Origins of Rocks: Where do rocks come from? Why do they form? How do they form? (Mineralology and Petrology) (L2 & L3)
What is a rock? • A naturally-occurring aggregate of minerals • CD: EM/A,B
What is a Mineral? • A naturally occurring, inorganic, crystalline solid with a specific chemical composition • A specific chemical composition means that the composition of a mineral can be expressed as a chemical formula • Example: halite (salt) is NaCl
Identify Minerals by Their Physical Properties [See CD: Earth Materials] Specific Gravity Color Streak Luster Acid Test Crystal Habit Cleavage & Fracture * Striations Hardness
What is a Crystal? • A form of matter which has a regular, repeating framework of atoms
What are atoms?[CD:Slides 1094-1105 in Geo Time] • The smallest unit of an element that retains the element’s physical and chemical properties • An element is a substance that cannot be broken down into a simpler substance • Made up of protons, neutrons and electrons • examples: gold, iron, hydrogen, oxygen, sodium, chlorine, carbon, silicon, helium
Protons have positive charge & mass of 1 Neutrons have no charge & mass of 1 Protons and neutrons make up the nucleus of an atom Electrons have a negative charge, NO mass and are most important for chemical reactions Protons, Neutrons, Electrons
Structure of an Atom Protons +/1 Neutrons 0/1 Electrons -/0
IONS +/- CHARGED ATOMS FORMERLY NEUTRAL ATOMS WHICH HAVE GIVEN UP (+) OR TAKEN ON AN ELECTRON (-) INVOLVED IN IONIC BONDING ISOTOPES ELEMENTS WITH VARIABLE ATOMIC WEIGHT DUE TO VARYING NUMBERS OF NEUTRONS IN THE NUCLEUS. IONS & ISOTOPES
Hydrogen atom: Mass = 2 1 proton, 1 electron Carbon atom: Mass = 12 6 protons, 6 neutrons, 6 electrons Electrons move in orbitals (not “orbits!”) forming shell-like spheres around the nucleus. ** If atoms loose or gain electrons they have an electrical charge (-/+) and are called ions; also, a group of atoms (molecule) can behave as an ion if they have too many or too few electrons.
What holds the atoms togetherin crystals? • Chemical bonds: • Ionic Bonds • atoms (ions) held together by their electricalcharges • weaker kind of chemical bond • Covalent Bonds • atoms held together because they share electrons • stronger kind of chemical bond
Example of covalent bonding: Carbon Naturally occurring crystalline carbon is the mineral diamond Covalent bonding between the carbon atoms make this the hardest known substance
WHAT FACTORS DETERMINE WHAT MINERAL WILLFORM? 1 WHAT ELEMENTS ARE AVAILABLE 2 ABUNDANCE OF EACH ELEMENT 3 SIZE & CHARGE OF EACH ELEMENT 4 ENVIRONMENT - SPACE, PRESSURE, TEMPERATURE
Ionic Radii & charge of some geologically important ions allows “ionicsubstitution” or SOLID SOLUTION
Average Composition of the Continental Crust O O Si Weight Percent Volume Percent
COMMON ROCK-FORMINGMINERAL FAMILIES NATIVE ELEMENTS - GOLD, COPPER HALIDES - PRECIPITATES (salts) OXIDES - 1 METAL + OXYGEN SULFIDES - 1 METAL + SULFUR SULFATES - (SO4)2- CARBONATES - (CO3)2- SILICATES - SUPER STARS!!!
Silicates • The largest group of minerals are the silicates • Silicates are distinguished by being composed of the silicate ion: (SiO4)4- • an ion of Silicon has a charge of 4+ • four ions of Oxygen have a charge of 8-
Silicate ions have a tetrahedral shape • In other words, they’re four-sided pyramids
Tetrahedrons can link together to make complex crystals isolated ring single chain double chain sheet framework--like sheet but in three dimensions
Where do rocks come from? • All rocks are made from other rocks! • Three basic families of rocks based on their origin: • Igneous • Sedimentary • Metamorphic • CD: EM/C.
Igneous Rocks • Igneous rocks formed when their constituent minerals crystallized out of molten rock as it cooled from a high temperature • Three ways to melt rocks: • increase in temperature - hot spots • decrease in pressure - mid-ocean ridges • add water - subduction zones
Where do Igneous rocks form? • Inside the earth (intrusive/Plutonic) • in cracks: dikes • in-between rock layers: sills, laccoliths • in magma chambers • magma is molten rock beneath the surface of the Earth • Surface of the earth (extrusive/Volcanic): • lava flows • pyroclastics: pumice and ash
Sedimentary Rocks? • Formed from sediments settling out of a fluid • sediments are: • particles worn off of other rocks • tiny mineral grains precipitated out of a fluid • for example, salt, gypsum, limestone • created by biologic activity • Fluid can be either air or water • mudstones form in water • petrified sand dunes form in air
Metamorphic rocks? • Formed when rocks are exposed for a long time to a different (usually higher) pressure / temperature than which they formed in • most minerals are only stable at a particular pressure and temperature range • Examples: • high pressure: downgoing slab in subduction zones • high temperature: rocks next to magma/lava • high pressure and temperature: roots of mountain ranges
high pressure high temperature
The Rock CycleCD: Earth Materials, C. • Since every rock is formed out of pre-existing rocks, the evolution of rocks can be shown to be a rock cycle, which illustrates how every rock type can be formed out of the other two types.
Igneous RocksCD:EM/D • There are a wide variety of igneous rocks, and they can be better understood if we sort them by their: • Texture • size distribution of mineral grains • Chemical composition • types of minerals present
Igneous Rock Textures • Aphanitic (Fine) Texture • Very tiny crystals • Indicates rapid cooling: extrusive or volcanic rocks • volcanic rocks form on the surface or shallow subsurface • Phaneritic (Coarse) Texture • Large crystals • Indicated slow cooling: intrusive or plutonic rocks • plutonic rocks form underground • Porphyritic (Mixed) Texture • Large crystals set in a matrix of tiny crystals • Indicates slow then fast cooling
Igneous Rock Chemistry • Most igneous rocks are made of: • Oxygen (O) • Silicon (Si) • Aluminum (Al) • Iron (Fe) • Magnesium (Mg) • Calcium (Ca) • Sodium (Na) • Potassium (K)
Igneous Rock Chemistry • All igneous rocks can be described as mafic, intermediate or felsic • Mafic Igneous Rocks • Dominated by dark-colored mineral grains • High Fe, Mg, Ca • Low Si • Felsic Igneous Rocks • Dominated by light-colored mineral grains • Low Fe, Mg • High Si
Igneous Rock Chemistry • Finally, igneous rocks can be further subdivided on the basis of how much Sodium (Na) and Potassium (K) they contain • If they have more Na and K than Ca, they are considered to be alkaline igneous rocks (Felsic)
Chemistry and Minerals • The kind of minerals an igneous rock contains results from the chemical composition of the parent magma, the molten rock from which the rock formed • The chemistry of the parent magma is the result of two processes: • Partial melting of a source rock • Fractional crystallization of the magma as it cools
Partial Melting • Any given rock is made of several different minerals with different melting temperatures • When a rock begins to melt, some minerals will melt before others • This process results in a melt which has a different chemical composition than the rock from which it is forming
Fractional Crystallization • As a magma cools, crystals will start to grow within it • These crystals remove elements from the magma, changing the chemical composition of the remaining melt • This remaining melt can then grow crystals different from those that have already grown from it
Classification of Igneous Rocks volcanic felsic intermediate mafic ultramafic plutonic
Hawaiian Igneous Rocks • It takes a great many steps of fractional crystallization to make a felsic rock out of a mafic one • In Hawaii, most of the magma was erupted too quickly for it to be anything other than a mafic, aphanitic rock: a basalt • Therefore, the most common minerals found in Hawaiian igneous rocks are olivine and Ca-rich plagioclase
Magma & Plate Tectonics • Ultramafic magmas - mantle source • Mafic magmas - partial melt of mantle • Intermediate magmas - “ “ of oceanic crust • Felsic magma - partial melt of continental crust • Don’t forget that the “mother of all rock” is PERIDOTITE.