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Geology: Most Popular Articles

These articles are the most popular over the last month.
Identify Almost Any Rock with This Table
Identify 43 of the most common rock types: igneous, sedimentary and metamorphic.
Pictures of Every Rock Type
My master alphabetical guide to igneous, sedimentary and metamorphic rock pictures.
Mineral Picture Index
My master guide to annotated pictures to help you identify minerals: typical examples of rock-forming, accessory, and rare or notable minerals.
Densities of Common Rock Types
Density, or specific gravity, of common rock types.
Green Minerals
A guide to the most common and significant green and greenish minerals.
Fault Types in a Nutshell
The three basic types of fault, with their nomenclature, in a nutshell.
Granite and Its Geology
Granite is the signature rock of the continents. Here are the basics of this important stone.
Blue and Purple Minerals
A guide to the most common and significant blue and purple minerals.
About Igneous Rocks
Introducing igneous rocks, first of the three great rock classes (along with sedimentary and metamorphic rocks).
Where to Look for Rocks and Minerals
Rocks and minerals can be found everywhere, but there is a learning process involved.
How to Look at a Rock
Notes on the most basic operation in geology.
About Sedimentary Rocks
Introducing sedimentary rocks, second of the three great rock classes (along with igneous and metamorphic rocks).
How to Identify Minerals: 10 Steps to Mineral...
First steps in mineral identification.
The Earth's Crust
The Earth's crust is exceptionally important, and not just because we live on it.
About Metamorphic Rocks
Introducing metamorphic rocks, third of the three great rock classes (along with igneous and sedimentary rocks).
Igneous Rock Types
The major igneous rock types, plutonic, intrusive and extrusive.
Basalt
Basalt is one of the major igneous rock types. Page 3.
Serpentinite
Serpentinite is a major metamorphic rock type from regional metamorphism. Page 16.
Schist
Schist is a major metamorphic rock type from regional metamorphism. Page 15.
Black Minerals
A guide to the most common and significant black minerals.
Quartzite
Quartzite is a major metamorphic rock type from regional metamorphism. Page 14.
Obsidian
Obsidian is one of the major igneous rock types. Page 13.
Gneiss
Gneiss is a major metamorphic rock type from regional metamorphism. Page 6.
Granite
Granite is one of the major igneous rock types. Page 8.
Slate
Slate is a major metamorphic rock type from regional metamorphism. Page 17.
Gabbro
Gabbro is one of the major igneous rock types. Page 7.
Seismic Hazard Maps of the World
The United Nations' global map of seismic hazard
Scoria
Scoria is one of the major igneous rock types. Page 22.
Diorite
Diorite is one of the major igneous rock types. Page 4.
What's the Most Common Mineral?
This question has several answers, depending on what a mineral is and what part of the Earth we're talking about.
The Mohs Scale of Mineral Hardness
The 10-mineral Mohs scale of relative hardness is based on what scratches what.
Siltstone
Siltstone is one of the major types of sedimentary rocks. Page 24.
About Feldspar
Feldspar is the commonest mineral in the Earth's crust, but identifying it can be tricky.
Metamorphic Rock Types
The major metamorphic rock types, from regional, contact and mechanical metamorphism.
Felsite
Felsite is one of the major igneous rock types. Page 6.
About Sandstone
Sandstone is sand cemented together into rock.
Sedimentary Rock Types
The major types of sedimentary rocks.
Marble
Marble is a major metamorphic rock type from regional metamorphism. Page 10.
Andesite
Andesite is one of the major igneous rock types.
Sandstone
Sandstone is one of the major types of sedimentary rocks. Page 22.
Pegmatite
Pegmatite is one of the major igneous rock types. Page 14.
Peridotite
Peridotite is one of the major igneous rock types. Page 15.
Porphyry
Porphyry is one of the major igneous rock types. Page 17.
Pumice
Pumice is one of the major igneous rock types. Page 18.
Phyllite
Phyllite is a major metamorphic rock type from regional metamorphism. Page 13.
Six Things to Know About the Earth's Mantle
A six-part look at the basics of the Earth's mantle, the part between the crust and the core.
Graywacke or Wacke
Wacke or graywacke is one of the major types of sedimentary rocks. Page 15.
About Quartz
Quartz is essential to the land as we know it.
Chert
Chert is one of the major types of sedimentary rocks. Page 7.
Shale
Shale is one of the major types of sedimentary rocks. Page 23.
Red and Pink Minerals
A guide to the most common and significant red and pink minerals.
The Tufa Towers of Trona Pinnacles, California
America's largest collection of tufa towers is in the Mojave Desert near Trona, California.
Limestone
Limestone is one of the major types of sedimentary rocks. Page 17.
Arkose
Arkose is one of the major types of sedimentary rocks. Page 2.
Tonalite
Tonalite is a felsic plutonic rock without alkali feldspar. Page 24.
Syenite
Syenite is one of the major igneous rock types. Page 23.
Landform Picture Index
Find and see geological landforms at About.com Geology.
Common Pseudofossils
Shatter cones may be mistaken for fossils. Page 10.
About Subduction in Plate Tectonics
Basics of plate tectonics: introduction to subduction.
Dunite
Dunite is a plutonic rock consisting of olivine. Page 5.
Chemical Weathering
A short explanation of chemical weathering.
Pyroxenite
Pyroxenite is one of the major igneous rock types. Page 19.
Before You Buy a Magnifier
What to think about and look for when you set out to get a nice magnifier.
Plutonic Rocks
Plutonic rocks defined and explained.
Greenstone
Grenstone is a major metamorphic rock type from regional metamorphism. Page 8.
Coquina
Coquina is one of the major types of sedimentary rocks. Page 11.
Granodiorite
Granodiorite is one of the major igneous rock types. Page 9.
Fossil Parks for Hands-On Digging
Public parks in the United States where you can collect fossils legally.
How to Read a Geologic Map
Understanding geologic maps starts with topography.
Quartz
Quartz is classified as a rock-forming mineral. Page 9.
Hornfels
Hornfels is a major metamorphic rock type from contact metamorphism. Page 9.
Tuff
Tuff is one of the major igneous rock types. Page 26.
All About the San Andreas Fault
Home page for everything about the San Andreas fault: where it is, how it behaves, what it means.
Soapstone
Soapstone is a major metamorphic rock type from regional metamorphism. Page 18.
Migmatite
Migmatite is a major metamorphic rock type from regional metamorphism. Page 11.
Conglomerate
Conglomerate is one of the major types of sedimentary rocks. Page 10.
Dolomite Rock or Dolostone
Dolomite or dolostone is one of the major types of sedimentary rocks. Page 14.
Exfoliation
How big rock domes peel off in shells. Page 6.
Rhyolite
Rhyolite is one of the major igneous rock types. Page 21.
The Rock-Forming Minerals
A handful of very abundant minerals account for the great majority of the Earth's rocks.
Mineral Pictures A to Z
A-to-Z list of minerals shown on About.com Geology.
Texas Geologic Map
Geologic map of Texas, part of a set of all 50 states. Page 43.
Breccia
Breccia is one of the major types of sedimentary rocks. Page 6.
Perlite
Perlite is one of the major igneous rock types. Page 16.
Fossil Picture Gallery
Pictures of fossils, part of the About.com Geology gallery of images.
Quartz Monzonite
Quartz monzonite is one of the major igneous rock types. Page 20.
Greenschist
Greenschist is a major metamorphic rock type from regional metamorphism. Page 7.
Amphibolite
Amphibolite is one of the major metamorphic rock types.
Gemstones to Minerals, Minerals to Gemstones
Mineral names and their corresponding gemstone names.
Argillite
A lightly metamorphosed claystone also called pipestone. Page 2.
Sediment Grain Size Categories
Sediment grades and grain sizes listed from boulders down to clay.
Rock Gypsum
Rock gypsum is one of the major types of sedimentary rocks. Page 20.
Current Earthquakes
Directions to information and data from earthquakes as fast as possible.
Eclogite
Eclogite is a major metamorphic rock type from regional metamorphism. Page 5.
Blueschist
Blueschist is a major metamorphic rock type from regional metamorphism. Page 3.
Quartz and Silica Minerals Gallery
Get familiar with quartz.
Mylonite
Mylonite is a major metamorphic rock type from mechanical metamorphism. Page 12.
Top 10 Gemstone Special Effects
The most important optical effects in gemstones, from fire to asterism.
What Is a Rock?
Everyone knows what a rock is, until you ask what it is exactly.
Geologic Maps of the 50 United States
Geologic maps of all 50 U.S. states.
Yellow Minerals
A guide to the most common and significant yellow minerals.
Rock Cycle Diagram
The rock cycle as I see it. Geology.
Coal
Coal is one of the major types of sedimentary rocks. Page 9.
Map of Plates and Their Boundaries
Details of the world's major tectonic plates and their boundaries.
Komatiite
Komatiite is a rare and ancient ultramafic lava. Page 11.
Rock Salt
Rock salt is one of the major types of sedimentary rocks. Page 21.
Travertine
Travertine is one of the major types of sedimentary rocks. Page 25.
Illinois Geologic Map
Geologic map of Illinois, part of a set of all 50 states. Page 13.
Sedimentary Textures
Sedimentary textures that can be seen in the field with the naked eye or a hand lens.
Mechanical Weathering
A short explanation of mechanical weathering.
Claystone
Claystone is a type of sedimentary rock. Page 8.
Anorthosite
Anorthosite is one of the major igneous rock types. Page 2.
Olivine
Olivine is one of the silicate minerals. Page 20.
Latite
Latite is the extrusive equivalent of monzonite. Page 12.
What Are Minerals?
Minerals meet four criteria, more or less strict.
About Transforms - Plate Tectonic Transform...
Basics of plate tectonics: what happens when plates move past each other.
Dating Planetary Surfaces with Crater Counts
How can we tell the age of features on other planets? It starts with counting impact craters.
Top 25 U.S. Colleges for Geology PhD's
These 25 American colleges grant the huge majority of the geology PhD's held by U.S. college faculty.
Granitoids
How geologists sort out granite from the rocks that look like it.
Chrysocolla
Chrysocolla is one of the silicate minerals. Page 7.
About Divergent Zones in Plate Tectonics
Basics of plate tectonics: divergent zones are where plates move apart.
Rock Hammers
Hammers that geologists and rockhounds use
Minerals with Metallic Luster
Minerals with metallic luster are easy to learn to identify.
What Is an Ophiolite?
Not a rock type, it's a whole assemblage of crustal rocks.
Kimberlite
Kimberlite is the peculiar deep-seated ultramafic rock most famous as the ore of diamonds. Page 10.
About the Earth's Core
The Earth's core: how we study it and what it may be made of.
Pictures of Geologic Features and Processes
Pictures of geologic features and the processes of geology.
Measuring Plate Motion
The five ways we track the motion of the lithospheric plates.
QAP Diagram for Plutonic Rocks
QAP ternary diagram showing IUGS classification of phaneritic igneous rocks.
Diamictite
Diamictite is a messy rock that is not quite conglomerate. Page 12.
Volcanic (Extrusive) Rock Types
Photos of the types of erupted igneous rocks.
Abrasive Minerals
A survey of the minerals whose properties make them useful abrasives.
Holes in Rocks
Geologists have named a variety of holes.
The Lithosphere in a Nutshell
What you need to know about the lithosphere.
Ironstone
Several types of sedimentary rocks are called ironstone. Page 16.
Diatomite
Picture and notes about the sedimentary rock diatomite. Page 13.
Peat
Peat is one of the major types of sedimentary rocks. Page 18.
Troctolite
Troctolite is one of the gabbroid rocks: igneous, plutonic and mafic. Page 25.
Michigan Geologic Map
Geologic map of Michigan, part of a set of all 50 states. Page 22.
The Bre-X Gold Scandal
First there is a mountain of gold, then there is no mountain, then there is no gold mining company named Bre-X.
Banded Iron Formation
Banded iron formation or BIF is one of the major types of sedimentary rocks. Page 4.
About Convergent Zones in Plate Tectonics
Basics of plate tectonics: convergent zones are where plates come together.
About Plate Tectonics
Start here to learn the basics of plate tectonics.
Uranium-Lead Dating
Uranium-lead method is the oldest and, when done carefully, the most reliable isotopic dating method.
North Carolina Geologic Map
Geologic map of North Carolina, part of a set of all 50 states. Page 33.
Index Fossils
The concept of index fossils is important in geologic dating.
The Comeback of Noah's Flood
Once flame-war fuel in the 1800s, the bibilical flood story of Noah is now just another marvel of modern science.
Gypsum Rose
Gypsum is one of the sulfate minerals. Page 6.
Obsidian Gallery
Obsidian is an extreme variety of igneous rock with a glassy texture.
Quaternary Geologic Map of Illinois
Quaternary geologic map of Illinois, courtesy of the state geological survey, with notes.

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