Igneous processes and volcanoes Flashcards
Recall the key rock-forming igneous minerals.
Feldspars, Quartz (SiO₂), Olivine (Mg,Fe)₂SiO₄, Pyroxenes, Amphiboles and Micas.
Explain where rock melts
Melting occurs in two places, the upper mantle and lower continental crust (magma production is very localised).
What is the Geotherm?
A heat profile of the Earth with depth.
What happens to the melting point of rock with depth and when does melting occur?
Melting point increase with pressure (so depth) and is marked by the anhydrous solidus. Melting can only occur if the geotherm intersects the solidus.
Decompression melting
-Most common cause of melting.
-No extra heat added only crust thinned.
-Shallows the geothermal gradient.
-How oceanic crust forms (70% of the earths surface).
Addition of water (melting)
-Adding water lowers the melting point.
-Solidus lowered below the geotherm.
-Occurs due to subduction.
-Changes the rocks chemistry.
Orogenic melting
-Convergent plate boundaries
-Cold under riding slab is warmed as rock is a poor conductor so can’t loose its heat.
-Very localised+rarely reaches the surface.
Does the geotherm cross the liquidus?
No, therefore mantle rock only ever undergoes partial melting where some mineral with lower melting points melt and rise.
explain why molten rock is typically either felsic (acidic) and mafic (basic) in composition
The composition of molten rock (magma or lava) is typically classified as either felsic (acidic) or mafic (basic) due to differences in silica content, mineral composition, and the tectonic settings where they form
Felsic Magma (Acidic)
-High silica content: >65% SiO₂.
-Silica-rich magma tends to be more viscous (thicker) and cools to form light-coloured rocks such as granite and rhyolite.
-High viscosity prevents gases from escaping easily, often resulting in explosive volcanic eruptions.
-Continental sources.
Mafic Magma (Basic)
-Low silica content: 45-52% SiO₂.
-Mafic magma is less viscous and flows more easily, forming dark-coloured rocks like basalt and gabbro.
-Low viscosity allows gases to escape more easily, typically leading to more effusive eruptions.
-Mantle sources.
Describe the key characteristic of intrusive igneous rocks in hand specimen and explain how they formed.
-Intrusive igneous rocks are coarse-grained, with visible crystals and an interlocking texture due to slow cooling beneath the Earth’s surface.
-They form in plutonic environments, where magma cools over long periods, allowing the growth of large, identifiable minerals.
Define a volcano.
A volcano is a geological feature where magma, gases, and volcanic ash escape from beneath the Earth’s surface through an opening, forming a vent, crater, or fissure.
Explain where volcanoes form.
Volcanoes form primarily at convergent boundaries (subduction zones), divergent boundaries (mid-ocean ridges and rift zones), and hotspots.
Define viscosity
A measure of a materials ability to flow. Controlled by temperature and composition (SiO2 and water content).
How does magma viscosity change with depth?
As magma ascends temperature decreases, H2O solubility decreases and crystal content increases. Therefore it gets more viscous as it ascends.
When does fragmentation occur?
When the total volume of gas exceeds the volume of magma.
What is the Vesiculation horizon?
The point when bubble begin to form in a rising magma.
Describe eruptive products of volcanoes.
-Lava (Pāhoehoe: Smooth, ropey lava flows. and ‘A‘ā: Rough, jagged, blocky lava.)
-Pillow lavas (submarine eruptions)
-Volatiles (Water Vapor, Carbon Dioxide, Sulfur Dioxide, Hydrogen Sulfide, Chlorine and Fluorine Gases.)
-Tephra (Volcanic Ash, Lapilli, Volcanic Bombs and Blocks and Pyroclastic flows.)
Controls on Eruptive Products.
-Magma Composition (higher silica content and gas content= more pyroclastic material.)
-Viscosity of the Magma (Low Viscosity: Magma flows easily, leading to lava-dominated eruptions.)
-Eruption Temperature (Higher Temperature: Lowers viscosity, favouring fluid lava flows.)
-Tectonic Setting (Convergent Boundaries: Produce andesitic and rhyolitic magma, leading to explosive eruptions.)
Name the four volcano types.
Shield, Strato, Lava domes and Cinder cones.
Shield volcanoes
-Basic in composition.
-Low viscosity.
-No pyroclastic.
-Large in size.
-Typically found at hotspots and divergent boundaries where mantle-derived magma surfaces.
Stratovolcanoes
-Acidic in composition.
-Moderate-high explosivity.
-High viscosity.
-Interstratified lavas and pyroclastic flows.
-Commonly found at convergent plate boundaries (subduction zones) where oceanic plates subduct beneath continental or other oceanic plates.
Lava Domes
-Basic-intermediate composition.
-Low explosivity.
-Moderate-high viscosity.
-Frequently occur within the craters of stratovolcanoes or as stand-alone features.
Cinder Cones
-Basic-acidic in composition.
-Low-moderate viscosity.
-Exclusively pyroclastic.
-Small, subsidiary to large volcanoes.
-Often found near larger volcanoes or in rift zones where magma rises through fractures.
