Igneous rocks and processes Flashcards by Shaira Ann Rueco

formed from the solidification of magma (intrusive) or lava which flows out from depths (extrusive) and/or through volcanic eruptions.

IGNEOUS ROCKS

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true or false

Rock can be composed of only one type of mineral

true

limestone is commonly made up of only calcite

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described as hot (temperature ranges from 600-1200oC) partially molten rock beneath the surface of the earth

magma

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Magma is composed of ?

Liquid components in the form of silicates (sometimes carbonate or sulfide)

Solid components that are either minerals and/or rock fragments formed at high temperatures

Dissolved gases dominated by water vapor and CO2 (and usually smaller amounts of other volcanic gases such as SO2, HCl, HF and other gases)

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When the magma reaches the surface of the earth through a vent, it is now called a

Lava

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the most basic and most commonly used classification of magma is dependent on its

composition

amount of silica it contains

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> 63% SiO2

Felsic, Silicic or Acidic

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52-63% SiO2

Intermediate

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45-52% SiO2

Mafic or Basic

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<45% SiO2

Ultramafic or Ultrabasic

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the property of substances to resist flow.

VISCOSITY

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The viscosity of magma is affected by different factors including

temperature, silica content, and water content

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Higher temperature

→ lower viscosity

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Higher SiO2 content

→ higher viscosity

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More H2O

→ lower viscosity

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In terms of temperature, the hottest type of magmas are

ultramafic magmas

they are also the densest type of magmas owing to the high concentration of heavier elements such as the ferromagnesians

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least dense magma due to high amount of silica

Felsic magmas

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has relatively low silica content

low gas content and low viscosity, or resistance to flow

Has high mean temperatures,

most fluid of magma types.

erupts non-explosively

lava cools into basalt

Mafic Magma

Typical mafic rocks are gabbro (intrusive) and basalt (extrusive)

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one of the most common rocks in Earth’s crust as well as the volcanic islands created by hot spots.

Basalt

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direct result of mafic magma eruptions

Hawaiian Islands

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has higher silica content than mafic magma

This results in a higher gas content and viscosity

As a result of its higher viscosity and gas content, it builds up pressure below the Earth’s surface before it can be released as lava.

explode violently and cools as andesite rock.

most commonly transforms into andesite due to the transfer of heat at convergent plate boundaries

Intermediate Magma

Typical intermediate rocks are diorite and andesite.

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often found at continental volcanic arcs, such as the Andes Mountains in South America, after which they are named.

Andesitic rocks

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has the highest silica content of all magma types,

has the highest gas content and viscosity, and lowest mean temperatures

Thick, viscous magma that can trap gas bubbles in a volcano’s magma chamber

eruptions eject lava violently into the air, which cools into dacite and rhyolite rock.

most commonly found at convergent plate boundaries where transfer of heat and flux melting create large stratovolcanoes

Felsic Magma

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Magma forms by melting of upper mantle and/or lower crust. Its formation is aided by factors such as

(1) increase in temperature,
(2) decrease in pressure, and/or
(3) addition of volatiles.

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true or false Earth’s natural temperature increases with depth

true

True or false At a depth of 100 to 250 km, the temperature becomes very close to the boundar

true At a depth of 100 to 250 km, the temperature becomes very close to the boundary, explaining why magma forms at these depths.

Why don't rocks melt despite having high temperatures at deeper conditions

due to increase in pressure

○ It usually happens when rocks ascend due to convection or rising plumes, or the crust is thinned. ○ This is commonly seen in divergent zones such as in the mid-oceanic ridges and rift valleys. a body of rock is held at approximately the same temperature but the pressure is reduced drop in confining pressure

adiabatic or decompression melting

true or false a little concentration of water can already trigger partial melting

true 0.2% water addition led to 25% melting of rocks assuming conditions similar to Japanese volcanic arcs

The composition of the magma relies on two major factors

○ the composition of the source rock ○ the degree of partial melting

Melting of mantle rocks generally results in

mafic/basaltic magmas

Crustal rocks yields

silica rich magmas

true or false during the movement of magmas towards the surface or during its residence in the crust, its chemical composition may change

true (e.g., mafic magma can transform into a more felsic magma) Due to certain processes collectively known as “magmatic differentiation”

- is the process of crystal formation and its removal from the magma (by gravity). occurs due to the differences in crystallization temperature of different minerals

Fractional Crystallization

Generally, ___________ (Fe-Mg-Ca rich) form first, are removed in early formed solids and settle (separated from the magma since their density is higher compared to the buoyant molten magma)

ferromagnesian minerals making the magma more silica rich and more felsic in composition.

literally means that only certain parts of the rock will melt. somewhat the opposite of fractional crystallization If solid rocks beneath the surface suddenly experienced an increase in temperature, crystals formed at lower temperatures will start to melt

Partial Melting - The molten materials become magma which separates from the rock due to its buoyancy as a function of temperature.

true or false Since silica rich minerals such as quartz has lower melting point, the resulting magma is typically more felsic in composition.

true In general, more siliceous magmas form by low degrees of partial melting. As the degree of partial melting increases, more mafic compositions can be generated.

melting a mafic source result in a

felsic to intermediate magma.

How are basaltic magmas formed

ultramafic (peridotite source) rocks such as those from the mantle should be melted.

occurs when two or more magmas with different compositions could mix with each other to produce compositions intermediate between the end members

Magma Mixing or Mingling this may happen when: ○ (1) two different magma types that formed the rock may have come into contact via the eruption process or they may have occupied the same chamber prior to the eruption or (2) there may have been two separate, but physically connected chambers.

When a molten body moves up through an existing basement rock, it assimilates rock (melts and incorporates elements from the surrounding rock). This changes the magma composition.

Assimilation

existing basement rock

country rock

○ In most cases, parts of the country rock are not completely melted leaving behind inclusions inside the cooled intruding rock. These inclusions are then called

xenoliths and xenocrysts (individual unmelted mineral).

The crystallization sequence of minerals was thoroughly described based on the experiment conducted by

Norman L. Bowen

When basaltic rocks are melted in a kiln then allowed to gradually cool down, certain minerals then formed depending on the temperature conditions. may also be used to identify what type of magma the rock came from based on its mineral composition.

BOWEN’S REACTION SERIES a rock dominated by olivine (and possibly few pyroxene) comes from an ultramafic source. A mafic rock on the other hand is dominated by pyroxene and Ca-rich plagioclase with significant amount of amphibole, biotite and possibly olivine

Olivine-pyroxene-amphibole-biotite transformation of minerals from one type to another

discontinuous series/branch.

only the mineral plagioclase develops but the amount of calcium and sodium in its composition changes.

continuous series at higher temperature, the plagioclase is more Ca-rich (anorthite) while at lower temperature, Na dominates (albite).

can cause rocks to melt at a lower temperature

Volatiles usually water vapor but can also be other compounds such as CO2

are molten rocks that solidified at the surface quickly these rocks are made up of very tiny crystals and volcanic glass that are not visible with the naked eye

Extrusive (volcanic) rocks) (e.g. basalt, andesite, rhyolite).

A special group of volcanic rocks formed from the explosive eruption of volcanoes (e.g. tuff, ignimbrite, etc.).

pyroclastic rocks

- rocks formed at depth (e.g. gabbro, diorite, granite). - Since magma at depth cool relatively slower compared to the surface, it is made up of coarse and more distinct crystals.

Intrusive (plutonic) rocks

solidified underground igneous bodies. generally classified based on shape (tabular or massive) and the orientation with respect to the host rock

Plutons

_______ if the pluton cuts across existing structures/layers of rocks

discordant Stock - small discordant pluton (their coalescence at depth might constitute a batholith).

________ pluton is parallel to the structures/layers

concordant

more than 100 km2 in outcrop area

Batholith

tabular body cutting across bedding

Dike/Dyke

concordant tabular body

Sill

blister-shaped sill that has pushed up the overlying rock layers

Laccolith

large lenticular sill. Flat concordant on top with funnel shaped dyke at the bottom

Lopolith

refers to the crystal size, crystal shape, interrelationship, and nature of component of the rock

Texture

○ coarse-grained; ○ mineral grains visible to naked eye ○ This texture is usually associated to a slow cooling rate. Most intrusive igneous rocks exhibit this texture since surrounding rock insulate the magma resulting to a slow cooling rate.

Phaneritic

very fine grained; mineral grains are not visible to naked eye and requires magnification ``` This texture is formed by fast cooling rates. Extrusive rocks (e.g., those formed from lava flows) commonly exhibit this. ``` The fast-cooling rate in lavas results from its heat exchange to the atmosphere and the ground where they come in contact with. However, some intrusive rocks (e.g., shallow intrusive) may also exhibit this texture.

Aphanitic

Consists of two distinct crystal sizes usually a combination of coarse- and fine-grained crystals. the visible mineral grains called phenocrysts are surrounded by glass or finer mineral grains The phenocrysts may consist of one or more minerals This texture signifies a change in the cooling rate of the magma (commonly slow cooling then suddenly shifts to fast cooling)

Porphyritic

phenocrysts are surrounded by glass or finer mineral grains called the

groundmass

has holes / vesicles that imply degassing or escape of gases during rock formation.

Vesicular

the vesicles are filled up by some other secondary minerals (formed after the original rock has formed) called “amygdules”.

Amygdaloidal

interlocking crystals greater than 1 cm in size. Aside from slow cooling, it usually form with the aid of mineralized hydrothermal fluids.

Pegmatitic

made up of fragmented grains (usually fragments of rocks and crystals of pre-existing rocks destroyed during an explosive eruption).

Pyroclastic The large fragments are usually referred to as “clasts” characterized by the lack of well-defined shapes, different colors and composition. The groundmass is usually made up of volcanic glass.

no minerals present, all glass; formed by very fast cooling or “quenching”

Glassy

true or false gneous rocks can have many different textures

true a rock may be aphanitic/porphyritic, at the same time vesicular such as in the case of most lava flows that can be seen in active volcanoes such as in Mayon volcano, Philippines.

This refers to the percentage of dark minerals (usually ferromagnesian minerals in the rock such as olivine, pyroxene, amphibole, and biotite) in the rock

Color index Mineral composition

In naming igneous rocks, we can now combine

texture (based on crystal size) with the magma type (based on composition).

a phaneritic felsic rock is called

Granite

an aphanitic felsic rock is called

rhyolite

an intermediate rock, exhibit a salt and pepper texture,

Diorite

dominated by black minerals as opposed to diorite

Gabbro

For the case of aphanitic and porphyritic rocks where very few crystals are visible, naming may be done by

(1) using the groundmass color as basis for the magma type (which is not always accurate) and (2) determining the composition of the phenocryst and associating it to the magma type. Geologists only use this as an initial classification scheme.

The composition of an aphanitic rock is examined either by

using a petrographic microscope or by getting its chemical composition (major elements) through geochemical analyses such as X-ray fluorescence.

true or false in naming pyroclastic fragments, dominant fragment size will determine the rock name to be given

true

true or false ○ Blocks and bombs have the same size but differ in shape or roundness.

true A block is extruded in a solid state A bomb is extruded partially or wholly molten making it rounded.

Average Particle size: | <2

Unconsolidated volcanic fragment (tephra): Ash Pyroclastic Rock name: Tuff

Average Particle size: 2-64

Unconsolidated volcanic fragment (tephra): Lapilli Pyroclastic Rock name: Lapilli stone

Average Particle size: >64, angular

Unconsolidated volcanic fragment (tephra): Blocks Pyroclastic Rock name: Volcanic breccia

Average Particle size: >64, rounded

Unconsolidated volcanic fragment (tephra): Bombs Pyroclastic Rock name: Agglomerate

What are the most abundant rocks on earth

Igneous rocks

rocks that are usually formed when the two types of crust interact in convergent boundaries.

Intermediate rocks

True or false Hotspots and mantle plumes are usually basaltic in compositions.

True

Resources from igneous rocks

Aggregates for construction Acts as a natural cement called pozzolan can be used as dimension stones (tiles and tabletops ore deposits (galena, sphalerite, chromite)