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

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

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

4. Intrusive 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

5. Intrusive Igneous Structures
Categorized by depth of emplacement
Epizonal
Mesozonal
Catazonal
Depth
Shallow <6-10 km
Intermediate ~8-14 km
Deep >~12 km
Contacts
Discordant
Variable
Concordant
Size
Small to moderate
Small to large
Contact metamorphism
Very common
Uncommon
Absent
Age
Cenozoic
Mesozoic-Paleozoic
Paleozoic or older

6. Intrusive Igneous Structures: Large Scale
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)

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

8. Intrusive Igneous Structures: Small 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)

9. 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

10. Extrusive Igneous Structures: Scale
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

11. Extrusive Igneous Structures: Eruption Styles
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

12. 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

13. 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

14. Types of Volcanic Products: Effusive
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
Lava Dome
Dacite or rhyolite (high viscosity, low gas content)
Thick, steep- sided flows

15. Types of Volcanic Products: Explosive
Tephra
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)
Bombs
Ash

16. 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

17. 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

18. Styles of Volcanic Eruption: Effusive
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

19. 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)
Mauna Loa, Hawaii

20. 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

21. Styles of Volcanic Eruption: Explosive
Cinder cone
Generally low viscosity, high gas content (basalt)
Subduction zone settings (also continental rifts and continental hot spots)
SP Crater, Arizona
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

22. Styles of Volcanic Eruption: Explosive
Composite cones and Stratovolcanoes
Generally higher viscosity, high gas content (andesites)
Dominantly subduction zone settings
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)

23. Styles of Volcanic Eruption: Explosive
Calderas and pyroclastic sheet (ignimbrite) deposits
Generally high viscosity, high gas content (rhyolite)
Subduction zone and continental hot spots
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)

24. Volcanic Hazards ~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