Geology 114A Lecture Notes

This is the first time I have taught this course; material given without dates is preliminary and subject to change.

What is a molecule?
Molecule: group of bonded atoms; e.g., H₂O, SiO₂, NaCl

What is a mineral?
Mineral: a solid of specific composition with a regular arrangement of atoms

How do atoms bond together to form minerals?
Elements bond by sharing or transferring electrons

Why don't elements prefer to remain alone, unbonded?
Elements like to have their outer electron orbital full of electrons, so elements with full orbitals are very stable (e.g., the noble gases He, Ar, Kr, Xe)
elements near the left side of the periodic table (e.g., K⁺, Mg²⁺) like to give up electrons (the next lower orbital becomes full), while those near the right side like to gain electrons to become full (e.g., S^2-, Cl^-)

ion: an element that has gained (anion) or lost (cation) electron(s)

How do bonds form among alkali metals and halogens?
ionic bonds form by the transfer of atoms from an element with an nearly empty orbital (e.g., Na) to an element with a nearly full orbital (e.g., Cl): this bond produces harmless table salt from an explosive metal and a poisonous gas!
elements in the center of the periodic table (e.g., C, Fe, Ni) with half-full outer orbitals tend to share electrons rather than donate or capture
covalent bonds form by the sharing of electrons among elements with half full outer orbitals

If Si and O are the most common elements in the outer part of the Earth, what are common minerals made of?
silicates are the most common minerals because O is the most common anion and Si is the most common cation

How do Si and O bond together to form 3-D structures?
Is SiO₄ a stable compound?
SiO₄ has a net negative charge of 4-; this must be balanced

What structure can be formed from pure Si and O?
Quartz: SiO₂ in a 3-D array of tetrahedra, each of which is joined to other tetrahedra at all 4 corners; quartz is 100% SiO₂ and has a density of 2.65 g/cm³

the minimum coordination number for an element that is part of a 3D mineral is IV, thus SiO₄, (cannot form 3-D structures from 3-coordinated things like CO₃) Are other structures possible for pure Si and O?
SiO₂ can also form other polymorphs, including:
coesite: coordination number = VI, density = 2.92 g/cm³, stable above 2 GPa;
stishovite: coordination number = VIII, density = 4.8 g/cm³, caused by meteorite impacts;
polymorphs: minerals with different structures but identical composition--usually form at different pressures or temperatures

What structures can be formed by adding elements to Si and O?

• albite, a plagioclase feldspar, NaAlSi₃O₈, can be formed from quartz via a substitution that maintains charge balance: Si4O₈ - Si⁴⁺ + (NaAl)⁴⁺
  albite contains less SiO₂ than quartz, 67%, and has a higher density of 2.6 g/cm³
  
• anorthite, also a plagioclase feldspar, CaAl₂Si2O₈, forms by a similar substitution scheme: Si4O₈ - 2Si⁴⁺ + (CaAl)⁴⁺
  anorthite contains less SiO₂ than albite, 58%, and has a higher density of 2.9 g/cm³

• mica, a platy mineral is composed of SiO₄ tetrahedra that share three corners with other tetrahedra; K⁺ between the layers provides charge balance
  
• pyroxene, (Mg,Fe)SiO₃, a blocky mineral composed of chains of SiO₄ tetrahedra that share two corners; its 2- charge is balanced by an M²⁺ anion;
  pyroxenes have only 50% SiO₂ and a density of 3.1 g/cm³
  ionic substitution (e.g., Fe <--> Mg) allows wide compositional variation in pyroxene
  
• olivine, (Mg,Fe)2SiO₄, is composed of independent tetrahedra balanced by 2^M2+ anions; it contains only 38% SiO₂ and has a high density of 3.3 g/cm³

How do pressure and temperature vary within the Earth?
P gradient within the Earth results from density*gravity*depth
T gradient within the Earth results from radioactive heating + cooling from nebular condensation + crystallization of the inner core from the outer cord

Is the Earth the same composition everywhere?
Crust-to-core figure
About 500 Ma after the beginning of the accretion to form the Earth, the temperature and pressure below ~500 km depth became high enough to melt Fe, which enabled this dense metal to sink to the core of the Earth
This gravity-driven movement of Fe toward the core produced additional heat and allowed other elements to melt (e.g., Ni)
thus the Earth became differentiated, principally by gravity, into a dense Fe-Ni core (11 g/cm³), a Mg-Si-O mantle (3-5 g/cm³) and a low-density Na-Al-O crust (2-3 g/cm³)

If we know the composition of the Earth, what minerals and rocks does it contain?
rock: an aggregate of minerals, glass or rock fragments

part of Earth	minerals	type	density
core	Fe + Ni minerals	metallic	>5 g/cm3
mantle	olivine + pyroxene	ultramafic	>3.3-4.5 g/cm3
oceanic crust	plagioclase + olivine	mafic (Magnesium + Ferrum)	3.0 g/cm3
continental crust	quartz + feldspar	felsic	2.7 g/cm3

Thursday, April 10, noon: UCSB alumna Nancy Emerson (Superfund Project Manager for Unocal) and associate Rita Rausch (Senior Project Geologist for the company Levine-Fricke Recon) speak on careers in geoscience.

How old is the Earth? (Recall that the universe is 15-20 Ga old)
Radiometric dating of the moon and meteorites indicate that the Earth is 4.6 Ga old

How does radiometric dating work?

isotope: atoms of a single element with the same number of protons, but different numbers of neutrons (e.g., ¹⁸O, ¹⁷O, ¹⁶O, all have 8 protons, but 10, 9, or 8 neutrons)
some (parent) isotopes are unstable and break down to form daughter isotopes at a constant rate of decay

Radioactivity was discovered about 100 years ago by Wilhelm Roentgen and Henri Becquerel, who noticed that uranium, just like sunlight, fogged photographic paper. Marie and Pierre Curie found other elements, like Th, Po, and Ra ("radioactivity" is named after radium), that behaved this way.

two common decay schemes are:
beta decay: a neutron loses an electron, becoming a proton, and radiant energy is expelled in the form of gamma rays; the atomic (proton) number (Z) of the daughter is the parent+1 and the neutron number (N) is the parent-1, but the atomic mass (A = Z + N) does not change (e.g., ⁴⁰₁₉K -> ⁴⁰₂₀Ca)
alpha decay: the loss of 2 protons and 2 neutrons that have combined to form He (e.g., ²³⁸₉₂U -> ²³⁴₉₀Th + ⁴₂He)
half life, t_1/2 , is the time required for half of a parent isotope to be consumed

alpha particle: helium nucleus
beta particle: electron emitted from a nucleus
gamma particle: electromagnetic radiation similar to x-rays
Table of the Nuclides

What radioactive elements exist and which parent-daughter decay schemes are commonly used and for what purposes?

decay scheme	material	age	half life
14C -> 14N	organic material	100-100,000 a	5730 a
40K -> 40Ar	felsic crustal rocks	100 ka to 10 Ga	1.3 Ga
238U -> 206Pb	mafic crustal rocks	1 Ma to 10 Ga	4.5 Ga

how old are the oldest minerals on Earth? 4.2 Ga (zircons)

how old are the oldest rocks? 3.8 Ga (sediments)

Geologic time scale from http://icecube.acf-lab.alaska.edu/~fsklb1/geo-time.html

EON	ERA	PERIOD		EPOCH	DATES (Ma)	AGE of
Phanerozoic	Cenozoic	Quaternary		Holocene	0-2	Mammals	Humans
				Pleistocene
		Tertiary	Neogene	Pliocene	2-5
				Miocene	5-24
			Paleogene	Oligocene	24-37
				Eocene	37-58
				Paleocene	58-66		Extinction of dinosaurs
	Mesozoic	Cretaceous			66-144	Reptiles	Flowering plants
		Jurassic			144-208		1st birds/mammals
		Triassic			208-245		First Dinosaurs
	Paleozoic	Permian			245-286	Amphibians	End of trilobites
		Carboniferous	Pennsylvanian		286-320		First reptiles
			Mississippian		320-360		Large primitive trees
		Devonian			360-408	Fishes	First amphibians
		Silurian			408-438		First land plant fossils
		Ordovician			438-505	Invertebrates	First Fish
		Cambrian			505-570		1st shells, trilobites dominant
Proterozoic	Also known as Precambrian				570-2,500		1st Multicelled organisms
Archean					2,500-3,800		1st one-celled organisms
Hadean					3,800-4,600		Approx age of oldest rocks 3,800