Geology for Archaeologists

Lithics Guide for Archaeologists

Purpose of Lithic Identification

Jump to Geology for Archaeologists

Prehistoric Native Americans extensively utilized the Carolina Slate Belt as a source of lithic raw material. Several studies have been conducted in North Carolina related to raw material acquisition and extraction. While these studies have helped improve the knowledge concerning the archaeology of the Slate Belt, there still remain many unanswered questions and broad gaps in our understanding of this geological region as a major source of lithic raw material during prehistory.

Many questions concerning the economic importance of this area to prehistoric groups through time remain enigmatic. One of the major problems continues to be the difficulty in positively identifying the locations of specific sources from specific raw material types found on sites away from the Slate Belt. This problem is directly related to the complexity of Slate Belt geology. As a result, there exists a basic identification problem.

In general, it is the high mineralogical and structural variability of Slate Belt rocks that makes it so difficult to recognize specific source locations from specific hand specimens or artifacts. As John Davis (1992) has so eloquently stated:

"The main problem is that the basic mineralogy and geochemistry of the lithic components are similar within a range of various elemental combinations, but highly differentiated as a result of successive metamorphic and depositional episodes. The lithic components are widespread across the Slate Belt and intermixed within formations rather than localized to specific outcrops."

Most archaeologists have struggled with this problem by using macroscopic variables such as color, grain size or texture, groundmass, inclusions, and fracture quality to describe raw material types from specific archaeological sites. Others have used geologic or mineral material types such as rhyolite, dacite, argillite, or tuff for descriptive purposes. Still others default to the use of generic type names such as metavolcanic, metasedimentary, felsic, or mafic when describing the raw materials associated with specific sites. As a result, there has been very little consistency among various studies regarding the definition of raw material variability. This makes comparison and synthesis of data nearly impossible.

There is some basic nomenclature in use by geologists to describe geological samples. In order to help address this "identification problem," it is suggested that archaeologists working with Slate Belt materials adopt a basic nomenclature when describing raw material types. In essence, we all need to know a little more geology. This means archaeologists have to work more closely with geologists, with an emphasis on achieving a baseline of comparability. Macroscopic analytical techniques to define lithic raw material variation should use the same basic language adapted from geology and in consultation with professional geologists.

The Office of State Archaeology has sponsored the creation of this website to provide archaeologists with a general source of information regarding basic geology with an emphasis on Slate Belt materials. The site provides a set of methods for identifying and classifying lithic raw materials and addresses the basic levels of identification regarding various rock types. To the side, one will find links to matrices that serve as guides through the Identification process. Also listed are links to a glossary with definitions of all the key terms found on the site.

Beyond identification, a major goal of the site is to further the development of a common set of terms and parameters to classify lithic raw materials. It is essential that archaeologists begin to create a more comprehensive system of this nature. Two case studies within the North Carolina Slate Belt region are provided to help illustrate an application of this system. One study is from Three Hat Mountain in Davidson County and the other is from the Uwharrie Volcanic Belt. Included in these case studies are pictures and spreadsheets outlining the details of the samples discussed.

This site assumes that the user has no formal training in geology and builds from that point. We feel this is the best and most effective approach to the issues involved. Everyone should benefit from the use of this site and gain better insight into the complexity of Slate Belt materials.

This website has been constructed as part of the 2007 Summer Internship Program coordinated by the North Carolina Youth Advocacy and Involvement Office. Kelly M. Tomlinson of the College of Charleston researched, designed, and constructed this site. Karen Browning, Webmaster of the Queen Anne's Revenge Shipwreck Project, Office of State Archaeology assisted Ms. Tomlinson in the layout and arrangement of the site. Ms. Browning is also responsible for posting and maintaining the site to the Office of State Archaeology web page. Phil Bradley, Senior Geologist with the North Carolina Geological Survey in Raleigh provided technical assistance to Ms. Tomlinson and graciously allowed the use of equipment and geologic specimens in the construction of this site. Mr. Bradley also led several specimen collection trips into the North Carolina portion of the Slate Belt. David Gunkle, Intern with the North Carolina Geological Survey, accompanied us on the collection trips and provided the GIS maps with the collection points plotted and referenced for inclusion in this site. Kelsey Zyvoloski, 2007 Intern with the Office of State Archaeology, also assisted our efforts on the collection trips. Lawrence Abbott of the Office of State Archaeology served as the supervisor for this project and provided technical assistance where needed.

Welcome to this site! We hope you find it of great interest and useful as a research tool. This site should be beneficial and provide comparative raw material data for artifacts collected at archaeological sites both within and outside the boundaries of the Slate Belt.

Please contact us with comments and feedback at lawrence.abbott@ncdcr.gov.

Lawrence Abbott,

Assistant State Archaeologist

Rocks 101

Rocks 101

What is a Rock?

A rock is made up of many different mineral crystals/fragments. A rock can be classified into three types:

A. Igneous Rocks
B. Sedimentary Rocks
C. Metamorphic Rocks

How are different rock types formed?

Rocks are formed through a system that constantly forms, deforms, transforms, destroys, and reforms them. This system can be best understood by looking at a Rock Cycle Chart. Here you can see that any one rock type can be transformed into either of the other two rock types or recycled within its own group.

If minerals have properties; do rocks also have properties?

The properties that a rock exhibits are our main clues to identify them. All three types of rocks exhibit different properties. However, there are some properties that are common among all three types of rocks. These properties are form, color, composition, and texture.

How do you tell the difference between an Igneous, Sedimentary, and Metamorphic Rocks?

As stated before different rock types have different properties. However also remember that because there are some common properties among all rocks that when identifying them you have to use many properties.

Igneous Rocks commonly have a composition consisting of olivine, pyroxene, and feldspar crystals. They also may exhibit have a ropy type of banding, this is caused by cooling lava.

Sedimentary Rocks are generally layered and exhibit a clastic texture. They also preserve fossils and other organic remains. Common places that sedimentary rocks occur at are river beds, deltas, beaches, sand bars, and extensive flat layers (depositional environments).

Metamorphic Rocks have textures such as folds, fractures, faults, and foliation. Foliation is the most common indicator of a metamorphic rock. Often the composition consists of garnet, tourmaline, and mica. It is a metamorphic rock if it contains serpentine, epidtoe, graphite, galena, or sphalerite because these minerals only occur in metamorphic rocks.

Rock Flow Charts

Igneous, Sedimentary, or Metamorphic Flow Chart Part 1
Igneous, Sedimentary, or Metamorphic Flow Chart Part 2
Rock Cycle Flow Chart

Igneous Rocks 101

Igneous Rocks 101

What is an igneous rock?

An Igneous rock is the product of cooled solidified magma/lava. As magma/lava cools it goes through the process of crystallization which turns it into an igneous rock. The amount of time it takes for an igneous rock to form depends on its location and its composition. There are two types of igneous rocks; Extrusive and Intrusive.

What is the difference between an Extrusive and Intrusive igneous rock?

The difference between an Extrusive and Intrusive igneous rock is the way in which they cool. An Extrusive igneous rock cools very fast on the surface and is created by lava. Since the cooling process is very fast extrusive igneous rocks have very small crystals (fine grained). On the other hand an Intrusive igneous rock cools very slowly beneath the surface and is created by magma. Since the cooling process is very slow intrusive igneous rocks have very large crystals (coarse grained). In some instances there is also a third type of igneous rock. Technically it is an extrusive rock, but it resembles glass. This glass type of igneous rock forms when magma/lava is instantly cooled.

What are the properties used to identify Igneous rocks?

Properties used to identify igneous rocks include mineral composition, texture, and color. There are four different mineral compositions; Felsic, Intermediate, Mafic, and Ultramafic. The texture of an igneous rock is determined by its grain size. There are eight terms relating to the type of texture an igneous rock has. They are pegmatitic, phaneritic, porphyritic, aphanitic, glassy, vesicular, frothy, and pyroclastic.

Why does the type of mineral composition matter when identifying an igneous rock?

By looking at what type of mineral composition an igneous rock has you can determine what type of magma made it and what environment it was made in.

What is the difference in composition between Felsic, Intermediate, Mafic, and Ultramafic igneous rocks?

Igneous rocks contain the following common minerals: Plagioclase feldspar, Olivine, Potassium feldspar, Pyroxene, Quartz, Amphibole, Biotite, and Muscovite. The percentage of these minerals present in an igneous rock determines whether it is felsic, intermediate, mafic, or ultramafic. Felsic rocks are those that are light in color and are mostly made up of feldspars and silicates. Mafic rocks are darker colored and are mostly made up of magnesium and iron. Intermediate rocks fall somewhere in between being Felsic and Mafic. Ultramafic rocks are very dark colored. For more detailed descriptions of each type click on its name to be redirected to the glossary.

What determines what type of texture an igneous rock has?

Texture is determined by the grain size of igneous rock which is determined by the rate of cooling. For example a smooth igneous rock was cooled very fast because it is has small crystals and is therefore an extrusive rock. The texture of this rock would be called fine grained or aphanitic. In the reverse situation an igneous rock that cooled very slowly and formed large grains would be described as having a coarse grained or phaneritic texture. To see the meaning of each type of texture click on its name to be redirected to the glossary (Pegmatitic, Phaneritic, Porphyritic, Aphanitic, Glassy, Vesicular, Frothy, Pyroclastic).

Where are igneous rocks found?

The most common igneous rock Basalt (mafic) makes up most of the oceanic plates and is founds at divergent plate boundaries. In fact most igneous rock activity occurs at divergent plate boundaries. However, intermediate to felsic igneous rocks are most commonly found along continental margins.

Igneous Rock Flow Chart

Igneous Rock Flow Chart

Sedimentary Rocks 101

Sedimentary Rocks 101

What is a sedimentary rock?

sedimentary rock is an accumulation of fragments of pre-existing rocks (sediments). There are three types of sedimentary rocks; Clastic (detrital), Chemical, and Biochemical (organic) rocks.

How is a sedimentary rock made?

Since sedimentary rocks are pieces of many different rocks they must be lithified together to form a solid rock before they can actually be called a sedimentary rock. There are three types of lithification; compaction, cementation, and crystallization.

What is the difference between compaction, cementation, and crystallization?

Compaction happens when weight from the materials above compact sediments together to form a solid rock. Cementation happens when a precipitation from a mineral cements surrounding sediments together making them a solid rock. Crystallization happens when a solid rock is created from a solution.

Where do the sediments that make up sedimentary rocks come from?

Sediments are the result of weathering which breaks up pre-existing rocks. There are two types of weathering; Mechanical and Chemical. After the rocks are weathered and broken down into sediments they are transported via water, wind, glaciers, or gravity to different depositional environments. Finally after being deposited the sediments accumulate to form a sedimentary rock through lithification. Sediments come in all different shapes, sizes, and sortings. The way sediments are sized, shaped, and sorted can tell us about their depositional environment and how they journeyed to get there.

What’s the difference between Clastic, Chemical and Biochemical sedimentary rocks?

Clastic or detrital sedimentary rocks are made of sediments that are the result of weathering and transport. Chemical rocks are created from chemical precipitation. Biochemical or organic rocks are made up biological remains.

What are properties of Sedimentary rocks used to identify them?

Properties of sedimentary rocks used to identify them include texture, composition, particle size and sedimentary structures. Not only do these properties help you identify the type of sedimentary rock you have but also in what type of depositional environment the rock was formed.

What are the different textures associated with Sedimentary rocks?

Types of sedimentary textures include clastic, bioclastic, crystalline, amorphous, and oolitic. To see further definition of these textures click on them so you can be redirected to the glossary.

What matter is found in the composition of Sedimentary rocks?

The composition of sedimentary rocks can be made up of the following things: carbonates, silica, clay minerals, organic matter, evaporites, rock particles, heavy minerals, and feldspar.

What is a Sedimentary Structure?

A sedimentary structure is a structural feature in a sedimentary rock formed as a result of weathering or transportation. Types of sedimentary structures include: stratification, cross bedding, graded bedding, ripple marks, oscillation marks, mud cracks, raindrops, and trace fossils.

What is a depositional environment?

Depositional environments are the places where sediment accumulates. There are three different groupings of sedimentary environments: continental, marine, and transitional. They are then further broken down into sub groups such as dunes, marsh, mountains, etc.

How does looking at sedimentary rocks properties tell me about where the rock was formed?

In order to determine where a sedimentary rock was formed you must look at every aspect of it. From the sediment size, shape, and sorting to the structures present in it. For example lets say we are looking at a conglomerate. In it we see angular sediments that are poorly sorted. This tells us that the rock did not travel far from its source because the sediments are sharp and jagged and there are many different types of rocks. Most likely it came form a mountainous or piedmont region. On the other hand if we had sandstone with ripple marks and cross bedding we could say that it was by a source of water because of the ripple marks and that wind probably caused the cross bedding. Therefore it was probably formed in a costal area.

Sedimentary Rock Flow Charts

Sedimentary Rock Flow Chart Part 1
Sedimentary Rock Flow Chart Part 2

Metamorphic Rocks 101

Metamorphic Rocks 101

What is a metamorphic rock?

To understand metamorphic rocks you must first know what metamorphism is. Metamorphism is the changing of one rock into another rock. Metamorphism occurs under the influence of extreme heat, pressure, and/or chemical fluids. A rock undergoing metamorphism changes the texture and mineral composition of the rock but never leaves solid form. If the rock were to melt and be changed that would classify it as an Igneous rock. Metamorphic rocks are transformed through the solid state.

How many types of metamorphism are there?

There are four types of metamorphism. Contact metamorphism is a result of heat generated from a nearby magma source. Regional metamorphism consists of change through intense heat and pressure. Hydrothermal metamorphism is a chemical change that occurs when hot fluids circulate through out the rock. Finally, fault zone metamorphism is change that happens along fault lines.

What features are present in metamorphic rocks?

When looking at metamorphic rocks things to look for include Foliation and the type of foliation present. Foliation types include; rock cleavage, schistosity, gneiss, and non-foliated. Metamorphic rocks are broken down into two groups based on whether the rock is foliated or non-foliated.

Can any rock be changed into a metamorphic rock?

Yes, any rock can be transformed into a metamorphic rock. In fact the parent rock (the original rock) is very important in the metamorphic process. The parent rocks makeup determines what metamorphic agent will change the rock. Some common parent child match ups in rock include; Slate and Shale, and Marble and Limestone.

Where does metamorphism occur?

Metamorphism really occurs wherever the three agents; heat, pressure, and chemical fluids; are present. Commonly though metamorphism occurs along convergent plate boundaries and subduction zones. Also hydrothermal metamorphism occurs along the mid-ocean ridge.

Metamorphic Rock Flow Chart

Metamorphic Rock Flow Chart 

Minerals 101

Minerals 101

What is a Mineral?

Minerals are the building blocks of all rocks. Presently there are about 4,000 minerals that have been identified. But what is a mineral besides a building block? A mineral is defined as a naturally occurring inorganic solid that has a definite chemical structure. The important part of that definition is the last part that states a mineral has a definite chemical structure. This definite chemical structure is what provides a mineral with its physical properties which in turn allows us to identify them.

If minerals make up rocks, what makes up minerals?

Minerals are made up of complex structures of elements. There are eight elements that make up most of these minerals; Oxygen, Silicon, aluminum, iron, calcium, sodium, potassium, and magnesium; they account for about 98% of the earths crust. From this point we have determined that elements are the building blocks of minerals and minerals are the building blocks of rocks.

What are the physical properties of minerals?

The common physical properties used to identify minerals include crystal form, luster, color, streak, hardness, cleavage, fracture, and specific gravity. However there are other properties used to identify minerals as well. These include reaction to acid, taste, smell, and magnetism. Some of these tests are more accurate then others when identifying minerals. For example color is not as reliable test as hardness. Since minerals such as quartz come in a variety of colors it’s hard to use color to identify them. However, when using the hardness/scratch test quartz has a definite number making it very easy to identify it. To see further definitions of any of the terms listed simply click on the word to be redirected to a glossary.

What are the steps to follow in identifying minerals?

Step 1: Determine what group of minerals; silicates or non-silicates; your mineral belongs to.

What's the difference between minerals being silicates or non-silicates?

Since there are so many minerals on the earth they have been divided and sub divided to make them easier to identify. The first division is determining whether your mineral is of the Silicate group or the Non-Silicate group.

Silicate minerals have silicone and oxygen in their composition. More specifically a silica tetrahedron makes up their internal structure. There are four types of structures that center on a silica tetrahedron in silicate minerals. They are single chain, double chained, sheet, and 3-D framework. Further more silicate minerals can be broken down into Light silicates and Dark silicates.

Every other mineral that does not have a silica tetrahedron in its structure is placed as a Non-Silicate. There are six sub groups of non-silicates they are; Carbonates, Oxides, Sulfides, Sulfates, Halides, and Native metals.

Step 2: Using your knowledge of the physical properties of minerals determine which properties the mineral has.

Step 3: Using the properties go to the mineral identification chart and follow it to figure out which mineral you have.

Minerals Flow Charts

Metallic Minerals
Non-Metallic Light Hard Minerals
Non-Metallic Dark Hard Minerals
Non-Metallic Dark Soft Minerals
Non-Metallic Light Soft Minerals

Glossary

Glossary

Minerals
Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks

Mineral Terms

Mineral - A naturally occurring inorganic compound with a specific chemical makeup and a defined crystal structure

Mineral Tests

Hardness/Scratch Test - This test determines the hardness of the material (or its ability to resist scratching) by taking two materials; one where you know the hardness and the other where you do not. By scratching them together you can determine which is harder. The material with the higher hardness will be able to leave a scratch mark. For a reference of items and their hardness one can look to the Moh- Hardness Scale. The Moh- Hardness scale takes 10 common minerals and rates them. The one that ranks number 10 is the most resistant and the one that ranks 1 is the least resistant. This scale also defines hard minerals from soft minerals. All minerals above the mark of 5.5 are known as Hard Minerals all those under the mark of 5.5 are known as Soft Minerals.

Moh - Hardness Scale

Hardness

Mineral

Object of known hardness

10

Diamond

 

9

Corundum

 

8

Topaz

 

7

Quartz

 

6.5

 

Streak Plate

6

Feldspar

 

5.5

 

Glass, knife

5

Apatite

 

4

Fluorite

 

3.5

 

Penny (copper)

3

Calcite

 

2.5

 

Finger nail

2

Gypsum

 

1

Talc

 

Streak Test - Often the color of a mineral is different form the color that is left by the minerals streak (aka the powdered residue) against an object (most often a streak plate). When trying to identify two similarly colored minerals this test can come in handy. Often the streak colors of the minerals will differ allowing for accurate identification. However this test only works on minerals that have a hardness of 6 or less.

Acid Test - If a mineral is a carbonate it will fizz when acid is dropped on it.

Magnetic Test - The mineral will be drawn to a magnet, therefore showing the presence of magnetic material.

Taste Test - Some minerals have specific tastes to them. By licking the mineral you can sometimes determine what it is. For example Halite tastes like salt.

Smell Test - For those who do not wish to lick a mineral sometimes a mineral can also be identified by the smell it gives off. For example sulfur has a very distinctive smell.

Mineral Proprieties

Luster - The appearance that a mineral gives off when light is reflected off it. There are many terms defining the type of luster a mineral gives off, most are self explanatory.

  • Metallic- looks like a metal
  • Non-metallic - does not look like a metal
    • Adamantine - looks like a diamond
    • Resinous - looks like resin
    • Vitreous - looks like glass
    • Pearly - looks like a pearl
    • Silky - looks like silk, meaning it has a very fine texture
    • Waxy - looks like wax
    • Earthy - looks like something that would come from off the ground, like dirt

Color - The color that the mineral is on the outside. This is not always a good way to determine what mineral is. Often minerals can be comprised of many colors or have a variety of colors.

Cleavage - Minerals that exhibit cleavage are able to break along parallel to sub parallel surfaces. A common example when explaining cleavage is Mica. Mica is made up of thin individual planes that can break off easily because of their weak bonds. When a mineral breaks and has cleavage it will break into pieces that haves the same geometry as each other. The type of Cleavage a mineral has is determined by the number of planes it has and at what direction they are placed. Minerals that do not have any cleavage will Fracture when trying to be broken.

Planes and Directions

Example

1 (basal cleavage)

Mica

2 at 90°

Feldspar

3 at 90° (cubic cleavage)

Amphibole

3 not at 90° (rhombohedral cleavage)

Dolomite

4 (octahedral cleavage)

Fluortie

6 (dodecahedral cleavage)

Sphalerite

Specific Gravity - Using the ratio of the weight of a mineral to the weight of an equal volume of water to determine what mineral it is. For example if a mineral weighs 5 times as much as an equal volume of water then its specific gravity is 5.

Crystal Form - Each mineral has a distinct crystalline structure within it. By identifying the crystal structure of a mineral you can narrow down the types of mineral it could be. There are about 64 different types of structures that are broken down into 6 groups.

  • Isometric - equal measure
  • Tetragonal - square cross sections with rectangular faces
  • Hexagonal - six sided
  • Orthorhombic - rectangular faces and profile
  • Monoclinic - rectangular faces and trapezoid faces
  • Triclinic - trapezoid faces

Like cleavage there are many terms used in defining crystal forms.

  • Crystal Face - the flat surface of a mineral
  • Crypto crystalline- crystals in a mineral to small to see with the naked eye
  • Amorphous - non-crystalline; due to rapid cooling

Mineral Groups

Silicates - Minerals that have silicone and oxygen in their composition. There are four types of silicate structured minerals.

  • Singe Chain Silicate (ex. Augite)
  • Double Chain Silicate (ex. Hornblende)
  • Sheet Silicate (ex. Micas)
  • 3-D Framework Silicate (ex. Feldspars)

Silicates are also broken up into two different groups based on their color. Light (nonferromangesian) Silicates are light in color and have a specific gravity around 2.7. Light Silicates contain amounts of aluminum, potassium, calcium and sodium. The other group of silicates is Dark (ferromagnesian) silicates. These silicates are dark in color and have a specific gravity ranging from about 3.2 - 3.6. They also contain mostly iron and magnesium.

Common Silicate Groups

Light Silicates

Group Name

Silicate Structure

Cleavage

 

Feldspar Group

3-D Framework

Two planes meeting at 90°

 

Quartz

3-D Framework

None

 

Muscovite

Sheet

1 plane (Basal)

 

Clay Minerals

Sheet

1 plane (Basal)

Dark Silicates

 

 

 

 

Olivine Group

3-D Framework

None

 

Pyroxene Group

Single Chain

Two planes meeting at 90°

 

Amphibole Group

Double Chain

Two planes at 60° and 120°

 

Biotite

Sheet

1 plane (Basal)

 

Garnet

3-D Framework

None

Non-Silicates - All other minerals that are not silicates are put into the non-silicate group then broken down into subgroups of non-silicates. There are 6 subgroups.

  • Carbonates - minerals that contain carbon and oxygen
  • Oxides - minerals with an oxygen base
  • Sulfides - minerals that contain sulfur
  • Sulfates - minerals that contain sulfur and oxygen
  • Halides - minerals that contain a metal and a halogen element
  • Native Metals - copper, silver, gold, zinc, iron, and nickel

Igneous Rock Terms

Igneous Rocks - made from the rapid or slow cooling of magma/lava.

Igneous Rock Types

Intrusive Igneous Rock - Igneous rock formed inside the earth. This type of igneous rock cools very slowly and is produced by magma. It has large grains, contains gas pockets, and usually lots of silicate minerals.

Extrusive Igneous Rock - Igneous rock formed on the surface of the earth. This type of igneous rock cools very fast and is produced by lava. It has small grains and contains little to no gas.

Igneous Rock Properties

Grain Size - The size of the grains in an igneous rock is an indicator on how fast the rock cooled. To be considered a coarse grained rock it has a ruff exterior with grains between 1 and 10 cm. There are also fine grained rocks which often have grains that are less than 1mm and hard to see with the naked eye. These rocks are very smooth to the touch. Rocks that are neither extremely coarse but are not fine grained are called medium grained. To say something is coarse, fine, or medium grained are informal terms. To learn the more formal terms move on to the next definition.

Texture - The feel of a rock based on the size, shape, and arrangement of the grains and other parts of the rock. For igneous rocks this is determined by the cooling rate. An igneous rock that cools faster will have smaller grains and therefore a smoother texture then one that cools slowly and forms bigger grains. There are many different terms to describe the type of texture an igneous rock has.

  • Pegmatitic - Very coarse, very slow cooling. Grains greater then 1 cm.
  • Phaneritic - Coarse. Grains 1 – 10 cm
  • Porphyritic - Has large crystals that are set in smaller crystals.
    • Phenocrysts -Large crystals (slow cooling)
    • Groundmass - Small crystals (fast cooling)
  • Aphanitic -Very fine and fast cooling. Grains less than 1mm
  • Glassy - no crystals
  • Vesicular - rock that contains gas pockets, look like little tiny voids in the rock
  • Frothy - porous texture
  • Pyroclastic - made of pyroclasts

Igneous Rock Composition

Mineral Composition - The mineral composition of an igneous rock is dependent on where and how the rock was formed. Magma around the world has different mineral make up. There are four different composition types; Felsic, Mafic, Ultramafic, and Intermediate. Color and mineral make up are indicators of each type. These compositions all have varying amounts of common minerals found in igneous rocks.

Common Minerals: Plagioclase feldspar, Olivine, Potassium feldspar, Pyroxene, Quartz, Amphibole, Biotite, Muscovite

Felsic - Felsic igneous rocks are light in color and are mostly made up of feldspars and silicates. Common minerals found in felsic rock include Quartz, Plagioclase feldspar, Potassium feldspar, and Muscovite. They contain about 0-15% mafic mineral crystals and have a low density.

Mafic - Mafic igneous rocks are dark colored and consist mainly of magnesium and iron. Common minerals found in mafic rocks include Olivine, Pyroxene, Amphibole, and biotite. They contain about 46-85% mafic mineral crystals and have a high density.

Ultramafic - Ultramafic igneous rocks are very dark colored and contain higher amounts of the same common minerals as mafic rocks. They contain about 86-100% mafic mineral crystals.

Intermediate - Intermediate are between light and dark colored. They share minerals with both felsic and mafic rocks. They contain 16-45% mafic minerals.

Sedimentary Rock Terms

Sedimentary Rocks - Rocks which are an accumulation of fragments of many pre-existing rocks.

Sediment - Fragment of a rock on the earth’s surface

Weathering - The process by which rocks are broken down into sediments. There are two types of weathering:

  • Mechanical - weathering in which physical process such as frost wedging and unloading break down rocks.
  • Chemical - weathering in which chemical processes such as oxidation break down rocks.

Transport - method by which sediments are moved across the surface. Types of transport include fluvial, glaciers, wind, and gravity.

Depositional Environment - area in which the sediment comes to rest, there are many different groups and subgroups classifying depositional environments. Most often they are very straightforward. For example a marsh environment would be called a marsh depositional environment. The three main groups however are:

  • Continental - deserts, lakes, river beds, swamps, and caves
  • Continental and Marine - deltas
  • Marine - ocean

Lithification - process by which sediments come together to form a sedimentary rock. There are three ways in which this is done:

  • Compaction - intense weight and compression squishes sediments together to form a sedimentary rock
  • Cementation - process in which sediments are cemented together via a precipitation.
  • Crystallization - process where an existing solution creates a sedimentary rock.

Properties of Sedimentary Rocks

Texture - The feel of a rock based on the size, shape, and arrangement of the grains and other parts of the rock. Sedimentary rocks can be broken down into having five different textures:

  • Clastic - consists of broken fragments of preexisting rock. Well sorted.
  • Bioclastic - consists of the remains of organic material.
  • Crystalline (Nonclastic) - minerals are in a pattern of interlocking crystals.
  • Amorphous - no crystal structure
  • Oolitic - made of small round particles of calcium carbonate

Composition - materials commonly found in sedimentary rocks.

  • Silica
  • Carbonate
  • Clay Minerals
  • Organic Matter
  • Evaporites
  • Rock Particles
  • Heavy Minerals
  • Feldspar

Shapes of Sediments

  • Angular - sharp corners and edges
  • Rounded - smoothed edges

Sizes of Sediments

  • Clay - <1/256mm
  • Silt - 1/256mm – 1/16mm
  • Sand - 1/16mm – 2mm
  • Pebble- 2mm – 64mm
  • Cobble - 64mm – 256mm
  • Boulder - >256mm

Sorting of Sediments

  • Poorly Sorted - all different sized particles
  • Well Sorted - same sized particles

Sedimentary Structures - Features in sedimentary rocks that reflect on what type of transportation created the sedimentary rock.

  • Stratification - layers formed from sediment deposition
  • Cross Bedding - stratification at angle, usually caused by wind.
  • Graded Bedding - bedding in which the size of the sediment increases as the depth does; marine environments
  • Surface impressions - impressions left by nature
    • Ripple Marks - left by water flow in one direction
    • Oscillation Marks - left by water flow in a back and forth direction
    • Mud Cracks
    • Raindrops
    • Trace Fossils

Metamorphic Rock Terms

Metamorphic Rocks - rocks that change form through the influence of heat, pressure, and/or chemical activity

Types of Metamorphism

1. Contact metamorphism - changes in the rock due to heat from nearby magma

2. Regional metamorphism - causes change through intense heat and pressure

3. Hydrothermal metamorphism - chemical changes in the rock due to the circulation of hot liquids through the rock fractures

4. Fault Zone metamorphism - metamorphic changes caused by fault movements

Degrees of Metamorphism - The quality of the rock is based on the amount of heat and pressure it had applied to it during the metamorphic processes

A. High Grade - high amounts of heat and pressure

B. Intermediate Grade - medium amounts of heat and pressure

C. Low Grade - low amounts of heat and pressure

Changes That Occur during Metamorphism

  • Re crystallization - occurs when small crystals join together to create larger crystals of the same mineral
  • Neomorphism - new minerals are created from the original mineral composition
  • Metamorphism - new minerals are created by gaining or losing chemicals

Metamorphic Rock Properties

Texture - The feel of a rock based on the size, shape, and arrangement of the grains and other parts of the rock. Metamorphic rocks can be broken up into two texture groups:

  • Foliated Texture - layers due to heat and pressure; occurs mostly at regional metamorphism spots. Types of foliation can be used to correctly identify rocks.
  • Non-foliated Texture - no layers it is granular instead

Foliated Textures

Texture

Grade

Grains

Other Features

Slaty

Low grade

Fine grained micas

Sheet separation

Phyllitic

Low – intermediate grade

Fine grained mica and chlorite

Glossy with a wrinkled look

Schistose

Intermediate grade

Medium to coarse grained micas, chlorites, and quartz

 

Gneissic

Intermediate grade

Medium to coarse grained

Alternating dark and light layers

Migmatitic

High grade

 

 

Non-Foliated Textures

Texture

Grains

Other Features

Crystalline

Medium to Coarse

Equal sized visible crystals

Microcrystalline

Fine

Microscopic crystals

Sandy

Medium to Coarse

Fused grains

Glassy

No visible grains

 

There are also features that occur in both Foliated and Non-foliated Metamorphic Rocks

  • Stretched/Sheared grains - pebbles, fossils, or crystals that have been stretched, shortened, or sheared
  • Porphyroblastic - large crystals arranged in a fine grained groundmass
  • Hydrothermal veins - fractures filled in by minerals from fluids
  • Folds - bends in the rock layers
  • Lineations - lines on the rock at the edge of foliation, shear planes, slay cleavage, folds, or aligned crystals

Common Metamorphic Changes

Parent Rock

New Rock

Sandstone

Quartzite

Limestone

Marble

Basalt

Schist or Amphibolite

Shale

Slate

Granite

Schist

Rhyolite

Schist

Sample Surveys

Sample Surveys

Three Hat Mountain Quarry


Located in Davidson County, NC this prehistoric quarry is rich in knappalbe lithic raw materials. The objective of this report is to provide an assessment in the changes of extraction technology, reduction methods, and distribution patterns of the raw materials. In the links below you will find full report on the meta volcanic rich Three Hat Mountain Quarry.

An Assessment of Lithic Extraction Technology at a Metavolcanic Quarry in the Slate Belt of North Carolina

 

Uwharrie Volcanic Belt


Located in Central North Carolina the Uwharrie Volcanic Belt, most commonly known as the Carolina Slate Belt, is one of the most complex and least understand geological areas of North America. Stretching from Petersburg, VA to central Georgia this region was heavily utilized as a source for knappable stone by prehistoric man. Below you will find a full Field Trip guide written by Jeri L. Jones on the Uwharrie Volcanic Belt. 

Field Guide To The Geology and Archaeology of The Uwharrie Volcanic Belt Central North Carolina - Jeri L. Jones April 1977

 

Carolina Slate Belt 


The OSA would like to thank the North Carolina Geological Survey for providing its supply of hand samples of rocks from the Carolina Slate Belt region. Below you will find a descriptive excel file outlining each rock's geological features.

NC Geological Survey Carolina Slate Belt Rocks Descriptive Excel File

References Cited

References Cited

Jeri L. Jones
1977      Field Guide To the Geology and Archaeology of the Uwharrie Volcanic Belt Central North Carolina. Paper presented to the Second Lithic Symposium on the Uwharrie Mountains.

Lawrence E. Abbott, Jr.
2004      An Assessment of Lithic Extraction Technology at Meta volcanic Quarry in the Slate Belt of North Carolina. Paper presented at the annual meeting of the Society for American Archaeology in Montreal

Lynn S. Fichter
Declaration of Geology; July 2, 2004

Loren A. Raymond
1995      Petrology The study of Igneous Sedimentary Metamorphic Rocks. Wm. C. Brown, Dubuque, Iowa.

National Audubon Society
2000      National Audobon Society Field Guide to Rocks and Minerals. Alfred A. Knopf, New York.

Richard M. Busch (editor)
2006      American Geological Institute National Association of Geoscience Teachers Laboratory Manual in Physical Geology. 7th ed. Pearson Prentice Hall, Upper Saddle River, New Jersey.

Steponaitis, Vincas P. (editor), Theresa E. McReynolds (editor), Jeffrey D. Irwin (editor), and Christopher R. Moore (editor)
2006      Stone Quarries and Sourcing in the Carolina Slate Belt. Research Report No. 25, Research Laboratories of Archaeology, The University of North Carolina at Chapel Hill

Tarbuck, Edward J. and Frederick K. Lutgens
2005      Earth An Introduction to Physical Geology. 8th ed. Pearson Prentice Hall, Upper Saddle River, New Jersey.

Quick Study Academic
2001      Rocks & Minerals. Bar Charts, Boca Raton, Florida.

Recommended Texts

Recommended Texts

  • National Audubon Society, Field Guide to Rocks and Minerals  - An essential book to have. You don't necessarily have to get this one, most guides are very extensive and good.
  • Rocks & Minerals by Quick Study Academic - This is a laminated folding reference for rocks and minerals. It's great for checking quick facts and for carrying around.
  • Earth: An Introduction to Physical Geology, Eighth Edition by Tarbuck and Lutgens - This is a first semester geology book. Recommended if you are interested in learning more about geology in general.
  • American Geological Institute National Association of Geoscience Teachers, Laboratory Manual in Physical Geology, Seventh Edition - This is a great lab manual to accompany the text book above. It includes not only excellent information but also interactive exercises to teach the basics as well as very detailed charts.
  • Petrology: The study of Igneous, Sedimentary, Metamorphic Rocks by Loren A. Raymond - Recommended only if you have some background knowledge of geology, its a bit more advanced then the other two textbooks listed on this site. It is very extensive and the charts in the back are extremely useful.
Other Geology Websites

Other Geology Websites

James Madison University - An excellent site! This site by a professor at James Madison University is extremely extensive on all aspects of geology. Providing easy to understand information and detailed charts and graphs.

Hawk College site for igneous identification. Provides a really cool practice resource for idenitfication skills.

Geomania - A lot of great introductory information. 

Rocks for Kids! -  This site is an excellent resource for terminology and for text recommendations