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.