Chapter 3, 2nd half: Minerals
Mineral species: exhibit similar internal structures and chemical compositions eg quartz, calcite, pyrite.
Mineral class: where mineral species belong to depending on their anions. Eg, Silicates, carbonates, halides
Minerals within each class tend to have similar structures, hence similar properties. They are also often found in the same rock.
Major mineral classes:
The silicates: silicon and oxygen accounting for 90% of the earth’s crust. Have the same siliconoxygen tetrahedron building block. Most silicate materials form when molten rock cools and crystallizes. Cooling can occur at earth’s surface (low pressure, low temp) or at great depths(high temp, high pressure)
Eg Olivine crystallizes at high temp while quartz crystallizes at low temp. Light silicates:
Nonferromagnesian silicates are generally light in color, varying amounts of AL, K, Ca, Na rather than Fe and Mg.
1. Feldspar: most common mineral group and forms under wide range of temp and pressures. Divided into two groups. Potassium feldspar with potassium ions in the structure and plagioclase feldspar with sodium and calcium ions.
2. Quartz: entirely made up of silicon and oxygen
4. Clay minerals
Ferromagnesian silicates are dark because of their iron content. Low in silica Most common are olivine
1. Olivine group: High temp silicate
2. Pyroxene group
3. Amphibole group
Chapter 4: Magma and igneous rocks
If you want to learn more check out How will the movement of water affect a cell if it is transferred from a hypotonic solution to a hypertonic solution?
Igneous rocks form as molten rocks and solidify. Parent material is magma which is formed by melting that occurs at various levels in the earth’s crust and upper mantle.
When molten rock reaches the surface it is called lava.
Magma is completely or partly molten rock which on cooling solidifies to form an igneous rock composed of silicate materials.
Liquid component called melt, consisting of mobile ions
Solid component are silicate minerals that have already crystallized from the melt.
Gaseous component are called volatiles, are materials that will vaporize (form a gas) at surface pressures
In crystallization as the temp drops ions pack more closely as their rate of movement slows. When cooled sufficiently the forces of the bonds confine ions to an orderly crystalline arrangement. When magma cools, the silicon and oxygen atoms link together first to form silicon oxygen tetrahedral, the basic building blocks of the silicate materials. The tetrahedral join with each other to form crystal nuclei. The earliest formed minerals have space to grow and tend to have better developed crystal faces than do later ones that occupy remaining spaces.
Igneous rocks in two settings:
1. When magma loses its mobility before it reaches the surface it crystallizes to form intrusive igneous rocks also known as plutonic rocks. Intrusive rocks are coarse grained and consist of visible mineral crystals. They are observed at the surface in locations where uplifting and erosion have stripped away overlying rocks. Don't forget about the age old question of What is the concept of risk and return?
2. When lava solidifies at the surface they are classified as extrusive igneous rocks also called volcanic rocks. They tend to be fine grained.
Igneous rocks are composed mainly of silicate minerals. Silicon and oxygen are the most abundant in igneous rocks
Granitic (Felsic) composition: composed almost entirely of light coloured silicates quartz and feldspar. Igneous rocks with these as the dominant minerals have a granitic composition. Rich in silica and makes up the continental crust.
Basaltic (Mafic) composition: rocks with substantial dark minerals and calcium rich plagioclase feldspar (no quartz) have basaltic composition. They have a high %age of ferromagnesian minerals so are also referred to as mafic. Make up ocean floor.
Intermediate (Andesitic) composition: rocks between granitic and basaltic composition
Ultramafic (Periodite): contains mostly olivine and pyroxene, composed mostly of ferromagnesian minerals.
Silica content influences composition of igneous rocks, 40% in ultramafic rocks and 70% in granitic rocks. Amount of silica in magma influences its behaviour. Granitic magma, with high silica is viscous (thick) and basaltic magma is low in silica.
Describes overall appearance of a rock based on its size, shape and arrangement of mineral grains. It reveals a great deal about the environment in which the rock was formed. We also discuss several other topics like What is needed to prove libel?
Factors influencing textures:
1. Rate at which molten rock cools
2. The amount of silica present
3. The amount of dissolved gases in the magma
Slow cooling allows ions to migrate freely until they eventually join one of the existing crystalline structures. Promotes the growth of fewer but larger crystals
When rapid cooling occurs the ions quickly loose mobility and readily combine to form crystals. Result in numerous embryonic nuclei all of which compete for available ions. The result is a solid mass of intergrown crystals. When molten material is quenched quickly, there is not enough time for ions to arrange into an ordered crystal. Rocks with unordered ions that are frozen in place are referred to as glass.
Types of igneous textures:
1. Aphanitic(finegrained) texture: rocks formed at the surface or as small intrusive masses within the upper crust where cooling is relatively rapid exhibit a fine grained texture. Common features are the voids left by gas bubbles that escape as lava solidifies and these openings are called vesicles and rocks with this feature are said to have vesicular texture.
2. Phaneritic(coarsegrained) texture: when large masses of magma slowly crystallize at great depth they form rocks with coarse grained texture. We also discuss several other topics like What are the different types of transport mechanisms?
3. Porphyritic texture (He said he likes asking questions about this one): bc diff minerals crystallize under diff environmental conditions, produced from slow cooling where crystals of some minerals become large before others even begin to form and then when the erupted lava cools it cools rapidly and the resulting rock has large crystals embedded in a matrix of smaller crystals. Large crystals are called phenocrysts and the matrixes of smaller crystals are called groundmass. Don't forget about the age old question of How do childhood growth and age of sexual maturity differ between chimps and humans?
4. Glassy texture: molten rock is ejected into the atmosphere where it is quenched quickly resulting in a glassy texture. Unordered ions are frozen in place before they are able to unite into an orderly crystalline structure. Eg Obsidian
5. Pyroclastic (fragmental) texture: Forms from the consolidation of individual rock fragments
ejected during explosive eruptions
6. Pegmatitic texture: Exceptionally coarse grained; form in late stages of crystallization of magmas (rocks are called pegmatites)
Naming igneous rocks: Don't forget about the age old question of What translates to genetic code?
Grouped on the basis of texture and mineral composition
Felsic (Granitic) igneous rocks:
1. Granite: coarse grained
2. Rhyolite: extrusive equivalent of granite. Pink, composed of light silicates 3. Obsidian: dark colored glassy rock.
4. Pumice: volcanic rock with glassy texture.
Intermediate (andesitic) igneous rocks:
1. Andesite: fine grained
2. Diorite: intrusive equivalent of andesite
Mafic (basaltic) igneous rocks:
1. Basalt: most common extrusive rock.
2. Gabbro: intrusive equivalent of basalt
Origin of magma:
Originates in uppermost mantle, when rocks melt. rocks melt when temp exceed melting temp of rock or some minerals in rock.
Controlling melting temp:
Pressure: increase in pressure raises melting point
Water content: increase lowers mp
Rock composition: felsic minerals melt at lower temp than mafic minerals Generating magma from solid rock:
Increase in temp: raise temp above rocks melting point to induce melting
Decrease in pressure: decompression melting: increase in pressure raises rocks mp but reducing pressure lowers rocks melting point. When confining pressure drops decompression melting occurs. This occurs where hot mantle rock ascends thereby moving to regions of lower pressure.
Addition of volatiles: lower the melting point of rocks
How magma evolves:
Bowens reaction series: minerals crystallize in a systemic fashion based on their melting points. Fors to crystallize is olivine
Magmatic differentiation: formation of one or more secondary magma from single parent magma
Crystal setting: earlier formed minerals are denser than the liquid portion of the magma and sink to the base of the magma chamber
Assimilation: as magma migrates through the crust it may incorporate some of the nearby rock into the chamber
Magma mixing: during ascent of two chemically diff magma bodies, the more buoyant mass may overtake the slower body merging them
Partial melting: incomplete melting of rocks
Partial melting of ultramafic rocks yields mafic magmas
Partial melting of mafic rocks yields intermediate magmas
Partial melting of intermediate rocks yields felsic magmas
Read the slides on magma composition and intrusive activity
Read the slides
Hawaiian type explosions: eruptions involving very fluid basaltic magmas are triggered by arrival of new batch of melt near surface magma reservoir. Detected bc summit of volcano begins to inflate for months or years before an eruption. Injection of this new melt causes magma chamber to swell and fracture the rock above.
Explosive eruptions: when magma rises a reduction in pressure occurs and the dissolved gases begin to separate from the melt forming tiny bubbles. Highly viscous magmas produce explosive clouds of hot ash and gases evolve into buoyant plumes called eruption columns extending 1000’s of meters into the atmosphere. This is bc of the high viscosity which means the volatiles (gas) remain dissolved until the magma reaches a shallow depth where tiny
bubbles grow. As magma moves up the fractures, a drop in pressure causes more bubbles to grow resulting in an explosive event where the magma is blown into fragments.
Viscosity of magma plus quantity of dissolved gases and the ease with which they can escape largely determine the nature of a volcanic eruption
Types of lava:
Basaltic (mafic): low viscosity and non explosive.
a. Pahoehoe lava flow: smooth surfaces that resemble twisted braids of ropes b. Aa lava flows: surfaces are rough jagged blocks with dangerously sharp edges and spiny projections.
Andesitic : more viscous than basaltic and erupts
Rhyolitic(felsic): highly viscous and erupts xplosively.
a. Pyroclastic flows: a mixture of hot gases infused with incandescent ash and lava fragments that flows down a volcanic slope. Also called a nuee ardente b. Surge: small amount of ash that separates from the main body of the pyroclastic flow. c. Lahar: mudflow of an active or inactive volcano. Volcanic debris becomes saturated with water and rapidly moves down a volcanic slope
d. Volcanic ash: hazard to airplanes.
e. Volcanic gases: respiratory health hazard because some volcanoes emit poisonous gases.
Mechanical weathering: break rocks into smaller pieces without changing the rocks mineral composition. Increases the surface area for chemical weathering.
a. Sheeting/unloading: large masses of rock exposed by erosion where concentric slabs begin to break loose. The layers are onion like. Unloading is when the overlying rock is eroded away. Outer layers separate from the rock body and continued weathering creates an exfoliation dome.
b. Frost wedging: water works its way into a rocks and freezes. This enlarges the cracks in the rocks. Another way is when moist soil freezes; they expand due to growth of ice lenses. These masses of ice grow larger when supplied with more water and gradually weakens the rock causing it to fracture.
c. Salt crystal growth: sea spray from breaking water or salty groundwater penetrates spaces in rock. As water evaporates, salt crystals form and as they grow larger they weaken the rock by enlarging the cracks
d. Biological activity: plant roots grow into fractures and wedge rock apart
Chemical weathering: a chemical transformation of rock into one or more new compounds. Most important agent is water.
a. Dissolution: some minerals dissolve in water. Halite readily dissolves in water. The presence of acid dramatically increases the corrosive force of water so it can dissolve some minerals that are insoluble in pure water. Know the formulas
b. Oxidation: iron rich minerals rust when oxygen combines with iron to form iron oxide. Can only occur after the iron has been freed from the silicate structure by hydrolysis.
c. Hydrolysis: reaction of any substance with water. Mostly affects silicates. Acid accelerates hydrolysis
d. Spheroidal weathering: gradual rounding of the corners and edges of angular rocks Rate of weathering:
Rock characteristics: for silicates weathering occurs in the same order as their crystallization hence olivine which crystallizes first is the least resistant to chemical weathering whereas quartz which crystallizes first is the most resistant.
Climate: temperature and moisture. Optimum for chemical weathering is warm temp and abundant moisture.