Geology: Most Popular Articles
These articles are the most popular over the last month.
Rock Identification Tables
Identify 43 of the most common rock types: igneous, sedimentary and metamorphic.
Identify 43 of the most common rock types: igneous, sedimentary and metamorphic.
List of State Gems
Thirty-five states have named official gemstones. Here they are, listed by state.
Thirty-five states have named official gemstones. Here they are, listed by state.
Rock Picture Index
Master guide to rock pictures and descriptions on About.com Geology.
Master guide to rock pictures and descriptions on About.com Geology.
Gneiss
Gneiss is a major metamorphic rock type from regional metamorphism.
Gneiss is a major metamorphic rock type from regional metamorphism.
Granite
Granite is one of the major igneous rock types.
Granite is one of the major igneous rock types.
Mineral Pictures Index
Master guide to pictures of rock-forming, accessory, and rare or notable minerals.
Master guide to pictures of rock-forming, accessory, and rare or notable minerals.
About Igneous Rocks
The three great rock classes--igneous, sedimentary and metamorphic--are introduced with igneous rocks.
The three great rock classes--igneous, sedimentary and metamorphic--are introduced with igneous rocks.
Basalt
Basalt is one of the major igneous rock types.
Basalt is one of the major igneous rock types.
Schist
Schist is a major metamorphic rock type from regional metamorphism.
Schist is a major metamorphic rock type from regional metamorphism.
Shale
Shale is one of the major types of sedimentary rocks.
Shale is one of the major types of sedimentary rocks.
Global Seismic Hazard Maps
The United Nations' global map of seismic hazard
The United Nations' global map of seismic hazard
Marble
Marble is a major metamorphic rock type from regional metamorphism.
Marble is a major metamorphic rock type from regional metamorphism.
Quartzite
Quartzite is a major metamorphic rock type from regional metamorphism.
Quartzite is a major metamorphic rock type from regional metamorphism.
Sandstone
Sandstone is one of the major types of sedimentary rocks.
Sandstone is one of the major types of sedimentary rocks.
Slate
Slate is a major metamorphic rock type from regional metamorphism.
Slate is a major metamorphic rock type from regional metamorphism.
Obsidian
Obsidian is one of the major igneous rock types.
Obsidian is one of the major igneous rock types.
About Sedimentary Rocks
The second of the three great rock classes is introduced with photos.
The second of the three great rock classes is introduced with photos.
About Granite
Granite is the signature rock of the continents.
Granite is the signature rock of the continents.
About Metamorphic Rocks
The three great rock classes--igneous, sedimentary and metamorphic--are introduced with photos, finishing with metamorphic.
The three great rock classes--igneous, sedimentary and metamorphic--are introduced with photos, finishing with metamorphic.
Common Rock Densities
Density, or specific gravity, of common rock types.
Density, or specific gravity, of common rock types.
Scoria
Scoria is one of the major igneous rock types.
Scoria is one of the major igneous rock types.
Igneous Rock Types
The major igneous rock types, plutonic, intrusive and extrusive.
The major igneous rock types, plutonic, intrusive and extrusive.
Diorite
Diorite is one of the major igneous rock types.
Diorite is one of the major igneous rock types.
Pumice
Pumice is one of the major igneous rock types.
Pumice is one of the major igneous rock types.
Mohs Hardness Scale
The 10-mineral Mohs scale of relative hardness is based on what scratches what.
The 10-mineral Mohs scale of relative hardness is based on what scratches what.
World Tectonic Activity
Areas of tectonic activity within the last million years.
Areas of tectonic activity within the last million years.
Gabbro
Gabbro is one of the major igneous rock types.
Gabbro is one of the major igneous rock types.
Serpentinite
Serpentinite is a major metamorphic rock type from regional metamorphism.
Serpentinite is a major metamorphic rock type from regional metamorphism.
Chert
Chert is one of the major types of sedimentary rocks.
Chert is one of the major types of sedimentary rocks.
Limestone
Limestone is one of the major types of sedimentary rocks.
Limestone is one of the major types of sedimentary rocks.
Andesite
Andesite is one of the major igneous rock types.
Andesite is one of the major igneous rock types.
Fault Types in a Nutshell
The three basic types of fault, with their nomenclature, in a nutshell.
The three basic types of fault, with their nomenclature, in a nutshell.
Phyllite
Phyllite is a major metamorphic rock type from regional metamorphism.
Phyllite is a major metamorphic rock type from regional metamorphism.
The Crust
The Earth's crust is exceptionally important, and not just because we live on it.
The Earth's crust is exceptionally important, and not just because we live on it.
Amphibolite
Amphibolite is one of the major metamorphic rock types.
Amphibolite is one of the major metamorphic rock types.
Peridotite
Peridotite is one of the major igneous rock types.
Peridotite is one of the major igneous rock types.
Felsite
Felsite is one of the major igneous rock types.
Felsite is one of the major igneous rock types.
Pegmatite
Pegmatite is one of the major igneous rock types.
Pegmatite is one of the major igneous rock types.
Hornfels
Hornfels is a major metamorphic rock type from contact metamorphism.
Hornfels is a major metamorphic rock type from contact metamorphism.
Granodiorite
Granodiorite is one of the major igneous rock types.
Granodiorite is one of the major igneous rock types.
Pyroxenite
Pyroxenite is one of the major igneous rock types.
Pyroxenite is one of the major igneous rock types.
Porphyry
Porphyry is one of the major igneous rock types.
Porphyry is one of the major igneous rock types.
Siltstone
Siltstone is one of the major types of sedimentary rocks.
Siltstone is one of the major types of sedimentary rocks.
Migmatite
Migmatite is a major metamorphic rock type from regional metamorphism.
Migmatite is a major metamorphic rock type from regional metamorphism.
Syenite
Syenite is one of the major igneous rock types.
Syenite is one of the major igneous rock types.
About Sandstone
Sandstone is sand cemented together into rock.
Sandstone is sand cemented together into rock.
Dunite
Dunite is a plutonic rock consisting of olivine.
Dunite is a plutonic rock consisting of olivine.
Graywacke or Wacke
Wacke or graywacke is one of the major types of sedimentary rocks.
Wacke or graywacke is one of the major types of sedimentary rocks.
Map of World Seafloor Ages
The evidence that made plate tectonics plausible: seafloor spreading.
The evidence that made plate tectonics plausible: seafloor spreading.
Dolomite Rock or Dolostone
Dolomite or dolostone is one of the major types of sedimentary rocks.
Dolomite or dolostone is one of the major types of sedimentary rocks.
Tonalite
Tonalite is a felsic plutonic rock without alkali feldspar.
Tonalite is a felsic plutonic rock without alkali feldspar.
World Hotspot Map
A convenient map showing the most commonly accepted hotspots.
A convenient map showing the most commonly accepted hotspots.
About the San Andreas Fault
Home page for everything about the San Andreas fault.
Home page for everything about the San Andreas fault.
Conglomerate
Conglomerate is one of the major types of sedimentary rocks.
Conglomerate is one of the major types of sedimentary rocks.
Greenschist
Greenschist is a major metamorphic rock type from regional metamorphism.
Greenschist is a major metamorphic rock type from regional metamorphism.
Sedimentary Rock Types
The major types of sedimentary rocks.
The major types of sedimentary rocks.
Quartz
Quartz is classified as a rock-forming mineral.
Quartz is classified as a rock-forming mineral.
Mylonite
Mylonite is a major metamorphic rock type from mechanical metamorphism.
Mylonite is a major metamorphic rock type from mechanical metamorphism.
Greenstone
Grenstone is a major metamorphic rock type from regional metamorphism.
Grenstone is a major metamorphic rock type from regional metamorphism.
Rock Gypsum
Rock gypsum is one of the major types of sedimentary rocks.
Rock gypsum is one of the major types of sedimentary rocks.
Rock Salt
Rock salt is one of the major types of sedimentary rocks.
Rock salt is one of the major types of sedimentary rocks.
About the Earth's Mantle
A look at six different aspects of the Earth's mantle.
A look at six different aspects of the Earth's mantle.
How to Identify Minerals
Cleavage, the way a mineral breaks, is crucial for identifying minerals.
Cleavage, the way a mineral breaks, is crucial for identifying minerals.
Rhyolite
Rhyolite is one of the major igneous rock types.
Rhyolite is one of the major igneous rock types.
Quartz Monzonite
Quartz monzonite is one of the major igneous rock types.
Quartz monzonite is one of the major igneous rock types.
Soapstone
Soapstone is a major metamorphic rock type from regional metamorphism.
Soapstone is a major metamorphic rock type from regional metamorphism.
Eclogite
Eclogite is a major metamorphic rock type from regional metamorphism.
Eclogite is a major metamorphic rock type from regional metamorphism.
What Are Fossils?
Fossils are precious gifts from the geologic past.
Fossils are precious gifts from the geologic past.
State Gemstones of the United States
The 27 official gemstones of 35 states.
The 27 official gemstones of 35 states.
Tuff
Tuff is one of the major igneous rock types.
Tuff is one of the major igneous rock types.
Current Earthquakes
Directions to information and data from earthquakes as fast as possible.
Directions to information and data from earthquakes as fast as possible.
Arkose
Arkose is one of the major types of sedimentary rocks.
Arkose is one of the major types of sedimentary rocks.
Breccia
Breccia is one of the major types of sedimentary rocks.
Breccia is one of the major types of sedimentary rocks.
Plates and Topography
A map showing the actual features that signify plate-tectonic entities.
A map showing the actual features that signify plate-tectonic entities.
Metamorphic Rock Types
The major metamorphic rock types, from regional, contact and mechanical metamorphism.
The major metamorphic rock types, from regional, contact and mechanical metamorphism.
Perlite
Perlite is one of the major igneous rock types.
Perlite is one of the major igneous rock types.
Coquina
Coquina is one of the major types of sedimentary rocks.
Coquina is one of the major types of sedimentary rocks.
Komatiite
Komatiite is a rare and ancient ultramafic lava.
Komatiite is a rare and ancient ultramafic lava.
Coal
Coal is one of the major types of sedimentary rocks.
Coal is one of the major types of sedimentary rocks.
Blueschist
Blueschist is a major metamorphic rock type from regional metamorphism.
Blueschist is a major metamorphic rock type from regional metamorphism.
How to Look at a Rock
Notes on the most basic operation in geology.
Notes on the most basic operation in geology.
Anorthosite
Anorthosite is one of the major igneous rock types.
Anorthosite is one of the major igneous rock types.
Latite
Latite is the extrusive equivalent of monzonite.
Latite is the extrusive equivalent of monzonite.
Exfoliation
How big rock domes peel off in shells.
How big rock domes peel off in shells.
About the Earth's Core
How we study the Earth's core and what it may be made of.
How we study the Earth's core and what it may be made of.
Benitoite
Benitoite is the state gem of California.
Benitoite is the state gem of California.
Chemical Weathering
A short explanation of chemical weathering.
A short explanation of chemical weathering.
Agate
Agate is the state gem of Louisiana, Maryland, Minnesota, Montana, Nebraska and North Dakota.
Agate is the state gem of Louisiana, Maryland, Minnesota, Montana, Nebraska and North Dakota.
Quartz and Silica Minerals Gallery
Get familiar with quartz.
Get familiar with quartz.
Texas Geologic Map
Geologic map of Texas, part of a set of all 50 states.
Geologic map of Texas, part of a set of all 50 states.
Banded Iron Formation
Banded iron formation or BIF is one of the major types of sedimentary rocks.
Banded iron formation or BIF is one of the major types of sedimentary rocks.
Peat
Peat is one of the major types of sedimentary rocks.
Peat is one of the major types of sedimentary rocks.
Landform Picture Index
Definitions and examples of important landforms from About.com Geology.
Definitions and examples of important landforms from About.com Geology.
Global Seismic Hazard Map
The first map to show the entire world's seismic hazard consistently is all here on this site.
The first map to show the entire world's seismic hazard consistently is all here on this site.
Gemstone-Mineral Tables
Mineral names and their corresponding gemstone names.
Mineral names and their corresponding gemstone names.
About Plate Tectonics
Start here to learn the basics of plate tectonics.
Start here to learn the basics of plate tectonics.
Earth Formation in a Nutshell
The straight story of how Earth began, as today's geologists tell it.
The straight story of how Earth began, as today's geologists tell it.
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.
This question has several answers, depending on what a mineral is and what part of the Earth we're talking about.
About Feldspar
Feldspar is the commonest mineral in the Earth's crust, but identifying it can be tricky.
Feldspar is the commonest mineral in the Earth's crust, but identifying it can be tricky.
Tombolo, California
Tombolos are a type of depositional landform.
Tombolos are a type of depositional landform.
Volcanic (Extrusive) Rock Types
Photos of the types of erupted igneous rocks.
Photos of the types of erupted igneous rocks.
Green Minerals
A guide to the most common and significant green and greenish minerals.
A guide to the most common and significant green and greenish minerals.
Alabaster
Alabaster or massive gypsum rock is one of the sedimentary rocks.
Alabaster or massive gypsum rock is one of the sedimentary rocks.
QAP Diagram for Plutonic Rocks
QAP ternary diagram showing IUGS classification of phaneritic igneous rocks.
QAP ternary diagram showing IUGS classification of phaneritic igneous rocks.
Cataclasite
Cataclasite is a major metamorphic rock type from mechanical metamorphism.
Cataclasite is a major metamorphic rock type from mechanical metamorphism.
Fossil Picture Gallery
Pictures of fossils, part of the About.com Geology gallery of images.
Pictures of fossils, part of the About.com Geology gallery of images.
Valley Glacier (Mountain or Alpine Glacier), Alaska
Bucher Glacier, a typical valley - or mountain or alpine - glacier.
Bucher Glacier, a typical valley - or mountain or alpine - glacier.
Horn, Switzerland
The type example of a horn is in the Swiss Alps.
The type example of a horn is in the Swiss Alps.
The Rock-Forming Minerals
A handful of very abundant minerals account for the great majority of the Earth's rocks.
A handful of very abundant minerals account for the great majority of the Earth's rocks.
Mount St Helens Eruption
Remembering a scientist who died in the 1980 eruption of Mount St Helens.
Remembering a scientist who died in the 1980 eruption of Mount St Helens.
Washington Geologic Map
Geologic map of Washington, part of a set of all 50 states.
Geologic map of Washington, part of a set of all 50 states.
Jade
Jade, specifically nephrite, is the state gem of Alaska and Wyoming.
Jade, specifically nephrite, is the state gem of Alaska and Wyoming.
Sedimentary Rock Gallery
An assortment of sedimentary rocks, from fine to coarse grained.
An assortment of sedimentary rocks, from fine to coarse grained.
Amphibole Minerals
Gallery of amphibole minerals.
Gallery of amphibole minerals.
What Is a Rock?
Everyone knows what a rock is, until you ask what it is exactly.
Everyone knows what a rock is, until you ask what it is exactly.
Olivine
Olivine is an important rock-forming mineral.
Olivine is an important rock-forming mineral.
Earthquakes in a Nutshell
Start here for the seismic basics.
Start here for the seismic basics.
Travertine
Travertine is one of the major types of sedimentary rocks.
Travertine is one of the major types of sedimentary rocks.
Diamictite
Diamictite is a messy rock that is not quite conglomerate.
Diamictite is a messy rock that is not quite conglomerate.
Natural Asphalt
Natural asphalt is one of the major types of sedimentary rocks.
Natural asphalt is one of the major types of sedimentary rocks.
Basalt Picture Gallery
A selection of annotated photos of basalt.
A selection of annotated photos of basalt.
Dinosaur Eggs
Dinosaur eggs for sale at a high-end rock shop.
Dinosaur eggs for sale at a high-end rock shop.
Top 25 US Geology PhD Schools
These 25 American colleges grant the huge majority of the geology PhD's held by U.S. college faculty.
These 25 American colleges grant the huge majority of the geology PhD's held by U.S. college faculty.
Nevada Silver Rush
Oh for the days when horn silver lay on the ground in the Nevada silver rush.
Oh for the days when horn silver lay on the ground in the Nevada silver rush.
Pictures of Geologic Features and Processes
Pictures of geologic features and the processes of geology.
Pictures of geologic features and the processes of geology.
Chert Nodule Exterior
Exterior of a chert nodule found in the Mojave Desert.
Exterior of a chert nodule found in the Mojave Desert.
Moonstone
Moonstone is the state gem of Florida.
Moonstone is the state gem of Florida.
Sumatra 26 Dec 2004 M9.3
The greatest earthquake in 40 years killed 300,000 people.
The greatest earthquake in 40 years killed 300,000 people.
Chalcedony or Cryptocrystalline Quartz
A fractured chert pebble displays silica in its cryptocrystalline form.
A fractured chert pebble displays silica in its cryptocrystalline form.
The Biggest Eruptions
The biggest eruptions in history and prehistory
The biggest eruptions in history and prehistory
The Acid Test
Every geologist uses this simple test to tell limestone from dolomite.
Every geologist uses this simple test to tell limestone from dolomite.
Basic Plate Map
The classic basic map from the U.S. Geological Survey.
The classic basic map from the U.S. Geological Survey.
About Tsunamis
Introducing tsunamis or tidal waves--whatever you call them, watch out.
Introducing tsunamis or tidal waves--whatever you call them, watch out.
What Are Minerals?
Minerals meet four criteria, more or less strict.
Minerals meet four criteria, more or less strict.
Chlorastrolite
Chlorastrolite, a variety of pumpellyite, is the state gem of Michigan.
Chlorastrolite, a variety of pumpellyite, is the state gem of Michigan.
Cement and Concrete
Cement is an artificial stone--or maybe lava.
Cement is an artificial stone--or maybe lava.
Plate Tectonics - The World's Crustal Plates
The Earth's crustal plates, from the U.S. Geological Survey
The Earth's crustal plates, from the U.S. Geological Survey
Sunstone
Sunstone is the state gem of Oregon.
Sunstone is the state gem of Oregon.
The Tibetan Plateau
The largest highest place on Earth is a showcase of plate tectonics.
The largest highest place on Earth is a showcase of plate tectonics.
California Geologic Map
Geologic map of California, part of a set of all 50 states.
Geologic map of California, part of a set of all 50 states.
Rhodonite
Rhodonite is the state gem of Massachusetts.
Rhodonite is the state gem of Massachusetts.
Soil Creep
Diagram of soil creep, the gentlest kind of landslide
Diagram of soil creep, the gentlest kind of landslide
Insect in Amber
An amber earring contains an insect fossil.
An amber earring contains an insect fossil.
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.
First there is a mountain of gold, then there is no mountain, then there is no gold mining company named Bre-X.
Actinolite
Actinolite is a major amphibole mineral.
Actinolite is a major amphibole mineral.
About Unconformities
In rocks, what is not there can be as informative as what is there.
In rocks, what is not there can be as informative as what is there.
Igneous Rock Textures
The major texture types found in igneous rocks.
The major texture types found in igneous rocks.
Ice Cave, Alaska
Ice cave in Guyot Glacier, southeastern Alaska.
Ice cave in Guyot Glacier, southeastern Alaska.
Calcite
Calcite is a rock-forming mineral, most important in limestone.
Calcite is a rock-forming mineral, most important in limestone.
Slate or Phyllite?
Slate and phyllite grade into each other, and sometimes the choice is arbitrary.
Slate and phyllite grade into each other, and sometimes the choice is arbitrary.