3: Igneous Processes and Volcanoes

recall the key rock-forming minerals

feldspars, quartz, olivine, pyroxenes, amphiboles and micas

explain where and how rock melts

crystallise from magma which is molten rock in the subsurface and usually consists of liquid silicate, crystals and gas bubbles

why do magmas form?

Interior of the earth is hot:

• accretion of planetesimals = kinetic energy = heat

• initially so hot that most of it melted

• gravitational energy from iron sinking to the core

• radioactive decay

continental crust

andesitic ‘intermediate’; quartz, feldspar, Fe, Mg

Upper Mantle

peridotite ‘ultrabasic’; olivine and pyroxenes

oceanic crust

basaltic ‘basic’; olivine, pyroxenes and plagioclase

what happens to melting point with increased pressure and depth

it increases

this is shown by the solidus line: melting occurs when the geotherm intersects this

explain why molten rock is typically either felsic (acidic) or mafic (basic) in composition

Magmas can be derived from mantle or crust. Composition is determined by:

1. Composition of the source rocks

2. Degree of partial melting

IRL total melting rarely happens due to differing melting points of minerals and Ts not high enough

Magmatic differentiation:

• lower T minerals melt, leaving behind residual higher T minerals

• results in a melt that is more felsic

Also, Melting usually occurs in the asthenosphere (ultrabasic) or lower continental crust (intermediate) (~100km) through the partial melting of mantle peridotite

mafic vs felsic

• mafic contains less silica and is darker in colour

• felsic rocks contain more silica

melting due to decompression (asthenosphere)

• most common cause in practice

• oceanic crust is extended and thinned - this removes the lid over the mantle and the shallow geothermal gradient is steepened

• no heat is introduced, basaltic melt is produced through partial meltinf of the peridotite mantle

• this is how oceanic crust is produced at mid-ocean ridges

• basic

melting due to addition of water (continental crust)

Adding of water to crystal structure of a mineral - usually creating a new mineral

• opposite of weathering reactions

• changes fundemental properties of rocks incl melting temperature

• water added via subduction

• felsic

melting in orogenic belts (continental crust)

complex, fundementally by heating

• continental-continental collision results in doubling of crustal thickness

• cold underlying slab is warmed, rock is a bad conductor of heat so the thich continental crust cannot lose radiogenic heat fast enough = results in heating of crust above granite solidus

• felsic melts

describe the key characteristic of intrusive igneous rocks in hand specimen and explain how they formed

• coarse-grained texture, resulting from the slow cooling of magma beneath the Earth's surface. This slow cooling allows large mineral crystals to form.

• less dense

• atoms are more randomly and less efficiently organised

• viscous (can flow) but this varies

• intrusive melts cool slowly as the rock is a poor conductor

melting distance/temp graphs

where do magmas go?

magmas rise (less dense, viscous) and can:

• crystallise beneath the earths surface

→ deep (>2km) - plutonic, fine grained

→ shallow (<2km) - hypabyssal, between plutonic and volcanic

• erupt at earths surface

  → volcanic, fine grained

what determines magma composition

1. Composition of the source rocks:
   → mantle peridotite = ultrabasic
   → continental crust = intermediate

2. Degree of partial melting
   → total melting of mantle peridotite = ultrabasic magma
   → total melting of average crustal rock = intermediate magma
   in reality total melting rarely happens

cooling and crystallisation of magmas

as temperature descends, high temp minerals form at their melting T, leaving behind a melt that is enriched in the components of lower T minerals

intrusive igneous rocks

• slow cooling results in formation of crystals visible to naked eye

• “phaneritic” texture

• cumulate and porphorytic textures from multi-stage cooling

name some other important differentiation processes

1. magma mixing : physical mixing of two to create one homogeneous one
   e.g. acidic magma mixes with basic magma to form intermediate

2. partial melting of wall rock and “stoping”

what are volcanoes and where do they form?

A rupture in a planets surface, through which molten rock, and/or ash and gases escape from an underlying magma chamber.

→ they can be extensional or contractional

→ lead to deposits on earths surface - both beneficial or hazardous

describe the eruptive products of volcanoes and the controls on these
1. Viscocity of magma and lava

Viscocity: measure of the capability of a magma to flow

Controlled by:

1. Temperature
   → viscocity decreases with increasing temperature

2. Composition
   → (SiO2 content) - acidic (felsic) melts are always more viscous than basic (mafic) melts
   → Water content - melts with high dissolved water content are always less viscous

describe three eruption style controls

3. Fragmentation: if viscocity and bubble pressure are high enough, an explosive eruption will occur

   2. Vesiculation horizon: H2O exolves and gas bubbles form. At the same time, T is decreasing and the crystal content increasing - viscocity increases

      1. High pressure: all H2O is dissolved in the melt

eruption style in submarine settings

common at:

→ mid ocean spreading centres: basic (mafic) melts

→ mantle hotspots beneath oceanic crust

→ early stages of Oceanic Arc formation (ocean-ocean subduction): often variable

submarine eruptions

• low viscocity lava erupts as pillow lavas: initially warm and ductile, inflating as magma enters it - eventually cracks and a new pillow is formed

• pillows reach angle of repose and begin to break and tumble down - forming aprons of pillow debris = stratification

• extends to lower water pressures, increasing bubble pressure

• eruption style becomes more explosive, pillows cease forming, replacing with hyaloclastites - glassy chaotic basaltic fragments

what are eruptions with magma-water interactions called

phreatomagmatic

on land eruptions

•Mantle hot spots beneath oceanic crust are dominated by partial melting of ultrabasic mantle peridotite

  → Melts are basic (mafic)

•Subducting ocean plates are dominated by partial melting of basic ocean crust and intermediate lower continental crust

  → Melts often variable

•Mantle hot spots beneath continental crust and continental rifting dominated by partial melting of the lower continental crust

→ Melts are acidic (felsic)

Lava flows and examples

• molten flowing rock on surface, more common when basic or intermediate lava is erupted

Pahoehoe: smooth, billowy lava surface. Hot, low-viscocity

‘A’a: rough, rubbly fragmented lava surface. Cooler, high-viscocity

• distruptive but not life threatening (slow)#

can form “columnar” jointing style

• colonnade: regular columns with 3-8 planar sides

• entablature: thinner, less regular with curving sides

Pyroclastics

• fragmented rock erupted solid from the volcanic event, more common in intermediate to acidic settings

• occur intwo forms: fall deposits and flow/surge deposits

what are the four main volcano types?

1. shield volcanoes

2. lava domes

3. composite cones/stratovolcanoes

4. cinder cones

shield volcanoes

• broad, gently sloping volcanoes formed by low-viscosity basaltic lava flows, typically resulting in large, wide edifices.

• e.g. Hawaii

• largest on earth and solar system

• little/no pyroclastics

stratovolcanoes

•Intermediate to acidic composition

• •Moderate to high explosivity (Strombolian to Plinian)

  •Moderate to high viscosity lavas and pyroclastics.

  •Made up interstratified pyroclastics and lavas.

  Archetypal volcano. Large, with concave-up, steep profile

  e.g. fiji, vesuvius

lava domes

•Basic to intermediate composition

•Low explosivity (Hawaiian)

•Moderate to high viscosity lavas.

•Little or no pyroclastics

•Made up interstratified lavas, and lava blocks.

•Small, often subsidiary (flank or caldera eruption) to larger stratovolcanoes.

cinder cones

•Basic to acidic composition

•Low to moderate viscosity lavas, which fountain, solidify in the air, forming lapilli-sized fragments which fall more-or less in place.

•Made up almost exclusively of these pyroclasts.

•Small, often subsidiary (flank or caldera eruption) to larger shield of stratovolcanoes. Often the product of a single eruption.

Chimborazo (furthest point from earths centre)

continent-ocean subduction (addition of water)

Mauna Kea, Hawaii

mantle hotspot (addition of heat)

Kilimanjaro (tallest mountain in Africa)

Continental rifting (removal of the lithospheric lid)

Mount Fuji, Japan

ocean-ocean subduction (addition of water)

Yellowstone, USA (last erupted 640 ka)

mantle hotspot (addition of heat)

hekla, iceland

• oceanic extension (removal of the lithospheric lid)

• mantle hotspot

Submarine eruption, Mid Atlantic

oceanic extension (removal of the lithospheric lid)