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
What are igneous rocks?
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.
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.
Related
How did Half Dome, the massive rock monument in Yosemite National Park, acquire its unique shape?
Is glacier ice a type of rock?
What are metamorphic rocks?
What are sedimentary rocks?
Are there geologic maps or publications for where I live?
Where can I find information about the geology and natural history of National Parks?
What is the difference between a rock and a mineral?

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.
Aa lava flows erupting from Mauna Loa.
Aa lava flows erupting from Mauna Loa.
Gold in igneous, sedimentary, and metamorphic rocks
Related
How did Half Dome, the massive rock monument in Yosemite National Park, acquire its unique shape?
Is glacier ice a type of rock?
What are metamorphic rocks?
What are sedimentary rocks?
Are there geologic maps or publications for where I live?
Where can I find information about the geology and natural history of National Parks?
What is the difference between a rock and a mineral?

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.
Aa lava flows erupting from Mauna Loa.
Aa lava flows erupting from Mauna Loa.