Textbook geology terms

Minerals

form in specific three-dimensional lattices, and the nature of the lattices and the type of bonding within them have important implications for mineral properties.

Mineral grouping

according to the anion part of their formula, with some common types being oxides, sulphides, sulphates, halides, carbonates, phosphates, silicates, and native minerals.

Silicate minerals

by far, the most important minerals in Earth’s crust.

They all include silica tetrahedra (four oxygens surrounding a single silicon atom) arranged in different structures (chains, sheets, etc.).

Ferromagnesian silicates

rock-forming silicate minerals that contain substantial amounts of iron (Fe) and/or magnesium (Mg) in their chemical structures

Formation of minerals

Most minerals in the crust form from the cooling and crystallization of magma. Some form from hot water solutions, during metamorphism or weathering, or through organic processes.

Properties of minerals

Some of the important properties for mineral identification include hardness, cleavage/fracture, density, lustre, colour, and streak colour.

Three types of rock

igneous, sedimentary, metamorphic

igneous

formed by magma

sedimentary

formed from fragments of other rocks or precipitation from solution

metamorphic

formed when existing rocks are altered by heat, pressure, and/or chemical action

rock cycle

processes that contribute to cycling of rock material among three types.

driven by Earth’s internal heat, and by processes happening at the surface, which are driven by solar energy.

magma

molten rock, and in most cases, it forms from partial melting of existing rock

main processes of magma formation

decompression melting and flux melting

Range of magma composition

ultramafic, mafic, intermediate, felsic

Mafic rocks

rich in iron, magnesium, and calcium, and have around 50% silica

Felsic rocks

rich in silica (~75%) and have lower levels of iron, magnesium, and calcium and higher levels of sodium and potassium than mafic rocks

Intermediate rocks

compositions between felsic and mafic

Crystallization of magma

1. As a body of magma starts to cool, the first process to take place is the polymerization of silica tetrahedra into chains.

2. This increases the magma’s viscosity (makes it thicker) and because felsic magmas have more silica than mafic magmas, they tend to be more viscous.

Bowen reaction series

allows us to predict the order of crystallization of magma as it cools

Fractional crystallization

separation of early-forming crystals

modifies magma

classification of igneous rock

based on their mineral composition and texture.

Phaneritic

crystals are visible to the naked eye

most intrusive igneous rocks

Porphyritic

larger crystals in a matrix of smaller crystals

this texture happens when there were were two stages of cooling (slow then fast)

Pegmatitic

very large crystals

this texture happens if the cooling was extremely slow, or if water was present during cooling

How magma intrudes into country rock

pushing it aside or melting through it

Types of intrusive igneous rock bodies

irregular (stocks and batholiths)

tabular (dykes and sills)

or pipe-like

Batholiths

have exposed areas of greater than 100 km²

Stocks

exposed areas less than 100 km2

Sills

parallel to existing layering in the country rock

Dykes

cut across existing layering in the country rock

Seismic waves

travel through earth

either P-waves or S-waves

P-waves

compression, or “push” waves

S-waves

shear waves

Differences in P-wave and S-wave motion

P-waves are faster than S-waves, and can pass through fluids.

Things seismic waves can tell us

the nature and temperature characteristics of the various parts of Earth’s interior

Earth’s internal temperature

increases with depth (to around 5000°C at the centre)

but there are significant variations in the rate of temperature increase.

These variations are related to differences in composition and the existence of convection in the mantle and liquid part of the core.

Magnetic field cause

outer-core convection

Magnetic force directions

different at different latitudes

Polarity of the magnetic field

not constant, and has flipped from “normal” (as it is now) to reversed and back to normal hundreds of times in the past

Isostasy

The “plastic” nature of the mantle, which allows for mantle convection, also determines the nature of the relationship between the crust and the mantle.

The crust floats on the mantle. Where the crust becomes thicker because of mountain building, it pushes farther down into the mantle.

Oceanic crust, being heavier than continental crust, floats lower on the mantle.

Evidence for continental drift in early 20th century

included the matching of continental shapes on either side of the Atlantic and the geological and fossil matchups between continents that are now thousands of kilometres apart

The established theories of global geology (before plate tectonics)

permanentism and contractionism

but neither of these theories was able to explain some of the evidence that supported the idea of continental drift.

Strides made in understanding Earth in mid 20th century

discovering magnetic evidence of continental drift

mapping the topography of the ocean floor

describing the depth relationships of earthquakes along ocean trenches

measuring heat flow differences in various parts of the ocean floor

mapping magnetic reversals on the sea floor

Earth’s lithosphere

made up of over 20 plates that are moving in different directions at rates of between 1 cm/y and 10 cm/y

Three types of plate boundary

divergent

convergent

transform

Divergent plate boundary

plates moving apart and new crust forming

form where existing plates are rifted apart, and it is hypothesized that this is caused by a series of mantle plumes

Convergent plate boundary

plates moving together and one being subducted

Subduction zones are assumed to form where accumulation of sediment at a passive margin leads to separation of oceanic and continental lithosphere

Transform plate boundary

plates moving side by side

Mechanisms for plate motion

widely believed that ridge-push and slab-pull are the main mechanisms , as opposed to traction by mantle convection

Mantle convection is a key factor for producing the conditions necessary for ridge-push and slab-pull

Ridge-push and slab-pull

push of oceanic lithosphere down from a ridge area

and
pull of oceanic lithosphere down into the mantle

Earthquake

the shaking that results when a body of rock that has been deformed breaks and the two sides quickly slide past each other

The rupture is initiated at a point but quickly spreads across an area of a fault, via a series of aftershocks initiated by stress transfer

Episodic tremor and slip

a periodic slow movement, accompanied by harmonic tremors, along the middle part of a subduction zone boundary

Location of most earthquakes

at or near plate boundaries

especially at transform boundaries (where most quakes are at less than a 30 kilometre depth) and at convergent boundaries (where they can be at well over a 100 kilometre depth)

The largest earthquakes

happen at subduction zones, typically in the upper section where the rock is relatively cool

Magnitude

a measure of the amount of energy released by an earthquake, and it is proportional to the area of the rupture surface and to the amount of displacement.

Although any earthquake has only one magnitude value, it can be estimated in various ways, mostly involving seismic data

Intensity

a measure of the amount of shaking experienced and damage done at a particular location around the earthquake

will vary depending on the distance to the epicentre, the depth of the earthquake, and the geological nature of the material below surface

Serious consequence of most large earthquakes

damage to buildings

amount of damage is related to the type and size of buildings, how they are constructed, and the nature of the material on which they are built

Other important consequences of earthquakes

fires, damage to bridges and highways, slope failures, liquefaction, and tsunami

Tsunami

almost all related to large subduction earthquakes

Stress

includes compression, extension and shearing

typically originates from plate-boundary processes.

Response of rock that is stressed

either elastic or plastic strain

may eventually break

response depends on its composition and structure, the rate at which strain is applied, and also to the temperature of the rock body and the presence of water.

Folding

generally a plastic response to compressive stress, although some brittle behaviour can happen during folding

Antiform

An upward  fold

Synform

A downward fold

The axis of a fold

can be vertical, inclined, or even horizontal

Fractures

joints

typically form during extension, but can also form during compression

Faulting

involves the displacement of rock

can take place during compression or extension

as well as during shearing at transform boundaries

Reverse faulting

a non-vertical fault along which the hanging wall (upper surface) has moved up relative to the footwall

Thrust faulting

a low angle reverse fault

Big bang

The universe began 13.77 billion years ago when energy, matter, and space expanded from a single point

Evidence for big bang

1. the cosmic “afterglow” from when the universe was still very dense

2. red-shifted light from distant galaxies, which tell us the universe is still expanding.

Elements produced by big bang

hydrogen, helium, and lithium

Production of heavier elements

come from nuclear fusion reactions in stars

Large stars make elements such as silicon, iron, and magnesium, which are important in forming terrestrial planets.

Large stars explode as supernovae and scatter the elements into space.

How to build a solar system

1. Solar systems begin with the collapse of a cloud of gas and dust.

2. Material drawn to the centre forms a star, and the remainder forms a disk around the star.

3. Material within the disk clumps together to form planets.

Ordering of our solar system

In our solar system, rocky planets are closer to the Sun, and ice and gas giants are farther away. This is because temperatures near the Sun were too high for ice to form, but silicate minerals and metals could solidify.

Earth’s first 2 billion years

heated by radioactive decay, collisions with bodies from space, and gravitational compression.

Heating melted Earth, causing molten metal to sink to Earth’s centre and form a core, and silicate melt to float to the surface and form the mantle and crust

Origin of the moon

collision with a planet the size of Mars knocked debris into orbit around Earth, and the debris coalesced into the moon

Origin of earth’s atmosphere

the result of volcanic degassing, contributions by comets and meteorites, and photosynthesis

Volcanism connection to plate tectonics

Most volcanoes are associated with convergent plate boundaries (at subduction zones), and there is also a great deal of volcanic activity at divergent boundaries and areas of continental rifting.

Volcanos at convergent boundary

magma is formed where water from a subducting plate acts as a flux to lower the melting temperature of the adjacent mantle rock.

Volcano at divergent boundary

magma forms because of decompression melting

Magmas in volcanic regions

initially mafic in composition, but they can evolve into more felsic types through interaction with crustal rock, and as a result of crystal settling within a magma chamber.

Felsic magma

tend to have higher gas contents than mafic magmas, and they are also more viscous. The higher viscosity prevents gases from escaping from the magma, and so felsic magmas are more pressurized and more likely to erupt explosively.

Cinder cone volcanos

relatively small volcanoes that were formed during a single eruptive event and are composed mostly of mafic rock fragments

Composite volcanos

normally associated with subduction, and while their magma tends to be intermediate on average, it can range all the way from felsic to mafic. The corresponding differences in magma viscosity lead to significant differences in eruptions style

Shield volcanos

are associated with mantle plumes, and have consistently mafic magma which typically erupts as lava flows

Pyroclastic density currents

some as hot as 1000˚C can move at hundreds of kilometres  per hour and will kill anything in the way

Lahars

volcano-related mudflows, can be large enough to destroy entire towns

Lava flows

destroy anything in their paths, but tend to move slowly enough so that people can get to safety

Volcanic ash and gases

can lead to short-term significant climate cooling.