Physical Geology

Physical Geology Chapter 2

Minerals

Introduction

• About 4,500 minerals identified.

• Some properties: colour, luster, hardness, chemical compostion, transmission of light.

• Mineral: naturally occuring, inorganic, crystalline solid, that has a specific chemical composition.

Minerals and Rocks

• Rocks: naturally formed aggregates of. minerals or mineral-like substances.

  • can be composed of a single mineral.

  • Rocks are composed of minerals

• Minerals composed of atoms of elements in orderly crystalline structure.

Atoms and Elements

• Element: defined by number of protons in nucleus or atomic number.

• Isotopes: atoms containing different # of neutron but same # of protons.

• Ionic bonding = most common type of bonding in minerals

• Covalent bonding also common

  • diamonds

• Polymorphs: different crystal structures having the same composition.

Crystalline Structures

• Ionic radii play improtant role in arrangement of atoms in crystalline structure.

• Important to crystal structure = oxygen and silicon (earth’s crust)

• Silicon = element used to make computer chips

• Silica = term for oxygen combined with silicon

  • Silicon = 2nd most abundant element in crust.

• Silicates: substances that contain silica. Most contain one or more other elements

The Silicon-Oxygen Tetrahedron

• Silicon and oxygen combine to form atomic framework for most common minerals on Earth.

  • bastic structural unit: 4 oxygen atoms (anions) packed around single silicon.

• Tetrahedron: 4-sided pyramidal. Repsent 4 oxygen atoms. Each corner represents center of oxygen atom.

  • Basic building block of silicon-oxygen tetrahedron (Silica tetrahedron)

  • strongly bonded together.

  • For Silicon-oxygen tetrahedron to be stable withing crystal structure must:

    1. be balanced by enough positively charged ions or

    2. share oxygen atoms with adjacent tetrahedra.

  • Strucutre of silicate range from

    • Isolated silicate structure: depend entirely on positively charged ions to hold shape

    • framework silicates (quartz): all oxygen atoms are shared by adjacent tetrahedra.

  • Most common silsicate structures:

    • Isolated Silicate Structure

      • none of oxygen atoms are shared by tetrahedra.

      • bonded together by + charged ions

    • Chain Silicates

      • forms when 2 of tetrahedron oxygen atoms are shared w/adjacent tetrahedra.

        • a chain

      • Single Chain: ration of silicon to oxygen is 1:3

      • Double chain: two adjacent single chain.

  • Sheet Silicates

    • each tetrahedron share 3 oxygen atom to form a sheet

      • mica/clay group

  • Framework Silicates:

    • all four oxygen ion shared by adjacent tetrahedra.

      • quartz

    • Substitution occurs, so additional positive ions may be needed.

Nonsilicate Minerals

• Minerals that don’t contain silica.

• native elements: only 1 element in their formulas.

The Physical Properties of Minerals

• Color: First thing to notice.

  • Ferromagnesian Minerals: Iron/magnesium-bearing—green or black

• Streak: more realiable than color

• Luster: quality and intensity of light that is reflected from the sruface of a mineral.

  • Either metallic or nonmetallic

  • Glass (Vitreous) luster: glazed appearance. Most silicates have this.

  • Earthy Luster: surface of unglazed pottery.

• Hardness: Scratch-ability.

External Crystal Form

• Crystal Form: a set of faces that have a definite gometirc relationship to one another.

• consist of several types of forms combined together to generate full body of each specimen.

• angle between two adjacent faces of quart always exact same.

• Law of constancy of interfacial angles(steno’s law): the angles between the crystal faces of a given species are constant, whatever the lateral extension of these faces and the origin of the crystal, and are characteristic of that species

  • each type of mineral made up of many builidng blocks w/geometric shape of crystal being function of how these builidng blcoks put together.

Cleavage

• Cleavage: ability of a mineral to break when sturck along preferred planar directions.

  • Minerals tend to break along certian planes cause bonding between atom is weaker there.

    • Quartz no cleavage cause bonding all equally strong

• 3 of most common mineral groups (feldspars, amphiboles, pyroxenes) have 2 directions of cleavage.

Fracture

• Fracture: the way substance breaks where not controlled by cleavage.

  • mineral not cleavage usually have irregular fracture.

• Conchoidal fractures: break along curved fracture, inside of clam shell.

Specific Gravity

• Specific gravity: raito of mass of substance to mass of equal volume of water.

Special Properitees

• Properties only apply to one or few minerals

• Smell

• taste

• Striations: straight, parallel lines on flat surface of one of two celavage dir.

• Magnatism

• Generating electricity. Piezoelectricity.

• Double Refraction: light splitting into two componenets when it enters crystalline matierals. Travelling through components at different velocities.

Chapter 3

The Rock Cycle

• Rock: naturally formed, consolidated material, composed of grains of one or more minerals.

• earth change cause of internal/external heat engines. forcing substances out of equilibrium. Earth surface ever changing/rocks/minerals change too

• 3 Major rock types: igneous, metamorphic, sedimentary.

  • Rock move from deep to shallow, high to low temp

• Tectonic forces required to transport sedimentary (and volcanic) rock to lower levels in the crust. Igneous rock weatherd, turns sediment, buried underground, further buried returned to magma through plate teteonic.

Igneous Rocks

• Igneous rock = rock that has solidified from magma.

• Magma = molten rock, usually rich in silica and containing dissolved gases.

• Lava = magma on the Earth's surface

• extrusive = form at the Earth's surface

• intrusive = magma solidifies underground.

• Granite = a coarse-grained rock composed predominantly of feldspar and quartz,

  • is an intrusive rock.

  • Felsic Rock

  • granite = most abundant intrusive rock found in continents.

• Volcanic rocks = fine-grained (or glass) due to rapid solidification;

• intrusive rocks are generally coarse-grained. which indicates that the magma crystallized slowly

• slower cooling of liquids results in larger crystals.

• contact = a surface separating different rock types.

• Country Rock: Preexisting solid rock, any older rock into which an igneous body intruded.

• xenoliths = fragments of rock that are distinct from the body of igneous rocks in which they are enclosed.

• chill zones = finer-grained rocks that indicate magma solidified more quickly cause of rapid loss of heat to cooler rock.

Igneous Rocks Textures

• Texture = rock's appearance with respect to the size, shape, and arrangement of its grains

• crystalline rocks = made up of interlocking crystals

  • 2 critical factors determine grain size of igneous rocks: rate of cooling and viscosity.

    • magma cools rapidly = small crystals.

    • rock viscous lava is more likely to be finer grained than one formed from more fluid lava. Cause thick slows down movement of crystals

  • fine-grained rocks = most of the grains are smaller than 1 millimeter. Extrusive

• glassy rocks = composed primarily of glass and contain few crystals,

• fragmental rocks = fragments of igneous material.

• plutonic rocks = formed deep; coarse-grained

• coarse-grained, coarsely crystal-line rocks = most of the grains larger than 1 millimeter

• pegmatite = extremely coarse-grained

• porphyritic = distinct difference in the size of their mineral crystals

Glassy Textures

• extremely rapid or almost instantaneous cooling, forming glass rather than crystals.

  • Obsidian = one of the few rocks that is not composed of minerals.

Textures Due to Trapped Gas

• when lava solidifies while gas is bubbling through it, holes are trapped in the rock (vesicular texture)

• Vesicles = cavities in extrusive rock resulting from gas bubbles that were in lava

• Scoria = highly vesicular basalt

• pumice = a frothy glass with so much void space

Fragmental Texture

• pyroclasts = fragments of volcanic material ejected during explosive volcanic eruptions

• tuff (volcanic breccia) = rock composed of fine-grained pyroclastic particles (dust and ash).

Chemistry of Igneous Rocks

• chemical composition of magma determines which minerals and how much of each will crystallize when an igneous rock forms.

• igneous rocks are composed primarily of the silicate minerals quartz, plagioclase feldspar, potassium feldspar, amphibole, pyroxene, biotite, and olivine.

• magma rich in silica, aluminum, potassium, and sodium will crystallize minerals that contain those elements (feldspar and quartz).

• magma rich in iron and magnesium/calcium will contain a lot of the dark-colored minerals

• all igneous rocks, SiO2 (silica) is the most abundant component.

Mafic Rocks

• rich in iron and magnesium, characterized by their dark color. silica poor.

• made up predominantly of gray plagioclase feldspar and the ferromagnesian minerals pyroxene and olivine

• gabbro = Mafic magma that cools slowly beneath the surface forms the coarse-grained

• basalt = mafic magma erupts on the surface, it forms the dark, fine-grained

Felsic Rocks

• felsic rocks = silica-rich igneous rocks with a relatively high content of potassium and sodium. lighter in color than other types of rocks

• Minerals: quartz/feldspar. Element: sillicon, aluminium

• Rhyolite = a fine-grained, extrusive felsic rock.

• Granite = coarse-grained intrusive equivalent of rhyolite.

Intermediate Rocks

• Intermediate Rocks = chemical content between that of felsic and mafic are classified. dark ferromagnesian and light-colored

• diorite = coarse-grained intermediate rock.

• Andesite = fine-grained intermediate rock, is typically medium-gray or greenish-gray in color.

Ultramafic Rocks

• ultramafic rock = contains less than 45% silica and is rich in iron, magnesium, and calcium. No feldspars, no quartz. rare on the surface. 

• Komatiite = rare, volcanic rock that is a signature of the early Earth's hotter climate.

• Peridotite = coarse-grained intrusive rock, composed of olivine and pyroxene, most abundant ultramafic rock

Identifying Igneous Rocks

• In order to identify an igneous rock you must consider both its texture and the minerals it contains.

• Granite, being coarse-grained, can identified by verifying that quartz is present.

• Rhyolite usually cream-colored, tan, or pink.

• Andesite, medium-gray or medium-green color.

INTRUSIVE BODIES

• Intrusions, or intrusive structures = bodies of intrusive rock whose names are based on their size and shape, as well as their relationship to surrounding rocks.

• various intrusions classified on of the considerations:

  • (1) Is the body large or small?

  • (2) Does it have a particular geometric shape?

  • (3) Did the rock form at a considerable depth, or was it a shallow intrusion

  • (4) Does it follow layering in the country rock or not?

Shallow Intrusive Structures

• small compared to those formed at depth. Intrusions that formed at depths of less than 2 kilometers

• volcanic neck = intrusive structure formed from magma that solidified within the throat of a volcano. weathering occurs

• Dike = tabular (shaped like a tabletop), discordant (not parallel to any layering), intrusive structure

  • shallow depths and be fine-grained, or greater depths be coarser-grained.

• sill = tabular intrusive structure, but it is concordant (parallel to any planes or layering in the country rock)

• If the country rock is NOT layered, a tabular intrusion = dike.

Intrusives that Crystallize at Depth

• pluton = body of magma or igneous rock that crystallized deep depth within the crust.

• stock = small discordant pluton, less than 100 square kilometers.

• batholith = greater than 100 square kilometers, plutonic rock. Mostly granite.

• diapirs = blobs of magma

• Silicic magma is much more viscous than mafic magma.

ABUNDANCE AND DISTRIBUTION OF PLUTONIC ROCKS

• Granite most abundant igneous rock in mountain ranges.

How Magma forms

• Most of the heat that contributes to the generation of magma comes from the Earth's core

• geothermal gradient = rate at which temperature increases with increasing depth

  • 3°C for each 100 meters

Decompression Melting

• melting point of a mineral generally increases with increasing pressure.

  • a rock that melts at a given temperature at the surface of the Earth requires a higher temperature to melt deep underground.

• Decompression melting = when a body of hot mantle rock moves upward and the pressure is reduced. pressure reduced lowers melting point.

Addition of Water (Flux Melting)

• Water sealed in under high pressure helps break silicon-oxygen bonds in minerals,

• flux melting = rock's melting temperature is significantly lowered by water under high pressure.

How Magmas of Different Composition Evolve

• Bowen's reaction series = sequence in which minerals crystallize in a cooling magma.

  • cooling magma, certain minerals are stable at higher melting temperatures/crystallize before those stable at lower temperatures.

  • Discontinuous branch: only ferromagesian minerals

    • olivine crystals react with the melt and recrystallize to pyroxene

    • If all of the iron and magnesium in the melt is used up before pyroxene recrystallizes to amphibole, then the ferromagnesian minerals in the solid rock will be amphibole and pyroxene.

• Crystallization in discontinuous and continuous branch takes place at same time.

Continuous branch: only plagioclase feldspar.

• sodium or calcium atoms, with aluminum, silicon, oxygen accommodated in its crystal structure,

• composition of plagioclase changes as magma is cooled and earlier-formed crystals react with the melt.

• mafic magma crystallize into pyroxene (w/ or w/o olivine) and calcium-rich plagioclase, basalt or gabbro

• intermediate magma crystallize into diorite or andesite

• minerals are separated from a magma, remaining magma is more felsic than the original

• heat a rock, the minerals will melt in reverse order.

Differentiation

• differentiation = process by which different ingredients separate from an originally homogenous mixture

• crystal settling = downward movement of minerals that’s denser than magma from which they crystallized.

Partial Melting

• a process that occurs when some minerals in a rock melt, but not all of them, due to high temperatures

  • basaltic magma product of partial melting of ultra-mafic rock in the mantle

Assimilation

• process by which magma incorporates solid or fluid material from the surrounding rock

Mixing of Magams

• If two magmas meet and merge within the crust, the combined magma should be compositionally intermediate

Explaining Igneous Activity by Plate Tectonics

• Divergent boundaries creation of basalt and gabbro of the oceanic crust.

• Convergent basalt, gabbro

Igneous Processes at Divergent Boundaries

• crust beneath the world's oceans: mafic volcanic and intrusive rock,

  • most basalt and gabbro created at mid-oceanic ridges, most common at oceanic crust

• mafic magma/basalt produced at divergent boundaries due to partial melting of asthenosphere.

Intraplate Igneous Activity

• mantle plumes = narrow upwellings of hot material within the mantle

Igneous Processes at Convergent Boundaries

• Intermediate and felsic magmas/granite/andesite are related to convergence of two plates and subduction

• melting occurs cause the subducted oceanic crust releases water into the asthenosphere.

• Partial melting produces a mafic magma.

• mafic magma evolves into an intermediate magma by differentiation and by assimilation of silicic crustal rocks.

• origin granite

  • partial melting of the lower continental crust must take place.

• felsic: silica rich, Mafic: silica poor.

Chapter 4: Volcanism and Extrusive Rocks

• strong correlation between chemical composition of magma/lava, its physical properties, size/shape of volcanos, lava flows, and violence of eruption.

• atmosphere created by degassing magma.

• hydrosphere: Condensation of water vapor during the degassing.

ENVIRONMENTAL GEOLOGY 4.1

• Volcanism = occur when magma makes its way to Earth's surface

• Volcanoes = landforms formed by extrusion of lava /ejection of rock fragments from vent.

• pyroclastic flows = fast-moving of hot ash and pyroclastic stuff

• Explosive eruptions (pyroclastic eruptions) = generation of solid volcanic fragments.

• Effusive eruptions = slower, lava flows.

• Volcanic activities: create new land, geothermal energy.

Eruptive Violence and Physical Characteristics of Lava

• explosivity: (1) amount of gas in the lava or magma (2) the ease/difficulty with the gas can escape to atmosphere.

• viscosity = resistance to flow how easily gas escapes.

  • more viscous the lava greater volume of gas trying to escape = more violent eruption.

• most gas released = water vapor.

• lava = too viscous will fragment, causing explosive eruptions blast ash and rock

• 3 factor influence viscosity of lava

  • (1) the silica (SiOz) content of the lava

  • (2) the temperature of the lava

  • (3) gas dissolved in magma

  • Mafic lavas, low SiO2 flow easily. felsic lavas more viscous (explosive eruptions)

    • slower lava cause silicon-oxygen tetrahedra linked to form small, framework structures in the lava.

The Eruptive products of Volcanoes

• volcano = opening in earth's crust molten lava, ash, and gases are ejected.

• vent = opening which an eruption takes place.

• crater = basin-like depression over a vent at summit of cone

• flank eruption = lava pours from vent on side of a volcano.

Effusive Eruptions

• commonly basaltic in composition cause mafic basalts are less viscous, and gases can escape easily.

Mafic Lava flows

• low silica content, basaltic lava typically low viscosity, flows easily.

• Pahoehoe = rapid cooling and solidification of surface of the lava flow

• 'A'a = flow that has a jagged, rubbly surface

• spatter cone = small, steep-sided cone built from lava sputtering out vent

• lava tube = tunnel-like conduit for lava that develops after lava fluid

Flood Basalts

• Flood Basalts = vast outpourings of mafic lava from fissures

Columnar Jointing

• Columnar jointing = mostly six-sided, vertical columns.

  • basalt contracts as it cools after solidifying.

Submarine Lava Flows

• pillow structure = rocks, generally basalt, pillow-shaped, rounded masses.

  • from lava erupts into water and cools rapidly

• Pillow basalts = Fluid lava flows into water. each blob squeezed out and solidifies and continues this process.

• basalt magma flows up the fracture that develops at a divergent boundary

• intermediate/felsic lava more viscous

explosive Eruptions

• pyroclasts = explosive eruption, expanding, hot gases fragments, cooling magma into fragments blasting into the air.

• soda can metaphor, magma rise toward surfece, decrease in pressure gas out and expand.

Pyroclastic materials (tephra)

• dust/ash. determined by size.

  • dust < 1/8 milimeter

  • ash 1/8-2 milimeter

• Cinder = bigger than dust/ash. general term for smaller pyroclasts.

• Lapilli = bigger, grain of rice to a peach.

• blocks = angular pyroclastic fragments, no corners, or rounded edges

• bombs = spindle, lens shaped. looks like a seed.

pyroclastic flow

• pyroclastic flow = dense mixture of gas and pyroclastic debris, flows rapidly to low areas.

  • develop by volcanic domes, exploding froth of gas/magma blast from side. gravitational collapse of gas/pyroclastic debris initially in air.

Types of Volcanoes

• Shield volcanoes = broad, gently sloping. solidified lava flows. Eruptions nonviolent lava spreads widely/thinly, low viscosity.

  • composition: basalt

• Cinder Cones (pyroclastic cone) = constructed of pyroclastic fragments from central vent. Exclusively of pyroclasts. steep (33 degrees). Smallest of 3 types. Mafic/intermediate lava. short life span.

  • composition: Pyroclastic fragments of any. Basalt most common.

• Composite Volcanoes (stratovolcano) = alternating layers of pyroclastic fragments, solidified lava flows. intermediate steepness. solidified lava protective cover over loose pyroclastic layers, composite volcanoes less vulnerable to erosion than cinders. explosive.

  • composition: Layers of pyroclastic fragments and lava flows. Mostly andesite

Distribution of Composite volcanoes

• Circum-Pacfic belt (ring of fire) = larger belt of composite volcanoes.

• Mediterranean belt = second major volcanic belt. Mount Vesuvius. An exceptionally violent eruption.

Lava Domes

• Lava domes = steep-sided, dome or spine-shaped masses of volcanic rock formed from viscous lava solidifies in or immediately above a volcanic vent. High in silica

• minerals crystallize, rock is rhyolite if from felsic magma, or andesite if from intermediate lava

Calderas

• caldera = volcanic depression much larger than original crater. summit blown off by explode gas/collpase due to empty magma chamber.

Volcanic Hazards

• ash clouds, pyroclastic flows, and mudflows.

• Pyroclastic flows, the most deadly volcanic hazards, Pyroclastic fall death by ash-covered roofs or by being hit by falling rock fragments.

• lahars = Volcanic mudflows, pyroclastic materials mixes with rainwater/snowmelt, forms slurry of water, ash, large boulders flows rapidly down slope

• Famine and other indirect causes, volcanic lightening

Monitoring Volcanoes

• active = currently erupting or has erupted recently.

• Dormant = not erupted in many thousands of years but expected to at some point

• Extinct = not erupted for a very long time and show no signs of ever again.

Volcanic Activity at Divergent Boundaries

• divergent plate boundaries, decompression melting of the asthenosphere generates basaltic magma contains small amounts of water.

• eruptions almost always consist of mafic lavas that create basalt.

• basaltic rock makes up virtually the entire crust underlying the oceans.

Volcanic Activity at Convergent Boundaries

• all larger/better-known volcanoes of the world located on convergent plate boundaries

• Melting occurs when the subducted oceanic crust releases water into the overlying asthenosphere, lowering its melting temperature, flux melting

• majority of lavas erupted along convergent margins are andesitic in composition. Andesite is more viscous than basalt.

• combination of viscous lava and large amounts of water vapor is what generates explosive eruptions.

Within-Plate Volcanic Activity

• Volcanic activity that away from plate boundaries, within tectonic plates, related to mantle plumes (hot spots) within plates.

• Hot spot melting associated w/large volumes of basaltic magma.