Formation of various rock types Igneous
Sedimentary, and Metamorphic
Rocks are aggregates of one or many minerals.
Three types of rocks:
Igneous
Sedimentary, and
Metamorphic
1. Igneous Rocks
§ Igneous rocks are formed from the crystallization of magma.
§ Igneous rocks form when molten rock cools to a solid state.
§ The term igneous comes from the Latin word ignis , which
means “fire”
Ø Magma is a slushy mix of molten rock, gases and mineral crystals within the Earth.
Ø Magma is called lava when it reaches the surface.
Ø Composition of magma
§ oxygen (O),
§ iron (Fe)
§ magnesium (Mg),
§ Calcium (Ca),
§ potassium (K), and
§ sodium (Na).
§ silica (Sio2)
Ø Of all the compounds found in magma, silica (Sio2) is the most abundant and has the greatest effect on magma characteristics.
What is Magma
Ø Magma are classified as basaltic, andesitic, and rhyolitic, based on the amount of Sio
2they contain.
Ø Silica content affects melting temperature and also impacts how quickly magma flows.
Basaltic magma
Andesitic magma
Rhyolitic magma
Viscosity:
A measure of how easily a fluid flows. Water has a low viscosity, molasses has a much higher viscosity.
Viscosity, in turn, controls the amount of gas that can be trapped in the magma.
The greater the viscosity the more gas in the magma.
The names are based on the rock type that forms when the
magma crystallizes.
Magma Type
Chemical Composition
Temperature (degrees C)
Viscosity Gas
Content Basaltic 45-55% SiO
2;
High in Fe, Mg, Ca; Low in K, Na.
1000 - 1200 Low Low
Andesitic 55-65% SiO
2; Intermediate Fe, Mg, Ca, Na, K
800-1000 Intermediate Intermediate
Rhyolitic 65-75% SiO
2; Low in Fe, Mg, Ca; High in K, Na
650-800 High High
Types of igneous Rocks
A. Extrusive Igneous Rocks: Fine-grained igneous rocks that cool quickly on Earth’s surface are called extrusive igneous rocks.
Eg. basalt, rhyolite, andesite, obsidian
B . Intrusive igneous: Some magma never reaches the surface but cools within the Earth. The igneous rocks formed are then termed intrusive. OR
Coarse grained igneous rocks that cool slowly beneath Earth’s surface are called intrusive igneous rocks.
Eg. granite, pegmatite, diorite , gabbro, peridotite, dunite
Intrusive igneous rocks are divided into 1. Hypabyssal igneous rocks:
Subvolcanic rocks, also known as hypabyssal rocks, are
intrusive igneous rocks that emplaced at medium to shallow depths within the crust, and have intermediate grain size.
Eg. Dolerite, peridotite 2. Plutonic igneous rocks:
Intrusive igneous rocks that emplaced at comparatively deeper depth , and have coarse grain size.
eg. gabbro, dunite
Magma from a single chamber may reach the Earth's surface as volcanic lava or it may cool inside the crust in a variety of different shaped intrusions. Over a very long period the magma in the chamber may itself cool into rock.
• The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting.
• Extrusive rock is produced in the following proportions:
• divergent boundary 73%
• convergent boundary (subduction zone) 15%
• hotspot 12%
Igneous Rock Mineral Compositions (Assemblages) Felsic rocks:
Generally the lighter-colored igneous rocks; enriched in silica and aluminum bearing minerals (e.g. quartz and potassium feldspar).
Rock types granite, rhyolite, pegmatite Mafic rocks
Generally the darker-colored igneous rocks; enriched in magnesium and iron bearing minerals (e.g. olivine and pyroxene).
Rock types basalt, gabbro.
Sedimentary Rocks
• Sedimentary rocks are formed on the earth’s surface under normal surface temperature and pressures.
• They result from the accumulation of the products of weathering of other rocks and organic materials.
• Weathering is a general term used for the physical and chemical breakdown of rocks at the earth’s surface by rain, wind, abrasion etc.
• Products of weathering are either transported or may accumulate where they are formed.
• The processes of transforming loose fragmented rocks into a compact solid cohesive mass is called lithification ( a process that includes compaction through burial).
• This process is also known as consolidation, and the resultant rock is said to be consolidated. Sandstone is a consolidated rock.
Classified by particle size Boulder - >256 mm
Cobble - 64 to 256 mm Pebble - 2 to 64 mm
Sand - 1/16 to 2 mm
Silt - 1/256 to 1/16 mm Clay - <1/256 mm
Sediment
METHODS OF SEDIMENT EROSION AND TRANSPORT
• There are five main agents of sediment erosion and transport. These are:
• Rivers – (or fluviatile effects)
• Sea – (marine effects)
• Glaciers – (glacial effects)
• Wind – (Aeolian effects)
• Landslides
DIAGENESIS
§ Sediments are derived by weathering and erosion of the surface rocks of the crust. Sediments are usually transported to a place where they accumulate or deposited to become a sedimentary rock.
§ The following are the processes involved:
1. The sediments may not undergo transport but may be deposited at its point of weathering. This is a sedentary rock.
2. If this undergoes transport, it becomes a transported sediment.
3. When it accumulates it is usually a loose mass e.g. sand and pebbles.
These are the ones referred to as unconsolidated rocks.
4. After accumulation the diagenesis and lithification process convert the unconsolidated sediment into an indurated or consolidated sedimentary rock which is hard, compact and coherent.
5. Diagenesis process describes all the processes that occur between deposition/ accumulation and lithification. Diagenesis occur in relatively low temperature and pressure environments near the surface of the earth.
DIAGENESIS AND LITHIFICATION
LITHIFICATION
This is the final induration of the sediments where chemical and physical reactions convert it from an unconsolidated rock into a consolidated rock.
This involves three main processes:-
A. Compaction - as more sediment is being deposited, there is an increase of weight or pressure that usually expels much of the connate water and forces the rock grains to come much closer together. As the grains are forced against each other, their outer surfaces usually dissolve and re- crystallize thus welding the grains together.
B. Cementation: Deposition of substances from aqueous solutions usually occurs in the voids or other spaces between the grains. When these solutions crystallize they bind the sediments and in the process they converted the loose aggregate into a solid coherent rock .
CLASSIFICATION OF SEDIMENTARY ROCKS OR TYPES OF SEDIMENTARY ROCKS
Sedimentary rocks are divided into three classes which include:
1. Clastic sedimentary rocks – consists of grains that of mechanical products of weathering.
Eg. Breccia
Sedimentary breccia composed of coarse, angular rock fragments cemented together
Conglomerate composed of rounded gravel cemented together Sandstone Medium-grained clastic sedimentary rock
Shale Fine-grained clastic sedimentary rock Siltstone Slightly coarser-grained than shales Claystone Predominantly clay-sized grains Mudstone Silt- and clay-sized grains
2. Chemical sedimentary rocks – formed dominantly by chemical processes and more so from direct precipitation of minerals from solutions.
Eg.
§ Carbonates
Contain CO3 as part of their chemical composition Common examples are Limestone and dolomite)
§ Chert
Hard, compact, fine-grained, formed almost entirely of silica
§ Evaporites
Form from evaporating saline waters (lake, ocean) Common examples are rock gypsum, rock salt
3. Organic sedimentary rocks– formed from organic debris such as mollusks, shells, plant debris etc.
§ Coal
Sedimentary rock forming from compaction of partially decayed plant material
METAMORPHIC ROCKS
Metamorphic rocks and Processes Metamorphism comes from the Greek words
“Meta” - change
“Morphe” – form
Ø Rocks created by heat, pressure and/or chemically reactive fluids Ø Metamorphic rocks are produced from
• Igneous rocks
• Sedimentary rocks
• Other metamorphic rocks
Ø Rocks that form when a pre-existing rock (protolith) changes due to temperature or pressure, and/or as a result of squashing or shearing.
Ø Metamorphic grade refers to the temperature and pressure under which a rock was metamorphosed.
Ø Metamorphic rocks form by solid-state (no melting) transformation of preexisting rock by processes that take place beneath Earth’s surface.
Ø Metamorphism progresses incrementally from low grade to high-grade Ø During metamorphism (transformation) the rock remains essentially solid
Examples of metamorphic rocks;
Slate, phyllite, schist, gneiss, quartzite, marble
1. Heat/Temperature as a metamorphic agent
§ Most important agent – it provides the energy needed to drive the chemical reactions that results in recrystallization.
§ Promotes recrystallization →increased grain size
§ consumes unstable mineral(s) and produces new minerals that are stable
Ø Sources of heat:
1. Contact metamorphism—heat from magma, the rock is “baked”
2. An increase in temperature with depth— geothermal gradient 3. Increasing pressure with depth
4. Radioactive decay
Agents Metamorphism
2. Pressure and differential stress
• Pressure can also alter the texture of a rock, resulting in an increase in grain size.
• Directed pressure or differential stress results in the formation and alignment of flat (platy) minerals; such as micas (e.g. biotite, muscovite).
• Micas are therefore characteristic of metamorphic rocks which have been affected by directed pressure. This texture is known as foliation. Fossils, or the pebbles in a conglomerate, can also become elongated by directed pressure.
In a depositional environment, as confining pressure increases, rocks deform in volume
Differential stresses are greater in one direction than in others.
During mountian building, rocks subjected to differential stress shortened in the direction perpendicular to that force.
Metaconglomerate is an example of differential stress.
3. Chemically active fluids
• Mainly water
• Enhances migration of ions
• Aids in recrystallization of existing minerals Ø Sources of fluids
– Pore spaces of sedimentary rocks – Fractures in igneous rocks
– Hydrated minerals such as amphiboles, clays and micas – These hydrated minerals contain water.
Elevated temperatures and pressures causes dehydration.
Once expelled, these hot fluids promote recrystallization by enhancing the migration of mineral matter.
– The metamorphism of shale to slate involves clay minerals that recrystallize to form mica and chlorite minerals.
