W5 Igneous rocks and volcanoes
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what is it called when magma solidifies underground?
an intrusion
how does magma rise into pre-existing rock
it percolates between grains or forces open cracks
what do you call the boundary between intrusive rock and wall rock?
intrusive contact
what is the baked zone? (intrusive contact)
where the rim of country rock is heated and altered by magma contact
what is the chill margin? (intrusive contact)
magma at the contact cools rapidly compared to the chamber’s interior
what is a dike? what does it do? where does it dominate?
• tabular intrusion cutting up through pre-existing layering • they spread rocks sideways • they dominate in extensional tectonic settings (rift zones, mid-ocean ridges)
what is a sill? where does it intrude? what does it do?
• a tabular intrusion that parallels rock fabric • intrudes along horizontal discontinuities • lifts up the entire landscape
what is a laccolith (what does it resemble?)? what happens to the overlying strata? what causes a laccolith?
• a lens-shaped intrusion resembling an umbrella or mushroom • overlying strata are pushed upward into a dome • magma blocked by overlying rock accumulates instead of spreading laterally
what is a pluton? how do plutons form? what do they create?
• Pluton is a large underground body of igneous rock that forms when magma cools and crystallizes beneath the Earth’s surface. • They form as magma chambers cool and solidify underground • They create vast batholiths that are eventually exposed by erosion
What is a batholith?
a vast granitic landscape from ancient intrusions
What are the 4 characteristic features of volcanoes?
• magma chamber • fissures and vents • craters • calderas
Where are magma chambers located? What are they? How do they move?
• They are located in the upper crust • they are underground cavities or highly fractured zones storing molten rock • they inflate and deflate as magma accumulates or is expelled
What is a vent? what is a fissure? (volcanic outlets)
• vent is a discrete lava outlet in a volcano • fissure is a linear tear in the crust
Where can vents form? give names
Anywhere on the volcano – summit vent is located on the top, flank vent is located on the side
What is a crater? How is it formed? Where does its vent often lie?
• bowl shaped depression at volcano’s summit • forms as lava and tephra accumulate around the vent or when the summit collapses into a drained conduit post-eruption • vent often lies at the floor of the crater, serving as the primary eruption outlet
what is a caldera? how does it form? what can happen to it?
• large volcanic depression, much bigger than a crater • forms when a magma chamber empties, causing the volcano to collapse into the evacuated space • some calderas fill with water, forming lakes
What are 3 products of volcanic eruptions?
• volcanic gases (water vapor, CO2, sulfur-rich aerosols) • lava flows (molten rock that spreads over the ground, cooling to form solid rock) • pyroclastic debris (fragments of lava, ash, and rock explosively ejected from a volcano)
What is an effusive eruption? What does it form? What type of lava?
Low-viscosity basaltic lava flows from vents in molten rivers, travelling 10-500 km before solidifying, forms shield volcanoes and large igneous provinces
What is the difference in emplacement of basalt above and below the water surface?
Under water: forms pillow basalts Above water: forms columnar basalts from lava cooling over 10-100 years, contracting into hexagonal columns due to thermal shrinkage
Why do explosive eruptions occur?
high viscosity felsic or andesite magma traps volcanic gas and builds pressure
what is a pyroclastic flow?
it is the collapse of a silica-rich eruption cloud; superheated air cushions downwards flow of hot ash, gas, and debris making them travel rapidly and destructively
what controls volcano shape and size?
magma composition
what are the names of volcano types, in order of size from biggest to smallest? what are they made of?
• shield volcanoes - repeated basaltic lava flows • stratovolcanoes - build from repeated eruptions with alternating layers of lava and tephra • cinder cones - built from ejected lapilli-size fragments as a result of explosive eruptions of viscous intermediate-to-felsic lava
What is a phreatomagmatic eruption?
An eruption that occurs when magma chambers admit water through infiltration which causes instantaneous vaporization and explosive expansion