How and where does magma form?

Class 2 - From rock to mush to magma

Sébastien Biass

sebastien.biasse@unige.ch

Group of physical volcanology and geological risk - 1st floor

February 23, 2026

Last week on…

• What is the structure of the Earth?
• Where does magma/lava come from?
• What allows it to rise to the surface?

The Earth’s structure

The world in 1665

Mundus Subterraneus, Kircher, 1665

The source

• Debate between liquid (→eclogite) and solid (→peridotite) source
• In the 50’s:

1. Seismology
   • S-wave through mantle → must be solid
   • Velocity profiles → elastic properties
2. Petrology
   • Basalts can be produced by partial melting of peridotite

S and P waves (Source)

Reminder: Seismic waves

The layered structure

	Masse (kg)	%
Croûte continentale	1.38 x 1022	0.23
Croûte océanique	0.69 x 1022	0.11
Manteau	398 x 1022	66.6
Noyau	197 x 1022	32.9
Total	5.97 x 1024

Velocity and profiles (Source)

Classification and limits based on

• Composition
• Mineralogical phases

• Rheology
• Seismic behviours

• Last piece to the modern view of the Earth’s layered structure
• During evolution → Earth differentiated and stratified
  • Iron core
  • Silicate mantle
  • Light crust
• Seismic velocity jumps = phase transitions, not chemical boundaries

Structure of the Earth: Crust

	Oceanic crust	Continental crust
Thickness	6–7 km	30–70 km
Density	~2.9–3.0 g·cm⁻³	~2.7 g·cm⁻³
Composition	Mafic	Felsic to intermediate
Age range	0–200 Ma	Up to ~4 Ga
Recycling	Easily subducted	Rarely subducted

→ The Mohorovičić discontinuity - or Moho:

• Region in the lithosphere defined by distinct changes in velocities of seismic waves
• Depth: Ocean floors: 5-10 km - Continental crust: 20-90 km

The Moho is the boundary between: - Crust (felsic to mafic rocks) - Upper mantle (ultramafic peridotite) It was discovered by Andrija Mohorovičić in 1909 from a sudden increase in seismic wave speeds.

Property	Crust	Upper Mantle
Rock type	Basalt / Granite	Peridotite
Density	~2.7–3.0 g/cm³	~3.3–3.4 g/cm³
Vp (P-wave)	~6–7 km/s	~8–8.5 km/s
Vs (S-wave)	~3.5–4 km/s	~4.5–4.8 km/s

Structure of the Earth: Upper mantle

• Lithosphere: Rigid part of crust + upper mantle (10-280 km thick)
  • Long term → Ductile deformation
  • Short term → Brittle failure
  • = Plate tectonics → earthquakes

• Astenosphere: Upper part of upper mantle below lithosphere (100-700 km thick)
  • Deeper → warmer
  • Ductile deformation though solid
  • Acts as a viscous fluid over long timescales

• Thickness of the lithosphere is considered to be the depth to the isotherm associated with the transition between brittle and viscous behavior.[2] The temperature at which olivine becomes ductile (~1,000 °C or 1,830 °F) is often used to set this isotherm because olivine is generally the weakest mineral in the upper mantle
• Lithosphere + astenosphere ± = same
  • Same composition
  • Solid
• Behaviour changes

Parameter	Lithosphere	Asthenosphere
Temperature	Lower	Higher
Viscosity	Very high	Lower (but still huge)
Dominant deformation	Elastic / brittle	Ductile creep
Stress storage	Yes	No
Earthquakes	Yes	No
Long-term flow	No	Yes

Structure of the Earth: Mantle

• Compositional layer
• ± solid but can flow
• Different rheologies

• Upper mantle → 410 km deep
  • Directly sampled!
• Transition zone → 670 km deep
  • Mineralogical phase boundary
• Lower mantle → 2890 km deep

Transition zone - Impacts convection - Some subduction slabs cannot penetrate deeper

What is the structure of the Earth?

• What is the structure of the Earth?
  • Mantle is ± solid
  • Flows like a ± liquid over long time scales

• Where does magma/lava come from?
• What allows magma to rise to the surface?

Where does magma/lava come from?

Melting the source: Peridotite

Lherzolite - Source

Source

Dense, coarse-grained ultramafic igneous rock composed primarily of olivine, with lesser amounts of pyroxene. It forms in the upper mantle and is high in magnesium and iron, containing less than (45%) silica

Simple fluids → water

• One phase / composition
• One melting curve (solid–liquid equilibrium)

Key features:

• Sharp phase boundaries
• No temperature interval of partial melting
• Solid ↔︎ liquid transition occurs at a single T for a given P

Complex fluids → magma

• Several phases / compositions
• Different minerals melt at different temperatures
• Melting → temperature interval

Key features:

• Solidus = temperature where first melt appears
• Liquidus = temperature where last crystal disappears

• Many components (SiO₂, Al₂O₃, FeO, MgO, CaO, Na₂O, K₂O, H₂O…)

Peridotite solidus vs geotherm

Melting it happen through…

→ Shift geotherm upwards

→ Shift geotherm to the right

→ Decrease solidus

Decompression

Increased T°C

Hydratation

Broad geodynamic contexts

Fusion conditions: Mantle

• Liquidus is never reached
  • Fusion is always partial
  • Partial melting
  • 2-20% extracted from solid matrix

Fusion conditions: Crust

• Crustal anaxtesis
  • Thickening of continental crust
  • Intrusion
  • 100% melting possible (though rare)

• Orogénèse → accumulation d’éléments radioactifs dans les roches granitiques → prolonge géotherme
• Aussi possible si intrusions

Melting initiation

Chazot et al. (2017)

Parfitt et al. (2023)

• Melt starts accumulating at triple junctions
• Lithostatic + pore pressure → stress → crack → vein

How does the melt escape the matrix?

Darcy’s law: \(q = \frac{K}{\eta} \frac{dP}{dx}\)

\(q\): flux of melt volume of melt per unit time passing a unit cross-sectional area perpendicular to the flow direction

Under pression/temperature conditions in the upper mantle, melts have lower densities than solid.

• Driver: Density contrast

• Allows: Porosity + permeability

• Prevents: Viscosity

• What drives the melt to escape?

  • Density contrast → buoyancy
  • Pressure contrast → pressure gradient in upper mantle → crust

• What allows the melt to escape?

  • Melting creates porosity
  • Coalescence connects pores and creates permeability
  • Percolation = movement of melt through an already connected pore network

• What prevents the melt to escape?

  • Viscosity
  • Permeability

Formulate an intuition-based model!

Using Darcy’s law as a framework, use your intuition to identify what quantity/physical processes

• drive the melt to escape?
• allow the melt to escape?
• prevent the melt from escaping?

Try and construct a narrative of what happens, highlighting when which process/quantity is important

Two-phase porous flow

Darcy’s law: \(q = \frac{K}{\eta} \frac{dP}{dx}\)

• \(q \ [LT^{-1}]\): Flux of melt
• \(K \ [L^2]\): Permeability
• \(\eta \ [ML^{-1}T^{-1}]\): Fluid viscosity
• \(dP/dx \ [ML^{-2}T^{-2}]\): Pressure gradient

Permeability: \(K = \frac{d^{2}\,\phi^{\,n}}{C}\)

• \(K \ [L^{2}]\): Permeability
• \(d \ [L]\): Grain size
• \(\phi \ [dimensionless]\): Porosity melt fraction
• \(n \ [dimensionless]\): Porosity exponent
• \(C \ [dimensionless]\): Geometric tortuosity

Melting the rock

Rock:

• Subsolidus: No melt
• Supersolidus: Unconnected melt pockets

Melt connectivity transition:

• Gabbro: ~ 1% melt
• Granite: ~7% melt

Subsolidus rock

Supersolidus rock, after Sparks et al. (2019)

From melt to mush

Melt → Mush:

• Large reduction in effective viscosity and strength
• Rheology controlled by crystalline network

Mush-magma transition:

• Typically 30–50% melt
• Depends on crystal size distribution and shape, and shear rate

Melt-poor mush

Melt-rich mush, after Sparks et al. (2019)

From mush to magma

Mush → Magma: Large reduction in effective viscosity and strength

• Rheology controlled by melt

• Mush: Non-eruptible
• Magma: Eruptible

Crystal-poor magma

Crystal-rich magma, after Sparks et al. (2019)

Where does magma come from?

• What is the structure of the Earth?
  • Mantle is ± solid
  • Flows like a ± liquid over long time scales
• Where does magma/lava come from?
  • Melting of upper mantle (dominantly), crust possible
  • Melting require extra mechanism for geotherm to cross solidus

• What allows magma to rise to the surface?

What allows magma to rise?

The Earth machine

Heat

Lockwood et al. (2022)

Density

Detay (2017)

The Earth gradients

 

Layer	Depth range (km)	Density min (g/cm³)	Density max (g/cm³)	Temp min (°C)	Temp max (°C)
Lithosphere	0–100	1.02	3.38	0	1091
Asthenosphere	100–250	3.36	3.44	1123	1371
Upper mantle	250–660	3.44	4.15	1373	1600
Lower mantle	660–2890	4.16	7.36	1603	3698
Outer core	2890–5150	7.70	12.45	3700	5198
Inner core	5150–6371	12.48	13.09	5200	6000

Mantle convection

Source

• Driven by temperature contrasts → primary heat from earth formation + radioactivity
• Simple to imagine, complex in practice
  • Mantle plumes are still debated
  • Probably different shallow/deep regions of convection
• If ridge push dominated, plates would move symmetrically away from ridges — they don’t.

Plate tectonics

Principal plates today and their velocities (cm yr^-1). After Detay (2017)

Divergent, convergent and transform plate boundaries

• Dozen major plates + numerous minor ones
• Compare the spreading velocities
  • Those attached to subducting plates are faster

Age of the oceanic crust

Where are the plates

• Young → creation
• Older → recycling
• Old → passive margin

Driving forces

Driving forces:

• Ridge push
• Slab pull

Resisting forces:

• Trench friction
• Basal drag

 

Source

→ What force (ridge push/slab pull) dominantly drives plate motion?

• In other words,
  • continental plates are slow (due to increased drag)
  • and oceanic plates that are attached to subduction zones are fast
• These results imply that slab pull is the primary driving force of plate motion

Geodynamic contexts of volcanism

 

Chazot et al. (2017)

Divergence: Mid oceanic ridges

• Most active volcanic feature on Earth
• 65’000 km long

Divergence: Types of ridges

Slow

Intermediate / fast

• 1-5 cm yr^-1 (→ Mid-atlantic ridge)
• 10-20 km width, focus of volcanism within graben
• Normal faults → hydrothermal systems
• Limited magma production → discontinuous in time and space
• No magma reservoir → coalescence of small magma pockets
• Usually small edifices → destroyed by plate movements

• Intermediate: (→ Juan de Fuca) to fast: 9-18 5-9 yr^-1 (→ Southeastern Indian ridge)
• Axial dome → central ridge <5 km width + no graben
• Off axis volcanoes up to tens of km from center
• Some ± mature magma reservoir
• Higher magma supply → diversity in emplacement processes

Convergence: Subduction

• 40’000 km
• Importance of age
  • Young → thin → hot → flat (10-30°)
  • Old → thick → cold → steep (up to 80°)

• Steep:
  • Slab rollback + break-off
  • Can reach lower mantle

Steep (top) vs flat (bottom) slabs (Source)

• Faible pendage: Cascades, Paama, Costa Rica, Chili central, Andes australes
• Pendage élevé: Mariannes
  • Slab retreat
  • Can reach lower mantle → crust recycling

Convergence: Types of arcs

Arc types after Chazot et al. (2017)

Classification based on geodynamic context + nature of the lithosphere

• Reflect accretion speeds
• Variable length + duration

• Arcs intraoceaniques: plaque oceanique
  • Usually one is colder/heavier → Isu-Bonin
  • Or there was the formation of an oceanic plateau
• Péri-continentaux: continental thickness of 20-40 km
• Andin: Plus epais → andin → altiplano

accretion speeds

• Atlantic + mediteranean: 2 arcs
• Indian: 1 huge (Sumatra + Java)
• Rest: Pacific

age Philippines/Indonesia = micro-plates, rapidly changing

Convergence: Complexity

Japan

Aeolian islands

• 4 subduction zones
• Different ages

• 2 subduction zones
• 3 different plates

Intraplate

Intraplate volcanoes younger than 1 Ma (Chazot et al. (2017))

Intraplate: Plumes & hot spots

• Tomography → down to 400-500 km depth
• Long- vs short-lived → differences in magma production
• Very complex, still debated → several models

3 main types:

1. Major → directly from D layer
2. Secondary → lower mantle dome
3. Shallow from astenosphere convection

Intraplate: Continental

Mostly lithospheric material

Massif central:

• Thermal anomaly → lithosphere thinning
• Astenosphere flux → Alpes
• Thermal erosion

Other sources of astenosphere flux:

• Slab break-off → Etna
• MOR subduction
• Convection around cratons
• Hot spots → Canary islands

Intraplate: Large igneous provinces

Self et al. (2014)

1. Large: A>10⁵ km², V>10⁵ km³,
2. Long: Up to 50 Ma, multiple paroxysms (1-5 Ma) → 75% of the volume
3. Intraplate

Intraplate LIPS: Traps and Oceanic plateaux

• Large volumes + short durations → Ma
• High (5-15%) fusion rates from plume head
• Diameter → 500’s km
• Importance for strategic minerals

Intraplate LIPS: Colombia River Flood

Kasbohm and Schoene (2018), Kasbohm (2022)

• Miocene → 17-15.5 Ma
• 175’000 km³ in 1.5 Ma, maximum thickness of 3’500 m
• 300 individual flows of mean volume = 580 km³
• Maximum flow length = 600 km without changes in composition

Intraplate LIPS: Rifts

Passive rifting

Driven by far-field plate extension

Active rifting

Driven by mantle plume

→ Ratio of tectonic vs volcanic drivers in accomodating extension

Intraplate LIPS: Active rift

Ethiopian rift

• Large mantle plume
• Mainly astenosphere → storage close to Moho

• Axial volcanoes → Erta Ale, Oldoino Lengay
  • Composition: plumes
• Marginal volcanoes → Kilimandjaro
  • Composition: lithospheric component → 40 km-thick crust
  • Convection around Tanzanian craton

• Afar plume (maybe merged with Kenya and Levant plumes)
• Lithospheric contamination by Tanzanian craton → craton splits the rift

Interacting contexts: MOR-Plume

Location of main current hot spot in relation to MOR (Chazot et al. (2017))

1. Independent, plume gets nearer
2. Onset of interaction→ geochemical indication of mixing
3. Overlap → increase in magma supply → oceanic plateau / island (Iceland)
4. Some overlap, decrease in magma supply (Galápagos, Açores)
5. Independent systems, no indication of mixing

Interacting contexts: Back-arc bassin

Stern and Dickinson (2010)

• Transition from convergence → divergence
• Often associated with slab rollback, retreat or break-off

How does magma rise??

• What is the structure of the Earth?
  • Mantle is ± solid
  • Flows like a ± liquid over long time scales
• Where does magma/lava come from?
  • Melting of upper mantle (dominantly), crust possible
  • Melting require extra mechanism for geotherm to cross solidus
• What allows magma to rise to the surface?
  • Heat gradient → convection
  • Density gradient → buoyancy
  • = Plate tectonics

Take home messages

Summary

Internal Earth Structure

The Earth is internally layered with distinct compositional and rheological boundaries. The mantle is solid but behaves as a viscous fluid over geological timescales, flowing through convection.

Partial Melting Requirements

Magma forms through partial melting of mantle rock (peridotite), which requires one of three mechanisms - i) decompression, ii) increased temperature or iii) hydration - to bring the geotherm across the solidus.

Degrees of Melting

Melting in the mantle is always partial (2–20% melt), never reaching the liquidus temperature. In contrast, crustal anatexis can occasionally produce complete melting (~100%).

Summary

Melt Escape Mechanics

Melt escapes from solid rock through interconnected pore networks, driven by density contrasts and pressure gradients → less dense melt buoyantly rises through the surrounding rock.

Melt Transitions

The progression from isolated melt pockets → mush → magma depends on melt fraction (~1% connectivity threshold, 30–50% mush-to-magma transition), determining whether material is eruptible or not.

Magma Ascent Balance

The rise of magma depends on the competition between buoyancy (driving force from density contrasts) and viscosity/mechanical resistance (resisting force), modulated by crustal stress and plate geometry.

Summary

Plate Tectonics as primary driver:

Plate motion, driven primarily by slab pull, controls where and how magma forms globally by modulating pressure, temperature, and stress in the crust and mantle.

Mid-Ocean Ridge Volcanism

The largest volcanic features on Earth (65,000 km), mid-ocean ridges continuously produce magma via decompression melting as plates diverge.

Subduction Zone Volcanism

Subduction generates volcanoes through hydration of the descending slab and decompression melting in the mantle wedge above.

Intraplate Volcanism

Intraplate volcanism is complex and reflects diverse processes (e.g., plume activity, lithospheric thinning, and astenospheric flow).

Exercise

Outline

• Some partial melting has created a batch of magma at depth

Aim:

• Identify processes/properties that drive/resist the magma to rise to the surface
  • Intrinsic → magma-related
  • Extrinsic → host rock-related

Intrinsic parameters

• Magma = melt + crystals + volatiles
• At depth:
  • Volatiles = dissolved
  • Crystals = absent/small

• How will these phase change with:
  • Time?
  • Depth?
  • Temperature?

• What properties do these phases control?

Extrinsic parameters

• Thermal gradient
• Pressure gradient
• Behaviour → ductile vs brittle
• Heterogeneities
  • Composition
  • Density
  • …
• Stresses → geodynamics
• Mature/immature systems

Questions

1. What allows melt to escape the deep reservoir?
2. What controls alert melt ascent?
3. What controls whether melt will rise, stop, create a new reservoir?
4. What controls melt evolution in shallow storage?
5. What causes magma to reach the surface?
6. What controls explosive vs effusive eruptions?

References

Bibliography

Chazot G, Lénat J-F, Maury RC, et al (2017) Volcanologie. De Boeck Supérieur, Louvain-la-Neuve Paris

Detay M (2017) Traité de volcanologie physique, 2nd edn. Lavoisier, Paris

Kasbohm J (2022) Flood basalt buildup warms climate. Nature Geoscience 15:342–343. https://doi.org/10.1038/s41561-022-00944-z

Kasbohm J, Schoene B (2018) Rapid eruption of the Columbia River flood basalt and correlation with the mid-Miocene climate optimum. Science Advances 4:eaat8223. https://doi.org/10.1126/sciadv.aat8223

Lockwood JP, Hazlett RW, de la Cruz-Reyna S (2022) Volcanoes: Global perspectives, 2nd edn. Wiley-Blackwell

Parfitt E, Wilson L, Kerber L (2023) Fundamentals of physical volcanology, 2nd edn. Wiley-Blackwell

Self S, Schmidt A, Mather TA (2014) Emplacement characteristics, time scales, and volcanic gas release rates of continental flood basalt eruptions on Earth. In: Volcanism, Impacts, and Mass Extinctions: Causes and Effects. Geological Society of America

Sparks RSJ, Annen C, Blundy JD, et al (2019) Formation and dynamics of magma reservoirs. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 377:20180019. https://doi.org/10.1098/rsta.2018.0019

Stern RJ, Dickinson WR (2010) The Gulf of Mexico is a Jurassic backarc basin. Geosphere 6:739–754. https://doi.org/10.1130/GES00585.1