerals in Granite:

    • Typically contain quartz, feldspar (K-spar + plagioclase), biotite, and muscovite.

    • Characterized by phaneritic texture (large visible crystals).

  • Common Minerals in Gabbro:

    • Usually contains Ca-rich plagioclase, pyroxene, and sometimes olivine.

    • Also exhibits phaneritic texture.

  • Description of a Stock:

    • Defined as a small intrusive body (<100 km²).

    • Notable example: Stone Mountain in Georgia (granite).

  • Characteristics of Stone Mountain:

    • Identified as a large granite rock dome (a stock).

  • Elberton Batholith:

    • Rock Type: Granite.

    • Uses: Commonly utilized for monuments, gravestones, and building stone.

    • Size Comparison: Much larger than Stone Mountain.

  • Sierra Nevada Batholith:

    • Described as very large, extending for hundreds of kilometers.

    • Rock type present in Yosemite National Park: Granite.

  • Phaneritic Rocks:

    • Both granite and gabbro form deep underground from slow cooling.

  • Difference Between Stocks and Batholiths:

    • Batholith: Large intrusive body (>100 km²).

    • Stock: Smaller intrusive body (<100 km²), with both types commonly composed of the same rock types.