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Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.
Intrusive Igneous Rocks:
Intrusive, or plutonic, igneous rock forms when magma is trapped deep inside the Earth. Great globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the Earth's surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies. Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture. Some common intrusive igneous rocks are granite, diorite, gabbro and peridotite.
Extrusive Igneous Rocks:
Extrusive, or volcanic, igneous rock is produced when magma exits and cools above (or very near) the Earth's surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere. Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture. Some common extrusive igneous rocks are rhyolite, andesite, basalt and obsidian.
At the head of the valley in Yosemite National Park - as if on a pedestal - stands Half Dome. It is smoothly rounded on three sides and a sheer vertical face on the fourth. Half Dome, which stands nearly 8,800 feet (2,682 meters) above sea level, is composed of granodiorite, and is the remains of a magma chamber that cooled slowly and crystallized thousands of feet beneath the Earth's surface. The...
Glacier ice, like limestone (for example), is a type of rock. Glacier ice is actually a mono-mineralic rock (a rock made of only one mineral, like limestone which is composed of the mineral calcite). The mineral ice is the crystalline form of water (H 2O). Most glacier ice forms through the metamorphism of tens of thousands of individual snowflakes into crystals of glacier ice. Each snowflake is a...
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates...
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Common Sedimentary Rocks: Common sedimentary rocks include siltstone...
Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations. Geologic maps at many scales and from many sources are listed in the National Geologic Map Database. Some geologic maps can be purchased in hard copy through the USGS Store. Download digital geologic maps for entire states from the USGS Mineral Resources Online Geospatial Data...
Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history. A source of information from the USGS is our Geology and Ecology of National Parks website. The website has listings for regions of the country; selected parks are listed within each region...
A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. Common minerals include quartz, feldspar, mica, amphibole, olivine, and calcite. A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral matter. Common rocks include granite, basalt...
This outcrop is along Highway 175, near Hobergs, and is comprised of andesite. Andesite contains more silica than basalt, but less than dacite.
This outcrop is along Highway 175, near Hobergs, and is comprised of andesite. Andesite contains more silica than basalt, but less than dacite.
Looking southwest from the Steens Mountain summit area into the Alvord desert of eastern Oregon. This fault-block escarpment reveals the oldest lava flows of the Columbia River Flood Basalts, which erupted about 17 million years ago. Three or four vertical dikes that feed these flows are evident in the midground ridge where they stand as rock walls cutti
Looking southwest from the Steens Mountain summit area into the Alvord desert of eastern Oregon. This fault-block escarpment reveals the oldest lava flows of the Columbia River Flood Basalts, which erupted about 17 million years ago. Three or four vertical dikes that feed these flows are evident in the midground ridge where they stand as rock walls cutti
Geologists from Montana State University study an outcrop of diorite (a silicic igneous rock that cooled slowly beneath the ground). An outcrop of this size is important because it is easier to see any structural relations along with rock composition.
Geologists from Montana State University study an outcrop of diorite (a silicic igneous rock that cooled slowly beneath the ground). An outcrop of this size is important because it is easier to see any structural relations along with rock composition.
The photo was taken by Erin Todd during helicopter-supported geologic fieldwork in Lake Clark National Park this past summer.
The photo was taken by Erin Todd during helicopter-supported geologic fieldwork in Lake Clark National Park this past summer.
Mineral: Graphite (C) in pegmatite rock
Mineral Origin: Ticonderoga, NY
Primary Mineral Commodity: Graphite
Mineral Commodity Uses: brake linings, foundry operations, heat-resistant lubricants, refractory applications, and steelmaking
Mineral: Graphite (C) in pegmatite rock
Mineral Origin: Ticonderoga, NY
Primary Mineral Commodity: Graphite
Mineral Commodity Uses: brake linings, foundry operations, heat-resistant lubricants, refractory applications, and steelmaking
Overlying Spor Mountain rhyolite caprock and underlying Beryllium Tuff member.
Overlying Spor Mountain rhyolite caprock and underlying Beryllium Tuff member.
A close-up view showing the ropy texture that forms on the surface of pāhoehoe lava.
A close-up view showing the ropy texture that forms on the surface of pāhoehoe lava.
A pink granite monument at the Lyndon Baines Johnson Memorial Grove on the Potomac. It was hewn from a quarry in the Texas Hill Country, where Johnson was born and raised.
A pink granite monument at the Lyndon Baines Johnson Memorial Grove on the Potomac. It was hewn from a quarry in the Texas Hill Country, where Johnson was born and raised.
A granite cliff, looking east into the Gulf of Maine, at Thunder Hole in Acadia National Park
A granite cliff, looking east into the Gulf of Maine, at Thunder Hole in Acadia National Park
A specimen of gabbro, an igneous mineral that was used as a purifying agent in the iron smelting process at the Saugus Iron Works.
A specimen of gabbro, an igneous mineral that was used as a purifying agent in the iron smelting process at the Saugus Iron Works.
Huckleberry Ridge Tuff deposit exposed on Mt. Everts, near the northern boundary of Yellowstone National Park. The deposit was created by ash falling from the plume early in the eruption sequence, 2.08 million years ago. Photo by Madison Myers, Montana State University.
Huckleberry Ridge Tuff deposit exposed on Mt. Everts, near the northern boundary of Yellowstone National Park. The deposit was created by ash falling from the plume early in the eruption sequence, 2.08 million years ago. Photo by Madison Myers, Montana State University.
This photo shows the ropey texture of a recently crusted pahoehoe flow, as well as its superior strength as a natural insulator. The crack is still glowing hot but the lava is no longer moving under the crust.
This photo shows the ropey texture of a recently crusted pahoehoe flow, as well as its superior strength as a natural insulator. The crack is still glowing hot but the lava is no longer moving under the crust.
Obsidian Cliff exposes the interior of a thick rhyolite lava flow erupted about 180,000 years ago. The vertical columns are cooling fractures that formed as the thick lava flow cooled and crystallized. The flow consists of obsidian, a dark volcanic glass.
Obsidian Cliff exposes the interior of a thick rhyolite lava flow erupted about 180,000 years ago. The vertical columns are cooling fractures that formed as the thick lava flow cooled and crystallized. The flow consists of obsidian, a dark volcanic glass.
At the head of the valley in Yosemite National Park - as if on a pedestal - stands Half Dome. It is smoothly rounded on three sides and a sheer vertical face on the fourth. Half Dome, which stands nearly 8,800 feet (2,682 meters) above sea level, is composed of granodiorite, and is the remains of a magma chamber that cooled slowly and crystallized thousands of feet beneath the Earth's surface. The...
Glacier ice, like limestone (for example), is a type of rock. Glacier ice is actually a mono-mineralic rock (a rock made of only one mineral, like limestone which is composed of the mineral calcite). The mineral ice is the crystalline form of water (H 2O). Most glacier ice forms through the metamorphism of tens of thousands of individual snowflakes into crystals of glacier ice. Each snowflake is a...
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates...
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Common Sedimentary Rocks: Common sedimentary rocks include siltstone...
Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations. Geologic maps at many scales and from many sources are listed in the National Geologic Map Database. Some geologic maps can be purchased in hard copy through the USGS Store. Download digital geologic maps for entire states from the USGS Mineral Resources Online Geospatial Data...
Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history. A source of information from the USGS is our Geology and Ecology of National Parks website. The website has listings for regions of the country; selected parks are listed within each region...
A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. Common minerals include quartz, feldspar, mica, amphibole, olivine, and calcite. A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral matter. Common rocks include granite, basalt...
This outcrop is along Highway 175, near Hobergs, and is comprised of andesite. Andesite contains more silica than basalt, but less than dacite.
This outcrop is along Highway 175, near Hobergs, and is comprised of andesite. Andesite contains more silica than basalt, but less than dacite.
Looking southwest from the Steens Mountain summit area into the Alvord desert of eastern Oregon. This fault-block escarpment reveals the oldest lava flows of the Columbia River Flood Basalts, which erupted about 17 million years ago. Three or four vertical dikes that feed these flows are evident in the midground ridge where they stand as rock walls cutti
Looking southwest from the Steens Mountain summit area into the Alvord desert of eastern Oregon. This fault-block escarpment reveals the oldest lava flows of the Columbia River Flood Basalts, which erupted about 17 million years ago. Three or four vertical dikes that feed these flows are evident in the midground ridge where they stand as rock walls cutti
Geologists from Montana State University study an outcrop of diorite (a silicic igneous rock that cooled slowly beneath the ground). An outcrop of this size is important because it is easier to see any structural relations along with rock composition.
Geologists from Montana State University study an outcrop of diorite (a silicic igneous rock that cooled slowly beneath the ground). An outcrop of this size is important because it is easier to see any structural relations along with rock composition.
The photo was taken by Erin Todd during helicopter-supported geologic fieldwork in Lake Clark National Park this past summer.
The photo was taken by Erin Todd during helicopter-supported geologic fieldwork in Lake Clark National Park this past summer.
Mineral: Graphite (C) in pegmatite rock
Mineral Origin: Ticonderoga, NY
Primary Mineral Commodity: Graphite
Mineral Commodity Uses: brake linings, foundry operations, heat-resistant lubricants, refractory applications, and steelmaking
Mineral: Graphite (C) in pegmatite rock
Mineral Origin: Ticonderoga, NY
Primary Mineral Commodity: Graphite
Mineral Commodity Uses: brake linings, foundry operations, heat-resistant lubricants, refractory applications, and steelmaking
Overlying Spor Mountain rhyolite caprock and underlying Beryllium Tuff member.
Overlying Spor Mountain rhyolite caprock and underlying Beryllium Tuff member.
A close-up view showing the ropy texture that forms on the surface of pāhoehoe lava.
A close-up view showing the ropy texture that forms on the surface of pāhoehoe lava.
A pink granite monument at the Lyndon Baines Johnson Memorial Grove on the Potomac. It was hewn from a quarry in the Texas Hill Country, where Johnson was born and raised.
A pink granite monument at the Lyndon Baines Johnson Memorial Grove on the Potomac. It was hewn from a quarry in the Texas Hill Country, where Johnson was born and raised.
A granite cliff, looking east into the Gulf of Maine, at Thunder Hole in Acadia National Park
A granite cliff, looking east into the Gulf of Maine, at Thunder Hole in Acadia National Park
A specimen of gabbro, an igneous mineral that was used as a purifying agent in the iron smelting process at the Saugus Iron Works.
A specimen of gabbro, an igneous mineral that was used as a purifying agent in the iron smelting process at the Saugus Iron Works.
Huckleberry Ridge Tuff deposit exposed on Mt. Everts, near the northern boundary of Yellowstone National Park. The deposit was created by ash falling from the plume early in the eruption sequence, 2.08 million years ago. Photo by Madison Myers, Montana State University.
Huckleberry Ridge Tuff deposit exposed on Mt. Everts, near the northern boundary of Yellowstone National Park. The deposit was created by ash falling from the plume early in the eruption sequence, 2.08 million years ago. Photo by Madison Myers, Montana State University.
This photo shows the ropey texture of a recently crusted pahoehoe flow, as well as its superior strength as a natural insulator. The crack is still glowing hot but the lava is no longer moving under the crust.
This photo shows the ropey texture of a recently crusted pahoehoe flow, as well as its superior strength as a natural insulator. The crack is still glowing hot but the lava is no longer moving under the crust.
Obsidian Cliff exposes the interior of a thick rhyolite lava flow erupted about 180,000 years ago. The vertical columns are cooling fractures that formed as the thick lava flow cooled and crystallized. The flow consists of obsidian, a dark volcanic glass.
Obsidian Cliff exposes the interior of a thick rhyolite lava flow erupted about 180,000 years ago. The vertical columns are cooling fractures that formed as the thick lava flow cooled and crystallized. The flow consists of obsidian, a dark volcanic glass.
