7. igneous rocks Flashcards
What is the difference between volcanic and plutonic rocks?
Intrusive rocks are the coarsely crystalline rocks, which formed by intrusion of magma into rocks and therefore cooled slowly, having enough time to grow large crystals.
Volcanic rocks glassy (amorphous) or finely crystalline rocks, which are extrusive and cooled rapidly in Earth’s surface. They may have pores.
Where do igneous rocks form and occur?
- Increase the temperature.
- Lower the confining pressure.
- Add water
The two major magmatic geosystems are the mid-ocean ridges, where basalt wells up from the upper mantle and melts during decompression to form oceanic crust, and subduction zones, where subducting oceanic lithosphere partially melts by addition of fluid to generate differentiated magmas that rise through the crust and form island or continental volcanic arcs.
- Mid-ocean ridges
- Intracontinental rifts
- Island arcs
- Continental arcs
- Back-arc (oceanic) / retro-arc (continental)
- Ocean island basalt (hot spots underneath the ocean)
- Miscellaneous intracontinental activity
Where do specific igneous rocks form?
- Volcanic rocks form if the magma reach the Earth surface
- Plutonic rocks form if the magma does not reach the surface (= most common case)
How does magma is formed?
Magma forms at places in the lower crust and mantle where temperatures and pressures are high enough for at least partial melting of water-containing rock.
- The original material of igneous rocks - molten rock material
- Forms by partial melting of rocks in the Earth mantle or in the lower crust
- Melting due to increase of temperature, decrease of pressure, or decrease of melting temperature (usually due to H2O input)
- Mostly contains elements that build up silicate minerals + gas (ex: H2O)
- The magma body rises upwards due to a lower density than the surrounding rock
How are rocks formed from magma?
Volcanic rocks are formed by the fast solidification of magma when it is expelled in the crust as lava.
Intrusive rocks are formed by the slow solidification of magma inside of intrusion of rocks in the surrounding of the magma chamber.
What are the three main magma types? Which rocks can form from them?
- Basaltic melt (olivine, pyroxene, plagioclase) - dry magma
- mafic composition
- Partial melting of (dry) peridotite in the upper mantle
(Volcanic) basalt or (plutonic) gabbro is formed without magma differentiation. - Andesitic melt (plagioclase, pyroxene, hornblende) - chemical composition close to the average of the continental crust
- intermediate composition
- Partial melting of (wet) oceanic crust (basalt) in subduction zones
- Andesite typically occurs on the continental crust in volcanic arcs
(Volcanic) andesite or (plutonic) tonalite is formed without magma differentiation. - Rhyolitic melt (quartz, alkalifeldspar, mica)
- felsic composition
- Partial melting of (wet) continental crust (of andesitic composition)
(Volcanic) rhyolite or (plutonic) granite is formed without magma differentiation.
How does Bowen’s reaction series work?
Not all of the melt in a magma crystallises at the same time: simple silicates (ex: olivine - a nesosilicate) crystallises at higher temperatures than complicated silicates (ex: quartz - a tectosilicate).
- During cooling, small crystals of minerals with high crystallisation temperature grow larger and larger.
- Further cooling leads to the crystallisation of next stable minerals (and so on) until no magma is left.
- Crystals grow larger the more time they have. Volcanic rocks usually only contain crystals of minerals that crystallises at high temperature.
How do we classify igneous rocks?
- By texture
- By mineral and chemical composition
All igneous rocks can be divided into two broad textural classes: the coarsely crystalline rocks, which are intrusive and therefore cooled slowly; and the finely crystalline rocks, which are extrusive and cooled rapidly. Within each of these broad categories, the rocks are classified chemically as felsic, mafic, or intermediate on the basis of their silica content, or mineralogically, based on their proportions of lighter-colored, felsic minerals and darker, mafic minerals.
What are the methods used to describe and identify igneous rocks?
- General impression/main characteristics
Colour, density, solubility, magnetic effect… - Components (minerals, groundmass, matrix)
- Fabric of the components
- Structure (morphology of components - crystal size, crystal shape)
- Texture (arrangement of the components)
distribution in space (uniform or irregular)
position in space (non-aligned or aligned; layered, stratified, banded, …)
filling of space (massive or porous) - Rock group
- Rock name
What is the difference between the main lava types?
- Rocks in the upper mantle might partially melt to produce basaltic magma.
- A mixture of sedimentary rocks and basaltic oceanic rocks such as those found in subduction zones might melt to form andesitic magma.
- A melt of sedimentary, igneous, and metamorphic continental crustal rocks might produce granitic magma.
Andesitic melt
- Plagioclase, pyroxene, hornblende.
- Chemical composition close to the average of the continental crust
- Intermediate composition
- Partial melting of (wet) oceanic crust (= basalt) in subduction zones (ca. 80 km depth)
- Andesite typically occurs on the continental crust in volcanic arcs (Andes-ite)
- (Volcanic) andesite or (plutonic) tonalite is formed without magma differentiation.
Basaltic melt
-Olivine, pyroxene, plagioclase
-dry magma-mafic composition
- Partial melting of (dry) peridotite in the upper mantle (ca. > 100 km depth) - (Volcanic) basalt or (plutonic) gabbro is formed without magma differentiation.
Rhyolitic melt
- Quartz, alkalifeldspar, mica
- Felsic composition
-Partial melting of (wet) continental crust (of andesitic composition)
- (Volcanic) rhyolite or (plutonic) granite is formed without magma differentiation.
What is the relationship between lava type and volcano type? Why some volcanoes are explosive and others are not?
Volcanos like shield volcanic are less explosive because it erupts basaltic lava.
Basaltic magma has
- Low silica content
- High temperature
- Low viscosity
- Dissolved gas easily can escape
All these factors result in a non-explosive eruption.
Volcanoes like cinder cone are more explosive because of the rhyolitc lava that it erupts.
Rhyolitic magma has:
- High silica content
- Low temperature
- High viscosity
- Dissolved gas cannot escape resulting in rapid gas expansion when pressure decreases at the surface.
All theses factor result in a non-explosive eruption.
What is the definition of different volcanic particles?
• Lavas
Volcanic rocks formed from lavas range in appearance from smooth and ropy to sharp, spiky, and jagged, depending on the conditions under which the rocks formed.
• Pyroclastic rocks
In more violent eruptions, pyroclasts form when broken pieces of lava are thrown high into the air.
The finest pyroclasts are volcanic ash, extremely small fragments, usually of glass, that form when escaping gases force a fine spray of magma from a volcano.
Bombs are larger particles, hurled from the volcano and streamlined by the air as they hurtle through it.
All volcanic rocks lithified from these pyroclastic materials are called tuff.
What is the most common rocks in dikes and sills?
Diabase (dolerite) = gabbro - basalt
Pegmatite = granite - rhyolite
Aplite= granite - rhyolite
