Gas bubbles play an important role in the behavior of magma beneath the Earth’s surface. As magma rises towards the surface, the decreasing pressure causes dissolved gases to exsolve and form bubbles. The growth and ascent of these bubbles strongly influences processes such as volcanic eruptions and the solidification of igneous rocks. Understanding where in the crust gas bubbles have the greatest effect on magma properties is key to unraveling these complex systems.
What causes gas bubbles to form in magma?
Magma contains significant amounts of dissolved volatile species, predominately water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), and hydrogen sulfide (H2S). At the high pressures found deep underground, these gases are soluble in the silicate melt. However, as magma ascends towards lower pressures, the capacity of the melt to hold dissolved volatiles decreases substantially. This results in the nucleation and growth of gas bubbles, which accumulate and rise through the magma.
The exact depth where gas exsolution and bubble formation begins depends on factors like the original dissolved gas content, the rate of ascent, and the magma composition. However, for typical magmas, bubble formation often starts at depths of 3-10 km. At shallower depths, bubbles continue to grow as more gases exsolve from the surrounding melt.
How do rising bubbles affect magma properties?
The growth and ascent of gas bubbles have several important effects on the physical properties of the host magma:
- Density decrease – Bubble growth decreases the density of the magma. At low bubble fractions, magma density decreases linearly with increasing bubble content. At high bubble fractions (>50 vol%), magma density drops exponentially as the bubbles impinge and coalesce.
- Vesiculation – Accumulation of bubbles causes the magma to vesiculate (develop vesicles). High bubble fractions transform it into a foam or froth.
- Viscocity decrease – Bubbles reduce magma viscocity, causing it to flow more readily. Viscous magmas can be transformed into low-viscosity foams by vesiculation.
- Permeability increase – Bubbles enhance the permeability of magma, allowing easier outgassing. The permeability increase can be exponential at high bubble fractions.
- Buoyancy increase – Density reduction increases the buoyancy of magma, enhancing its ability to rise towards the surface.
These physical changes promote the ascent of magma through the crust and have major effects on its eruption behavior. Magma is essentially transformed into a mobile foam that can readily transport itself to the surface. Thus, bubble formation is a key prerequisite of volcanic eruptions.
How deep do bubbles influence magma properties?
The depth where bubbles begin to substantially influence the physical properties of magma depends on several factors:
- Gas content – Magmas with higher dissolved gas contents will begin vesiculating and changing properties at greater depths.
- Ascent rate – Faster rising magmas experience more rapid decreases in pressure, resulting in earlier bubble growth and property changes.
- Composition – More viscous, silica-rich magmas require higher bubble fractions before major property changes occur.
- Crystals – Extensive crystals hinder bubble growth and coalescence, delaying property changes.
- Shear zones – Flow through shear zones can promote localized degassing and bubble formation.
For typical magmas, major property changes due to bubble formation often begin at depths between 1-5 km. At shallower depths, effects like viscosity reduction and density decrease progressively intensify as the bubble fraction increases. Once a critical threshold is reached, the magma transitions to a low density, highly mobile foam.
Thus, the upper 1-5 km is generally considered to be the most critical “bubble formation zone” where gas exsolution has the greatest influence magma mobility and eruption potential. However, effects can be manifest at much greater depths in some systems.
How do bubbles affect volcanic eruptions?
Bubble growth produces fundamental changes that essentially prime magma for eruption. The key effects are:
- Reduced density increases buoyancy and enables magma rise.
- Lowered viscosity enables flow through cracks and conduits.
- Higher permeability allows efficient outgassing during ascent.
- Conversion to foam provides momentum for rapid ejection.
All these factors promote the acceleration and ascent of magma towards the surface. In many cases, it is the abrupt transition of magma to a vesicular foam that immediately precedes and triggers an eruption. Modeling studies show that this transition typically occurs at depths of <500 m for most eruptions. Shallower bubble formation allows more rapid outgassing, generating highly explosive eruptions like Vulcanian or Plinian events. In contrast, deeper bubble formation (<3 km) produces effusive events like Hawaiian eruptions. Thus, the depth where magma extensively vesiculates strongly controls the style and intensity of volcanic eruptions.
Effusive Eruptions
In effusive eruptions like those in Hawaii, bubble formation begins at great depth (>3 km) due to high gas contents in the magma. Extensive outgassing occurs during ascent, preventing the build-up of bubble overpressure. As a result, magma reaches the surface with a relatively low bubble content (<50 vol%), erupting relatively quiescent lava flows.
Explosive Eruptions
In explosive events, bubble formation begins at shallower levels (<1 km depth). Fast ascent prevents efficient outgassing, allowing bubble overpressure to build. This pressurized frothy magma accelerates into the surface, exploding from the volcanic vent in a highly violent manner. The expansion of bubbles is the driving force that generates the explosive energy.
How do bubbles affect crystallization of igneous rocks?
In addition to facilitating volcanic eruptions, gas bubbles also strongly influence the texture and mineralogy of igneous rocks that crystallize from magma. Bubbles provide nucleation sites for early-forming minerals and affect the growth patterns of crystals as magma solidifies.
Some key effects of bubbles on igneous textures include:
- Promoting groundmass microcrystals – Bubbles enhance nucleation of microlites.
- Inhibiting large crystal growth – Bubbles physically obstruct space and limit growth.
- Controlling mineral orientations – Bubbles align tabular crystals like micas perpendicular to their walls.
- Generating flow banding – Shearing around bubbles produces banding features.
- Forming amygdules – Secondary minerals infill cavities where bubbles resided.
- Causing auto-intrusion – Bubble growth can force late-stage melt into early formed crystals, fracturing them.
By influencing crystallization phenomena, bubbles play a key role in the phase transitions that produce the wide diversity of igneous rock types observed. Bubble density, size distribution, and deformation during crystallization have major effects on magmatic differentiation and final rock textures.
Observational Evidence
Various lines of evidence from real magma systems indicate that bubble formation and growth strongly affects magma properties at relatively shallow depths of <1-5 km:
- Seismicity – Increased seismicity is detected as bubble overpressure develops prior to explosive eruptions.
- Geodesy – Surface deformation indicates accelerated magma rise as bubbles decrease magma density.
- Petrology – Crystal fracture patterns record auto-intrusion driven by bubble growth.
- Volcanology – Transitions in eruptive style imply shifts in magma permeability due to vesiculation.
- Experimental – Bubble nucleation and growth observed in decompression experiments at relevant pressures.
Multiple disciplines studying active and extinct volcanic systems provide evidence that gas exsolution transforming magma rheology typically occurs at depths consistent with the upper crust “bubble formation zone.”
Conclusions
In summary, gas bubble formation exerts a major influence on magma properties and volcanic processes within the upper 1-5 km of Earth’s crust. The key conclusions are:
- Gas exsolution is driven by depressurization during magma ascent.
- Bubble growth substantially alters magma density, viscosity, permeability, and buoyancy.
- These changes promote magma rise towards the surface and volcanic eruption.
- Bubble effects also influence crystallization textures of igneous rocks.
- For typical magmas, the zone from 1-5 km depth represents a critical transition where bubble formation impacts magma mobility.
- Observational evidence from active and extinct systems confirms the significance of this shallow “bubble formation zone.”
Understanding bubble dynamics within this critical region provides fundamental insights into the inner workings of volcanic systems and processes by which magmas ascend and erupt on Earth.
