pennsylvania master naturalists geology workshop part 1 april 2012
TRANSCRIPT
Pennsylvania Master Naturalists Geology Workshop
part 1 April 2012
Sources • Chet & Maureen Raymo, Written in Stone, 1989• Joel Arem, Rocks & Minerals, Bantam Books, 1973• Bruce Goodwin, Geology of the Philadelphia Area, 1966• Commonwealth of Pennsylvania, Topographic and
Geologic Survey, Dept of Environmental Resources• Sarah West, “Gems of the Wissahickon,” 1995 • Wissahickon Photos by Sarah West• Aron Sando, Neree Wissahickon Worth Preserving 2007• Tarbuck & Lutgens, Earth, An Introduction to Physical
Geology, 2004• Stepanski & Snow Gem Trails of Pennsylvania and New
Jersey 1996
Program Overview
Minerals:
Rocks:
The Rock Cycle
Minerals• About 4000 different minerals have been identified• About 25 different minerals are common.• Over 90% are silicates• The building blocks of
rocks
MineralsMinerals are• Inorganic substances• Naturally occurring• Solid• Possess and orderly internal structure• Have a definite chemical composition, a definite
formula, such as NaCl or SiO2
Silicate Minerals
Identification of Minerals
• By Crystal Form
• By Hardness
• By Streak Color
• By Luster
• By Cleavage
• By Specific Gravity
Crystal FormIf space is available minerals form crystals
with a characteristic shape
Clear Quartz Garnet
Table Salt, NaCl
By Hardness
Fracture vs Cleavage• Cleavage: the tendency of a mineral to
break along a plane of weak bonding leaving a relatively smooth surface. Minerals that cleave will break into similarly shaped pieces.
• Fracture: Minerals that do not show cleavage will fracture into pieces with irregular surfaces. Minerals that fracture will break into dissimilar pieces.
Muscovite Mica & Biotite Mica cleave into thin sheets
Orthoclase Feldspar. Cleaves in 2 planes at close to a 90o angle
Fracture: Uneven breakage Concoidal Fracture Smooth fracture
glass or quartz Hematite
Types of Luster
Metallic
Non-Metallic
• Dull
• earthy
• Glassy
• Pearly
• Silky
Galena: metallic luster on right. Dull luster on left.
Asbestos: Silky luster
Orthoclase Feldspar: pearly luster
Plagioclase Feldspar
(showing striations)
Mineral Color is NOT a reliable identification property.
The color of a mineral depends on the inclusions that are present in the mineral crystal.
• Varieties of Quartz
Streak Color is used for identification• The color left from rubbing a mineral along a ceramic
plate may be very different from the apparent color of the mineral
• Some minerals leave no streak color• Pyrite has a metallic luster but a dark gray streak
Specific Gravity
• Ratio of the weight of a mineral to the weight of an equal volume of water
• Common rocks and minerals have a specific gravity of 2.5-3.0
• Metallic minerals have specific gravities 2 or 3 times greater. (Galena specific=7.6. Pure Gold =20)
• galena
Other Properties used in Identification
• Reaction with Acid: Identifies Carbonates such as calcite & dolomite
• Magnetism: identifies Magnetite
• Flourescence
Flourescence
• Sterling Hill Mine “Christmas Tree Ore”, Franklinite• Calcite fluoresces bright orange• Willemite: bright green• Hydrozincite: blue
Rocks are mixtures of Minerals. They will not have a single formula representing their composition
.
Rocks are classified according to the way they form as…
• Igneous
• Sedimentary
• Metamorphic
Two Major Categories of Igneous Rock
Felsic: Are highest in Silica, Feldspar & Quartz. Form viscous, explosive magma– Granite, Andesite, Rhyolite
Basaltic: Are lowest in silica. High in Iron and Magnesium. Form less viscous, more liquid magma– Basalt, Diorite, Gabbro
Felsic Igneous Rock
Basaltic (Mafic) Igneous Rock
Igneous Rocks are identified by
Component minerals
Texture
• Coarse Grained:
• Fine grained: Glassy
• Pyroclastic:
Glassy or Frothy Textures result when cooling is very rapid as in explosive lava eruptions
Texture reflects cooling rate or magma water content
When water content is high or cooling is very slow mineral crystals can grow large.
Pegmatite
Process of Sedimentary Rock Formation
• Sediment deposition or precipitation– In water or blown by wind
• Sediment Burial– Sediments pile up over time
• Compaction– Weight of overlying layers compress the lower layers
• Cementation– Dissolved substances fill in spaces and join or cement
the particles
Sedimentary Rocks are Identified by
Types of Sediments
• Detrital
• Chemical
• Organic
Particle Size
Presence of Fossils
Detrital Sedimentary Rocksvary according to particle size
• Shale: mud particles predominate
• Sandstone: sand particles predominate
• Breccia :gravel particles predominate
• Conglomerate: gravel particles predominate
Sediment Sorting by Particle Size
Heavy Particles can settle in turbulent water.
The lightest particles settle only in still water
Sandstone
Breccia & Conglomerate
Grand Canyon Bright Angel Shale
Crossbedding indicates formation from c wind blown sediments
Shale with fern fossil
Chemical Sedimentary Rock forms from material that is carried in solution and later precipitates
Inorganic Chemical sedimentary rock• Vein Quartz
• Geodes
• Chert
ChertAgate Jasper Flint
Geodes
Biochemical (organic) Sedimentary Rock
• Limestone CaCO3
• Dolomite CaMg(CO3)2
• Coral
• chalk
• Coquina
• Coal
Coquina
Chalk Cliffs of Dover
CoalFormation
Metamorphism produces:
• Layering
• Deformation– Uplift– Folding
• Recrystalization of minerals. Formation of gems such as mica, garnet. Staurolite, tourmaline etc.
Eroded Folds, Marcia Jones King
Metamorphic Rocks are classified by
Texture
• Foliated: layered
• nonfoliated
Grain Size
Folding or other deformation
Metaconglomerate: Note elongation of pebbles and beginning of alignment into
layers
Metamorphic Grades• Low
– Slate, phyllite
• Medium– Schist
• High– Gneiss, migmatite
The Rock Cycle
• illustrates the origin of the basic rock types
• explains how rocks can be transformed from one type to another
Rock Cycle
Metamorphosis occurs at subduction zones when crustal plates collide
Two Forces Drive the Rock CyclePart II
Plate Tectonics
• Produces new igneous rock
• Produces metamorphism of preexisting rocks
Weathering
• Produces sediments that are later compressed into sedimentary rocks
Seeing is BelievingOne important objective of our field trip is to
appreciate and enjoy the natural beauty all
around us.