5 - Igneous Petrology

Petrology

The science concerned with rocks, including their mode of occurrence, composition, classification, origin, and their relations to geological processes and history

Petrogenesis

interpretations of the origin of rocks

Petrography

places emphasis on the purely descriptive part of rock science from textural, mineralogical, and chemical points of view

Kaya mo na yan pero here's key terms: Igneous intrusive crystallize, igneous extrusive solidify, exposed to weathering, transportation, deposition, lithification to sedimentary to metamorphism (ign can be directly turned to met and sed, met can be directly turned to sed and ign, but sed can not be directly turned to ign as it should be met first)

Explain the rock cycle

Oceanic crust

3-10 km thick with relatively uniform stratigraphy and basaltic in composition

On mid-oceanic ridges

where on the oceanic crust where it is 10 km thick

Sediments, Pillow basalt, Sheeted dikes, more massive gabbro, ultramafic (mantle)

Ophiolite from top to bottom

Continental crust

20-90 km thick w/ average 35km and granodioritic or granitic in composition

Peridotite [(ultramafic) (Mg,Fe)2SiO4 ] , Silicate Perovskite [(Bridgmanite) (Mg,Fe)SiO 3 ], Ferropericlase [ (Fe,Mg)O ]

3 minerals in mantle

60-220 km

depth of low velocity zone

Transition zone

rapid velocity increase

Lower Mantle

more gradual velocity increase

Core

Iron Nickel alloy

Outer core is liquid and shear waves cannot be transmitted through liquids

Why does the outer core have no S-waves?

25 degree Celsius/km depth

Geothermal gradient

Fe, O, Si, Mg, S, Ca, Al

7 most common elements that comprise 97% of Earth's mass

Iron 35%, Oxygen 30%, Silicon 15%

3 most abundant elements in the whole Earth

Oxygen 46%, Silicon 28%, Aluminum 8%

3 most abundant elements in the crust

Divergent boundaries

where new crust is generated as the plates pull away

Continental boundaries

where crust is destroyed as one plate dives under another

Transform boundaries

where crust is neither produced nor destroyed as the plates slide horizontally past each other

Tectonic Plates or Lithospheric Plates

massive, irregularly shaped slab of solid rock generally composed of both continental and oceanic lithosphere

NA, SA, Pacific, African, Eurasian, Australian-Indian, Antarctic Plates

7 major plates

Caribbean, Nazca, Philippine, Arabian, Cocos, Scotia, Juan de Fuca plates

Minor plates

constructive margins

Divergent boundaries, also known as -???- where two plates move apart, resulting in upwelling of hot material from the mantle to create new seafloor

spreading centers

Divergent boundaries are also called -???-, because seafloor spreading occurs at these boundaries.

Oceanic Ridges

elevated areas of the seafloor that are characterized by high heat flow and volcanism

70,000 km

length of the longest topographic feature on Earth's surface

Mid-Atlantic Ridge, East Pacific Rise, Mid-Indian Ridge

comprises the global ridge system

Newly created oceanic crust is hot, making it less dense than cooler rocks found away from the ridge axis. As soon as new lithosphere forms, it is slowly yet continually displaced away fronm the zone of upwelling. Thus, it begins to cool and contract, thereby increasing in density. This thermal contraction accounts for the increase in ocean depths away from the ridge crest.

Why is the oceanic ridge in elevated position

Rift Valley

this structure is evidence that tensional forces are actively pulling the ocean crust apart at the ridge crest

5 cm (2 inches) per year

Typical rates of seafloor spreading

Continental rifting

Occurs where opposing tectonic forces act to pull the lithosphere apart

The initial stage of rifting tends to include mantle upwelling that is associated with broad upwarping of

the overlying lithosphere. As a result, the lithosphere is stretched, causing the brittle crustal rocks to break into large slabs. As the tectonic forces continue to

pull the crust apart, these crustal fragments sink, generating an elongated depression

Explain the mechanism of continental rifting

East African Rift

The region where scientist consider as the birthplace of the human race

destructive margins

Convergent boundaries, Also known as -???- where two plates move together, resulting in

lithosphere oceanic descending beneath an overriding plate, eventually to be reabsorbed into the mantle or possibly in the collision of two continental blocks to create a mountain system.

subduction zones

Convergent boundaries, also called -???-, because they are

sites where lithosphere is descending

(being subducted) into the mantle.

because the density of the descending tectonic plate

is greater than the density of the underlying asthenosphere

Why does subduction occurs

Deep-ocean Trenches

the surface manifestations produced as oceanic lithosphere descends into the mantle

Mariana Trench

Deepest trench

Partial melting

when a descending oceanic slab reaches a depth of about 100 km, melting is triggered within the wedge of hot asthenosphere that lies above it

Due to the presence of water

Why does wet rocks melt at lower temperature than dry rocks

The Philippines

good example of a volcanic island arc

Subduction

the sideways and downward movement of the edge of a plate of the earth's crust into the mantle beneath another plate

Obduction

the overthrusting of oceanic lithosphere onto the continental lithosphere at a convergent plate boundary where continental lithosphere is being subducted beneath the oceanic lithosphere

conservative margins

Transform boundaries, also known as -???- where two plates grind past each other without the production or destruction of lithosphere

Fracture zone

prominent linear breaks in the seafloor

Magma is the entire assemblage of melt, suspended crystals, and dissolved volatiles while Melt refers to the molten state of its own, excluding any solid material suspended on it.

Differentiate Magma and Melt

crystals become phenocrysts, gas bubbles become vesicles, melt becomes groundmass

Magma from molten to solid state

Petra

means rock

Logos

means disclosure or study

Petrology

the branch of geology dealing with the origin, occurrence, structure, and history of rocks

Igneous, Sedimentary, Metamorphic

3 main types of rocks

Igneous rocks

primary rocks, source is magma or lava

Sedimentary rocks

thin veneer above the Sial and Sima in Oceanic and Continental crusts, secondary rocks

Metamorphic rocks

change of forms of other rocks due to temperature, pressure, and chemical fluids

Silicon Aluminum

Sial

Silicon Magnesium

Sima

ignis

means fire

Intrusive and Extrusive igneous rocks

2 types of igneous rocks

rocks that directly solidify from molten or partially molten material

What is Igneous rocks

Rocks that came from crystallization of magma

What is intrusive igneous rocks

Rocks that came from solidification of lava

What is extrusive igneous rocks

rocks that result from the consolidation of loose particles (sediments) or the chemicals precipitating from solution at or near the Earth's surface; or organic rock consisting of secretions or remains of plants or animals

What is sedimentary rocks

sedimentum

means settling or sinking down

rocks derived from pre-existing rocks by mineralogical, chemical, or structural changes (especially in the solid state)

What is metamorphic rocks

metamorphosis

means change in form

Because it came from the primary source like an igneous rock but transportation is involved like in sedimentary rocks

Why is volcanic tuff a baklang bato

Because it came from alteration of ultramafic igneous rocks where it contains olivine and pyroxene that when undergoes metamorphism, it creates the mineral serpentine

Why is serpentinite a baklang bato

outcrop characteristics, general texture, and mineral assemblages present

How do we classify rocks?

Igneous rock

any crystalline or glassy rock that forms from the cooling of a magma

Magma

consists mostly of liquid rock matter but may contain crystals of various minerals and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase

Basaltic, Andesitic, Rhyolitic

3 general types of Magma based on chemical composition

Turns into basalt, has 45-55% SiO2, high Fe, Mg, Ca; low K, Na, ranges from 1000 to 1200 degrees Celsius, low viscosity and low gas content

What is a basaltic magma

Turns into andesite, has 55-65% SiO2, intermediate Fe, Mg, Ca, K, Na, ranges from 800 to 1000 degrees Celsius, intermediate viscosity and intermediate gas content

What is a andesitic magma

Turns into rhyolite, has 65-75% SiO2, low Fe, Mg, Ca; high K, Na, ranges from 650 to 800 degrees Celsius, high viscosity and high gas content

What is a rhyolitic magma

Chert

What material has 100% SiO2

It is generated from the Earth's mantle through partial melting

Where does magma come from

Heat from the early accretion and differentiation of the Earth, Heat released by the radioactive breakdown of unstable nuclides, and Heat from the Sun

Heat sources in the Earth

Radiation, Conduction, Convection

3 ways of Heat transfer

25 degrees Celsius/km depth

Geothermal gradient

the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way

What are the ways to generate magma in the solid part of the earth

It can be raised by upwelling of hot material from below either by uprising solid material (decompression melting) or by intrusion of magma (heat transfer).

How to raise the geothermal gradient

It can be achieved by adding H2O or CO2 (flux melting)

How to lower the melting temperature

Convection

What mechanism is needed to raise the geothermal gradient

Magmatic differentiation

refers to the process whereby an originally homogeneous magma changes its composition or becomes heterogeneous .

Fractional crystallization, magma mixing, magmatic assimilation

3 mechanisms of magmatic differentiation

Primitive magma

one which is close to its original composition and has therefore in theory not undergone crystal fractionation

Evolved magma

one in which crystal fractionation has taken place such the magma composition is different from the starting composition

Magmatic differentiation by fractional crystallization

when magma crystallizes it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change

If 2 magmas with different compositions happen to come in contact with one another, they could mix together

Define magma mixing

Assimilation

magma reacts with the country rock which is adjacent to the magma chamber

Inclusions

incompletely melted chunks of country rock.

xenolith

inclusion of rock in an igneous rock

xenocryst

crystal counterpart of xenolith

As a basaltic magma is cooled Olivine and Ca-rich plagioclase crystallize first. Upon further cooling, Olivine reacts with the liquid to produce pyroxene and Ca-rich plagioclase react with the liquid to produce less Ca-rich plagioclase. But, if the olivine and Ca-rich plagioclase are removed from the liquid by crystal fractionation, then the remaining liquid will be more SiO2 rich. If the process continues, an original basaltic magma can change to first an andesite magma then a rhyolite magma with falling temperature.

Explain the Bowen's Reaction Series

Distinct melting events from distinct sources, Various degree of partial melting from the same source, Crystal Fractionation/Fractional Crystallization, Magma mixing, Assimilation/Contamination of magma by crustal rocks, Liquid immiscibility

6 processes of magmatic differentiation

Temperature contrast, density contrast, viscosity contrast

Factors that can inhibit magma mixing

marble cake, disequilibrium mineral assemblages, reversed zoning in minerals, glass inclusions, chemical evidences

Evidences of mixing

Modal Composition or Mode

The most straightforward approach to determining rock mineralogy involves visually identifying the minerals and determining their percentages by volume.