Igneous Rocks and Temperature – Vocabulary Flashcards
Igneous Rocks: Intrusive vs Extrusive, Crystallization, and Magma Evolution
Overview
- Igneous rocks form by crystallization from molten rock (magma) when it cools and solidifies.
- Key divisions: extrusive (surface) vs intrusive (at depth).
- Extrusive rocks form on the surface from lava or pyroclastic material; intrusive rocks crystallize below the surface.
- Pyroclasts: rocks and fragments ejected during explosive volcanic activity; form under high heat/pressure and can travel as solid chunks through the air.
Extrusive vs Intrusive: definitions and implications
- Extrusive: formed at the surface; rapid cooling; textures tend to be fine-grained or glassy due to quick solidification.
- Intrusive: formed beneath the surface; cooling is slow; textures tend to be coarse-grained (visible crystals).
- Despite different textures, extrusive and intrusive rocks of the same chemical composition have related mineralogy (same mineral groups in different proportions depending on cooling history).
- Field and lab evidence help prove a common magma source for both, including magma interaction with preexisting rock (country rock).
Country rock, xenoliths, and magma interaction
- Country rock: rocks that existed before magma intrudes.
- Xenoliths (country rock fragments) can become incorporated into magma as it rises, often seen as inclusions within the magma or in erupted rocks.
- This incorporation implies that intrusions and eruptions can carry pieces of surrounding rock into the crystalizing body.
- In some cases, the magma can assimilate country rock and even melt parts of it, contributing to the magma’s evolving composition.
- Pyroclasts during explosive eruptions can also contain fragments of country rock.
Texture, grain size, and laboratory observation
- Texture is largely controlled by cooling rate:
- Slow cooling yields coarse grains (phaneritic textures).
- Rapid cooling yields fine grains (aphanitic textures).
- Very rapid cooling or quenching can produce glassy textures (not specifically covered in the transcript but relevant to texture).
- Lab work complements field observations to classify rocks by grain size and texture.
Composition-based classification (felsic to ultramafic)
- Rocks are divided into four broad compositional classes:
- felsic (silica-rich)
- intermediate
- mafic
- ultramafic (silica-poor)
- The primary divisor is silica content (SiO2). Silica content correlates with abundance of other oxides:
- As SiO2 increases, Fe and Mg typically decrease in abundance relative to Na, K, and Ca.
- High silica (felsic) rocks are generally more enriched in light-colored minerals (e.g., quartz, feldspars) and lighter colored minerals; ultramafic rocks are dominated by Mg- and Fe-rich minerals (e.g., olivine, pyroxene).
- Contemporary teaching often uses four classes for both intrusive and extrusive rocks:
- felsic
- intermediate
- mafic
- ultramafic
- The border among these classes is primarily defined by silica content, with other elements varying in a way that maintains relative chemical trends.
Magma as a hot, volatile-containing melt
- Magma composition is dominated by silicate framework structures built from
- silica tetrahedra:
\mathrm{SiO_4}^{4-} - allied cations and oxides: Fe, Mg, Na, K, Ca, Al, and trace constituents.
- Volatiles in magma are small in proportion but crucial: mostly water (H2O) and carbon dioxide (CO2); other gases and solids (crystals, xenoliths) are present in smaller fractions.
- Water and CO2 released from volcanoes can significantly influence atmospheric composition over geological timescales.
Where magma comes from: generation and sources
- The upper mantle contains rocks like peridotite; magma generation is linked to temperature and pressure conditions.
- Pressure tends to suppress melting; higher temperatures promote melting. Melting requires either heating, decompression (pressure drop), or increased volatile content.
- Mantle processes include:
- Decompression melting at tectonic boundaries (e.g., mid-ocean ridges, hot spots) where pressure decreases.
- Addition of volatiles (e.g., H2O) lowering melting points in rocks to produce magma.
- Xenoliths of mantle material (e.g., peridotite) can accompany magmas as they travel upward, offering direct clues about deeper sources.
- The mantle and crust interactions can produce magmas with a range of compositions through assimilation and mixing.
Crystallization sequences and the Bowen concept
- Crystallization is not random; minerals crystallize in a predictable sequence as temperature falls:
- Olivine → Pyroxene → Amphibole → Biotite → Feldspar (plagioclase and/or K-feldspar) → Quartz
- This sequence is driven by the silica tetrahedra framework evolution and the stability of minerals at given temperatures and pressures.
- As minerals crystallize, they often sink (fractional crystallization) due to higher density, excluding them from the remaining melt and changing the melt’s composition.
- The result of fractional crystallization is a progressive evolution from ultramafic to felsic compositions as cooling proceeds.
- The Bowen temperature window typically spans roughly from about
T \,\approx\, 1300^{\circ}C \quad \text{down to} \quad 750^{\circ}C
for systematic crystallization in magma bodies. - Quartz has one of the lowest crystal stability temperatures, so it tends to crystallize last; if the melt is heated again, quartz would melt first.
- The concept of fractionation explains how a single original magma body can produce a variety of rock types, depending on how crystals separate from the melt.
Fractional crystallization vs assimilation vs mixing
- Assimilation: incorporation and partial melting of surrounding country rock into the magma, altering its composition.
- Magma mixing: interaction and blending of magmas from different sources, creating intermediate compositions.
- All these processes contribute to the diversity of igneous rocks beyond the simple intrusive/extrusive dichotomy.
The ascent of magma: buoyancy, viscosity, and transit time
- Why magma rises: it is less dense than surrounding solid rock when hot, so buoyancy drives upward movement.
- The ascent rate is governed by viscosity (η): high viscosity slows ascent; lower viscosity speeds ascent.
- Viscosity is controlled by temperature and composition:
- Higher temperatures reduce viscosity (hotter magmas flow more easily).
- Silica-rich magmas (felsic) are more viscous than silica-poor magmas (mafic/ultramafic).
- Typical ascent speeds and timescales (as described in the lecture):
- Distances of hundreds of kilometers of crustal travel can occur over timescales of thousands of years, with speeds of a few centimeters per year to perhaps kilometers per hour during rapid ascent episodes.
- The path to eruption can involve fracturing of country rock, magma chamber dynamics, and interactions with existing rocks during ascent.
Rock types in the intrusive-extrusive spectrum
- Intrusive and extrusive rocks span a range from felsic to ultramafic, with characteristic textures:
- Granitic/gabbroic family (felsic to mafic) for intrusive rocks: e.g., granite (felsic), diorite (intermediate), gabbro (mafic).
- Rhyolite/andesite/basalt for extrusive counterparts: rhyolite (felsic), andesite (intermediate), basalt (mafic).
- Ultramafic examples (more common in the mantle, rarely at the surface):
- Dunite (mostly olivine)
- Peridotite (dominant mantle rock)
- Komatiite (ultramafic extrusive, largely of historical significance but rare today)
- The labs referenced in the course will typically feature granite, rhyolite, diorite/andesite, basalt, diorite/gabbro, and ultramafic rocks like peridotite or dunite; komatiite may appear in specific contexts.
The mantle, xenoliths, and mantle-crust interactions
- Mantle-derived rocks (e.g., peridotite) can be brought to shallower levels by tectonic processes and carried into eruptive products as xenoliths.
- Xenoliths provide a window into depth, pressure, and temperature conditions that are otherwise inaccessible.
- The occurrence of mantle-derived material in volcanic rocks supports the idea of magmas that originate deep in the Earth and migrate upward through the crust.
The role of magma in Earth’s system and the real-world relevance
- Magmatic processes influence atmospheric composition through volcanic outgassing (notably CO2 and H2O).
- Large-scale volcanic activity has climate and environmental implications over geological timescales, including links to mass extinctions and environmental crises.
- Understanding magma generation, ascent, crystallization, and differentiation helps explain rock diversity, plate tectonics, and crustal evolution.
Quick recap: key relationships to remember
- Cooling rate controls texture: slow cooling yields coarse grains; fast cooling yields fine grains.
- Composition controls mineralogy: higher SiO2 generally means more felsic minerals; lower SiO2 correlates with mafic/ultramafic minerals.
- Magma evolves through crystallization (Bowen sequence) and/or melting and mixing; fractional crystallization changes melt composition over time.
- Buoyancy and viscosity govern magma ascent; temperature and silica content significantly influence viscosity.
- Country rock interactions (assimilation and xenoliths) record magma’s path through the crust and its depth of origin.
Glossary of important terms
- Magma: hot, molten rock beneath the surface.
- Lava: magma that reaches the surface.
- Country rock: preexisting rock surrounding a magma intrusion.
- Xenolith: a fragment of country rock embedded in magma or erupted rocks.
- Pyroclasts: fragments of rock ejected during explosive volcanic eruptions.
- Intrusive (plutonic) rocks: rocks that crystallized below the surface.
- Extrusive (volcanic) rocks: rocks that crystallized at or near the surface.
- Felsic: silica- and light-element-rich, high silica rocks (often light-colored minerals like quartz and feldspars).
- Intermediate: composition between felsic and mafic.
- Mafic: silica-lower, magnesium- and iron-rich rocks.
- Ultramafic: very low silica content with high Mg and Fe content (e.g., olivine-dominated rocks).
- Bowen’s reaction series: the sequence of mineral crystallization with decreasing temperature.
- Fractional crystallization: removal of early-formed crystals from melt, progressively changing melt composition.
- Decompression melting: melting induced by pressure decrease rather than temperature increase.
- Peridotite: mantle rock composition; main source for many mantle-derived magmas.
- Komatiite: ultramafic extrusive rock associated with high degrees of mantle melting in Earth's history.
Equations and key numerical references (LaTeX)
- Magma temperature range:
630^{\circ}C \le T \le 1300^{\circ}C - Bowen’s crystallization temperature window (illustrative):
T \approx 1300^{\circ}C \rightarrow 750^{\circ}C - Silica tetrahedron unit (conceptual):
\mathrm{SiO_4}^{4-} - General trend: as silica content increases, Fe/Mg content tends to decrease while Na/K/Ca can vary, altering mineral stability and viscosity.
- Magma temperature range:
Notes for study and lab preparation
- Be able to distinguish intrusive vs extrusive textures by grain size and cooling rate.
- Recognize the four compositional classes and the role of silica in defining them.
- Understand how magma generation, ascent, and crystallization produce the diversity of rocks.
- Be familiar with the concept of country rock and xenoliths as field evidence for a shared magma source and crustal interaction.
- Know the basic Bowen sequence and its implications for mineralogy and rock type evolution.