Igneous Petrology

Ians stuff

How can we study the Earth’s interior?

• Direct sampling (xenoliths up to 420 km depth)

• Remote sensing (seismic data for whole Earth)

• Laboratory experiments (recreating pressure and temperature conditions)

What are the two types of deformation?

    1. Ductile deformation (occurs in lower crust where temperature is higher, causing folded rocks)

   

    2. Brittle deformation (occurs in cooler conditions, causing rocks to snap and create faults)

What are the main types of faults?

• Normal faults - associated with extension/tensional forces

• Reverse faults - associated with compression/shortening forces

• Thrust faults - low angle reverse faults found in mountain belts

• Strike-slip faults - associated with horizontal movement

What are the different types of seismic waves?

Body waves:

• P-waves (longitudinal, fastest, can travel through deep earth)

• S-waves (transverse, slower than P-waves, can't travel through liquids)

   

    Surface waves:

• Love and Rayleigh waves (slower, shallow but very destructive)

What affects seismic velocity ?

    - Generally increases with depth

    - Affected by changes in mineralogy due to phase transitions

    - significantly changes at boundaries (like core-mantle boundary)

What

What pressures can experimental rock squashing achieve?

    - 25 GPa - recreates upper mantle conditions

    - 50 GPa - recreates lower mantle conditions

    - 130 GPa - recreates core-mantle boundary conditions

Why are earthquakes important for studying Earth's interior?

    - They produce seismic waves that reveal Earth's structure

    - Wave behavior changes at different boundaries and materials

    - Help identify different layers and discontinuities in Earth's interior

What are seismic discontinuities?

They relate to phase changes in the deep Earth (density and/or change of state)

What is the age of the solar system and how does it compare to the Universe?

The solar system is 4.568 Ga old, while the Universe is approximately 13.8 Ga old

What is nucleosynthesis and where does it occur?

• Nucleosynthesis is the formation  of large stars (at least 4x the mass of the Sun)

• Occurs by three processes:

  • Fusion: elements fuse together up to Fe

  • Neutron capture (r and s processes): neutrons get added to the fused elements until unstable (radioactive)

  • proton capture (p processes): added to produce heavy isotopes

What is the solar nebula and how did it form?

    The solar nebula is a disk-shaped ring that forms around a new sun when:

    - Dust clouds from supernovas contract under gravity

    - Angular momentum causes the cluster to rotate faster as it contracts

    - This rotation creates the disk shape

What are the two main processes in planet formation?

1. Condensation:

    - High temperature compounds condense from gas

    - Temperature decreases with distance from sun

    - Results in rocky inner planets and gas/ice giant outer planets

   

    2. Coalescence:

    - Condensed droplets collide and merge

    - Larger objects develop gravity and sweep up smaller objects

    - Forms planet-sized objects

What are the main types of meteorites and what do they tell us?

    1. Chondrites:

    - Building blocks of planets

    - Record earliest solar system history

    - Contains CAIs (oldest material, 4568±0.2 Ma)

   

    2. Achondrites:

    - Processed/melted during planetary formation

    - Record information about planetary accretion

    - Show evidence of early chemical differentiation (4546±15 Ma)

What is special about pre-solar grains?

- Not vaporized at solar nebula stage

    - Include materials like diamond, graphite, and various oxides

    - Recognized by extreme isotopic anomalies

    - Pre-date the solar system

How do elements behave differently during Earth’s formation?

    - Volatile elements: lost during accretion to space

    - Siderophile elements: go to the core

    - Lithophile and refractory elements: redistributed between crust and mantle

How did the Moon form?

- Formed by collision of Mars-sized planet (Theia) with early Earth ~4.5 Ga

    - Both bodies had already differentiated into metallic core and rocky shell

    - Impact blew away rocky shell to form Moon

    - Impactor's core was absorbed by Earth

What are the differences between terrestrial and Jovian planets?

• Terrestrial Planets (Mercury, Venus, Earth, Mars):

    - Rocky composition

    - Rich in refractory compounds

    - Poor in volatiles

   

• Jovian Planets (Jupiter, Saturn, Uranus, Neptune):

    - Massive

    - Rich in volatiles

    - Have rocky components

    - Some moons still volcanically active

What is the oldest dated terrestrial material and how old is it?

Jack Hills Zircons from Australia, dated at approximately 4.4 Ga

What are the key temperature points in rock melting?

- Solidus: Temperature below which the system is entirely solid

    - Liquidus: Temperature above which the system is entirely liquid

    - Between these temperatures, rock is partially molten

What is a geotherm and how does it vary?

    - Geothermal gradient describes temperature variation within Earth (°C km-1)

    - Temperature rises faster with depth beneath oceans than continents

    - This is because oceanic crust is thinner than continental crust

What is the difference between one-component and two comonent systems?

One-component systems:

    - Single component (e.g., SiO₂, H₂O)

    - Change phase at specific temperatures at fixed pressure

   

    Two-component systems:

    - More complex melting/solidification

    - Melt over a temperature range

    - Have eutectic point where liquidus and solidus meet

What is the eutectic?

- Point where liquidus and solidus curves meet

    - Represents unique temperature and composition

    - System will first melt at this point

What is decompression melting and why is it important?

    - Occurs when hot asthenosphere rises and pressure decreases

    - Major cause of volcanism on Earth

    - Happens because melting point increases with pressure

    - Normal mantle melts 15-20% under ridges

    - Results in formation of oceanic crust through basalt extraction

How does basalt form from mantle periodtite?

- Peridotite partially melts at ~1300°C

    - Selectively dissolves clinopyroxene and Al-rich phases

    - Produces basalt magma enriched in FeO, CaO, and Al₂O₃

    - Leaves behind refractory residue of olivine and orthopyroxene (harzburgite)

What is chemical differentiation by melting?

    - Initial melt can have very different composition from original rock

    - Extracted melt leaves residue enriched in different components

    - This process has helped differentiate the Earth

    - Example: mantle peridotite melting to form basaltic oceanic crust

What is the lever rule used for?

• Used to determine the proportion of melt and solid crystals in two-phase fields between liquidus and solidus curves

• Based on geometric considerations of relative distances on phase diagram

How does melting affect entropy?

    - Liquids have greater entropy than solids of same composition

    - Gases have higher entropy than liquids

    - Entropy increases with disorder in the system

What are mid-ocean ridges and what percentage of world volcanism occurs there?

Mid-ocean ridges are constructive/divergent plate boundaries where new plates are created. 90% of the world's volcanism occurs at ocean ridges.

What are the characteristics of earthquakes at ocean ridges?

Ocean ridge earthquakes are:

    - Typically small magnitude (rarely > 7)

    - Occur at shallow depths (< 10 km)

    - Located in a narrow zone a few kilometers wide

    - Largest earthquakes occur along fracture zones

What is the typical thickness of oceanic crust and how is it formed?

Oceanic crust is 6-8 km thick and is formed by melting beneath ocean ridges, producing basaltic rocks.

 

What are ophiolites?

Ophiolites are fragments of oceanic crust that have been uplifted or obducted onto land, allowing scientists to study the structure of spreading centres.

How do magnetic stripes form at mid-ocean ridges?

When new oceanic crust forms, it becomes magnetized in the direction of Earth's magnetic field at the time. As the Earth's magnetic field reverses periodically, it creates a pattern of magnetic stripes parallel to the ridge, symmetrical on both sides

What are typical spreading rates at mid-ocean ridges?

    - North Atlantic: 2 cm/year

    - East Pacific Rise: 15-20 cm/year

    - Average global rate: 10 cm/year

    - Creates 4km² of new surface every year

    - Produces 30 km³ of lava annually

What are transform faults?

Transform faults are large fracture zones that break up ocean ridges into segments. They are only active between displaced ridge crests and their movement is controlled by the plates moving past each other.

What are black smokers and what type of ecosystems do they support?

Black smokers are hydrothermal vents where hot fluids mix with cold seawater, precipitating sulphides and metal oxides. They support unique ecosystems based on chemosynthetic bacteria that use sulphide oxidation rather than photosynthesis for energy

How does the depth of the ocean floor change with distance from the ridge?

The ocean floor deepens away from the ridge because as new sea-floor moves away from the hot zone, it cools and becomes denser. Following the principle of isostasy, this cooler, denser rock sinks lower than the hotter, lighter rock at the ridge.

What is sea-floor spreading and who proposed it?

 

Sea-floor spreading, proposed by Harry Hess, is the process where new oceanic crust is created at mid-ocean ridges as plates pull apart. The theory was supported by Vine and Matthews' explanation of magnetic stripes in 1962.

What are hot spots and where do they occur?

Hot spots are intra-plate volcanoes that occur well within tectonic plates, not at plate boundaries. There are approximately 40 active hot spots worldwide, including Hawaii, the Canaries, Ascension, Cape Verde, and Reunion.

What makes Hawaii unique as a hot spot?

    - One of the most active basalt volcanoes in the world

    - Located on old, thick oceanic crust (not at a mid-ocean ridge)

    - Produces ocean island basalts (OIBs)

    - Has magma coming from very deep that gets trapped beneath thick lithosphere before eruption

What are the Emperor Seamounts and what do they tell us?

 The Emperor Seamounts are a chain of underwater volcanoes (seamount chain) that:

    - Show an age progression from NW to SE

    - Demonstrate that the Hawaiian hot spot has remained stationary for 60 million years

    - Have a distinctive kink showing plate direction change 45 million years ago

    - Indicate Pacific plate motion of about 9cm/year

How do scientists monitor and predict volcanic activity at hot spots?

    - Surface inflation/tilt caused by magma chamber filling

    - Gravity field changes due to density changes

    - Seismic activity

    - Deep magma chamber imaging

What are mantle plumes and how do they relate to hot spots?

: Mantle plumes are upwellings of hot mantle material that:

    - Create hot spots at the surface

    - Often originate at the core-mantle boundary

    - Can be detected using seismic tomography (appear as areas of slower seismic velocity)

    - Produce ocean island basalts (OIBs) with distinct compositions from MORBs

What are Large Igneous Provinces (LIPs)?

Large Igneous Provinces are:

    - Huge volcanic eruptions recorded in geological history

    - Linked to modern-day hot spots

    - Examples include Deccan Traps and Siberian Traps

    - Often associated with mass extinctions (e.g., End-Permian extinction)

    - Initial manifestations of mantle plumes reaching the surface

How do hot spots affect continental break-up?

Hot spots can initiate continental break-up by:

    - Causing deep upwelling under continents

    - Eroding the bottom of the lithosphere

    - Initiating rifting

    - Example: North Atlantic rifting and formation of Iceland

What makes Iceland unique as a hot spot?

Iceland is unique because it:

    - Sits on both a hot spot and a mid-ocean ridge

    - Is above sea level due to enhanced melting

    - Has intermediate mantle temperatures

    - Can generate more melt due to combined decompression melting from ridge spreading and hot spot activity

How do Ocean Island Basalts (OIBs) differ from Mid-Ocean Ridge Basalts (MORBs)?

OIBs:

    - Are produced by deep upwelling from near core-mantle boundary

    - Have distinct compositions due to their deep origin

    - Form through decompression melting of hotter material

    - Are characteristic of hot spot volcanism

What evidence shows that hot spots remain relatively fixed while plates move?

: Evidence includes:

    - Linear age progression along hot spot chains

    - Consistent paleolatitude measurements

    - Cross-cutting relationships with ocean floor ages

    - Systematic age dating of volcanic rocks along chains

 What are the main characteristics of subduction zones?

Subduction zones (destructive plate margins) have:

    - Deep ocean trenches

    - Volcanism (often explosive)

    - Earthquakes (shallow, intermediate and deep)

    - Occur where old lithospheric plates sink into the mantle

    - Are marked by the Benioff-Wadati zone of earthquakes reaching ~700 km depth

How does the age of oceanic crust affect subduction?

The age of oceanic crust affects subduction by:

    - Older crust becomes colder and denser

    - Older, denser slabs tend to subduct at steeper angles

    - Younger, hotter crust (like in S. Chile) is more difficult to subduct

    - The steepness of subduction depends on both age and velocity of the subducting plate

What is back-arc spreading and when does it occur?

Back-arc spreading:

    - Occurs above subduction zones

    - Associated with steeply dipping slabs that 'roll-back'

    - Happens when trenches migrate away from the arc

    - Creates extensional forces in the arc

• Example: Marianas arc has split multiple times due to back-arc spreading

How does flux melting work in subduction zones?

Flux melting in subduction zones:

    - Occurs when subducting plates dehydrate

    - H₂O-rich fluids rise into the mantle wedge

    - Water reduces the melting temperature of the mantle

    - Adding 0.5 wt% water can reduce solidus temperature by 100°C

    - Produces hydrous basaltic magmas (~4 wt% H₂O)

    - Oceanic slabs rarely melt; instead, they cause mantle melting

What are the differences between ocean-ocean and ocean-continent subduction zones?

Key differences:

    Ocean-Ocean:

    - Examples: Mt. Pinatubo, Rabaul

    - Generally more direct volcanic activity

   

    Ocean-Continent:

    - Examples: Villarica (Chile), Popocatépetl (Mexico)

    - Thicker continental crust makes magma passage harder

    - More intrusive rocks

    - Magmas often stall and fractionate, evolving from basalts to rhyolites and andesites

How do decompression melting and flux melting differ?

Decompression Melting (at MORs):

    - Produces dry basaltic magmas (<0.4 wt% H₂O)

    - Anhydrous minerals dominate

   

    Flux Melting (at Subduction Zones):

    - Produces wet basaltic magmas (~4 wt% H₂O)

    - Creates more SiO₂-rich magmas (andesites, rhyolites)

    - Forms granodiorites and granites at depth

    - Contains hydrous minerals like amphiboles and micas

What are the different types of continental margins?

1. Andean Margin:

    - Features accretion of exotic terranes

    - Long-lasting subduction (100s of millions of years)

   

    2. Californian Margin:

    - Evolution from subduction zone to transform fault

    - 30 Ma of dextral strike-slip motion

   

    3. Continental Collision (e.g., Himalayas):

    - Results from closure of oceans between continents

    - Produces orogenic belts

    - Creates crustal melting and granitic melts

    - Features rare volcanoes

    - Marked by ophiolites at suture zones

   

What happens during continental collision?

During continental collision:

    - Continental crust is too buoyant to subduct

    - Crust becomes thickened

    - Base of crust becomes very hot

    - Produces orogenic belts

    - Causes metamorphism of thickened crust

    - Creates granitic melts through crustal melting

    - Results in few volcanoes

    - Forms ophiolites (obducted oceanic lithosphere) at suture zones

What is the Benioff-Wadati zone?

The slab delineated by a broad inclined zone of Earthquake - which reaches ~700km

What is a glacial rebound?

When land rises up afetr ice melts following glaciation. As ice sheets form, they increase the downward force causing land to sink and form a root. When ice melts, the land rises and the root disappears

What is the viscosity of the Earth’s mantle?

   

    The mantle has a very high viscosity of 10²¹ Pa s, which is why it takes tens of thousands of years to respond to changes like glacial rebound.

This is billions of times more viscous than molten sugar.

What’s the difference beween the lithosphere and asthenosphere?

• Lithosphere: Elastic outer shell that flexes, up to 100-200km thick, includes all crust and upper mantle, base defined by ~1200°C isotherm

  

• Asthenosphere: Deeper plastic/ductile shell that flows, made of upper mantle rocks, also known as low-velocity zone

What are the three principal mechanisms of heat loss from Earth?

1. Radiation

   • Heat loss through radiation of energy from hot surfaces

2. Conduction

   • Heat transfer through molecular collisions

   • Rocks are poor conductors

3. Convection

   • Heat transfer through fluid motion due to density changes

When does convection occur?

Occurs when hot fluid becomes buoyant, rises and is replaced by sinking cold dense fluid

    - Happens when Rayleigh Number exceeds 1700

    - Very efficient method of heat loss

    - Forms thermal boundary layers where fluid is heated or cooled

How does plate tectonics relate to convection?

Hot mantle rises at ridges

Cold plates sink at subduction zones

Plates act as coldthemal boundary layers

Deep circulation occurs through hot mantle plumes

What does seismic tomography tell us about Earth’s interior?

• Maps Earth’s interior using data from thousands of earthquakes

• Shows velocity differences relative to Earth Reference Model

• Shows subducted slabs can reach lower mantle

• subducted slabs = faster seismic waves

What is the low velocity zone?

•     - Part of the asthenosphere

      - Contains small amount of partial melt (<1%)

      - Underlying mantle is solid (transmits S-waves and P-waves)

      - Beginning of melting leads to reduction in seismic velocity

What role does water play in the mantle?

Water changes the viscosity of the mantle, making it easier to convect

What is magma?

    A high temperature mixture of liquid (or melt), crystals and gases (or volatiles). Most magmatic liquids are silicates, though carbonate varieties (carbonatites) are rare.

Define intrusive and extrusive rocks?

    Intrusive: Formed from magma that has not reached Earth's surface, cools slowly underground

    Extrusive: Formed from magma (lava) that has erupted on Earth's surface, cools relatively quickly

How are magmas classified by SiO₂ content?

    | Basalt | <53% SiO2 | Basic |

    | Andesite | 53-63% SiO2 | Intermediate |

    | Dacite | 63-70% SiO2 | Silicic (or Acid) |

    | Rhyolite | >70% SiO2 | Silicic (or Acid) |

What are the main magmatic volatile species?

H₂O and CO₂ are the most dominant, followed by C, SO_2, H₂S and CH₄

What affects magma viscosity?

• Higher SiO₂ content increases viscosity

• Prescence of crystals increases viscosity

• dissolved volatiles (especially water) decreases viscosity

What are the two main processes in crystallisation?

    Nucleation: Formation of crystal "nucleus" - critical number of atoms that represent a proto-crystal

    Growth: Further atoms diffuse through melt to reach nuclei and attach themselves

What are the two main styles of volcanic eruption?

    Effusive:

    - Predominantly basaltic flows

    - Low viscosity allows easy flow

    - Slow, allowing gases to escape easily

   

    Explosive:

    - More silica-rich melts

    - Very dangerous

    - Example: Mount St. Helens

What are the 4 types of radioactive decay?

Beta decay, alpha decay, positron decay, and electron capture.

What happens during beta decay?

A neutron converts into a proton, emitting an electron and an antineutrino. The mass number stays the same, but the atomic number increases by 1.

What happens during alpha decay?

An alpha particle (⁴He nucleus) is emitted, containing 2 neutrons and 2 protons. The parent decreases its mass number by 4 and atomic number by 2.

What is the relationship between decay constant and hlaf-life?

λ = ln(2)/t^1/2

Why is radioactive decay useful for dating rocks

It is a first order process that only depnds on the amount present and is not affected by extermal factors like temperature, pressure or gravitational force.

What is the isochron equation and what used for?

D = D* + D⁰

D = N(e^λt - 1) + D₀

where:

D = total number of daughter isotopes

D* = number of daughter isotopes from decay

D₀ = number of daughter isotopes originally

It's used to calculate the age of a rock by plotting daughter against parent isotope ratios. The age can be calculated from the slope of the line.

What are the three main rules for using isochrons?

1. All samples must be the same age

2. All samples must have the same initial isotope composition

3. The isotope ratios must only change due to radioactive decay

What type of rocks are easiest to date?

Igneous rocks are easiest to date. Sedimentary rocks are difficult to date, and metamorphic rocks can be dated but are tricky.

When does the radiometric dating clock start for rocks?

The clock starts when the rock gets cool enough for all the elements to be fixed in their mineral, which occurs fairly rapidly after the rocks formed

What is teh decay equation?

N = N₀e^-λt