IGNEOUS ROCKS form when magma/lava cools and solidifies

Cleveland Volcano, Alaska IGNEOUS ROCKS are “born of fire” Cleveland Volcano, Alaska

How Igneous Rock Is Formed They are formed by the crystallization of magma or lava Igneous rocks may form deep inside the Earth or at the Earth’s surface when a volcano erupts.

Color of Igneous rocks depend on type of lava and mineral composition Mafic Lava Hot, thin, and dark in color (similar to flowing water) Rich in iron and magnesium low in silica content

Felsic Lava Cooler, thicker, and light in color Contains much silica Explosive eruptions!!

Extrusive Rocks Rapid cooling near or at the Earth’s surface, produces many small crystals that are not readily seen by the unaided eye. This group of igneous rocks is called EXTRUSIVE and are typically volcanic in origin. Cooling may be so rapid that crystals do not have a chance to form and instead a glass is produced. Ryolite is an example

Intrusive Rocks (Plutonic) Slow cooling deep beneath the Earth’s surface allows crystals to grow to large size (1/8” or more). These crystals are easily visible and distinguish this group of igneous rocks as INTRUSIVE. GRANITE is a coarse to medium-grained rock that forms from the cooling of magma deep within the Earth (intrusive).

Igneous rock textures depend on cooling history Intrusive textures: 1. Fine-grained texture -- due to fast cooling (at or near surface) 2. Coarse-grained texture -- due to slow cooling at depth 3. Porphyritic texture (Combination) -- coarse crystals surrounded by fine-grained matrix forms due to initial slow cooling, then magma rising to (or close to) surface and the remaining magma cooling quickly Extrusive textures: 4. Glassy texture -- due to very rapid cooling -- magma cools so fast crystals don't have time to form. Obsidian (volcanic glass) forms this way. 5. Vesicular texture (Frothy) -- full of rounded holes (vesicles) -- forms due to escape of gas bubbles during cooling of lava. Pumice is a light-colored rock with this vesicular texture. 6. Pyroclastic texture (Fragmental)- chunks of molten material that fuse together

Igneous Rock Textures Coarse-grained Fine-grained Combination Glassy Frothy Pyroclastic (Fragmental)

Types of Igneous Intrusions   Igneous intrusions are rock bodies that form from crystallization of magma at depth within earth's crust.  They are categorized based on their shape and overall size. They come in a variety of shapes and sizes: Dikes are small igneous intrusions that cut across rocks into which the magma intrudes. They are commonly sheet-like, only a few meters wide. Think of magma invading a vertical or near-vertical fracture in rock. Sills are also small igneous intrusions. They are sheets of rock that, unlike dikes, are parallel to pre-existing rocks. Think of magma invading sedimentary rocks by spreading out between rock layers. That magma would cool to form a sill. Batholiths are huge igneous intrusions. Their size is on the scale of an entire mountain range (100’s of miles). Laccoliths are rather large, mushroom-shaped intrusions that “puff up” in the center due to gases. Volcanic Necks are igneous rock forming the feeder pipe of a volcanic vent

The volcanic neck rises dramatically 1,267 feet (386 m) above the surrounding terrain and the summit is 5,112 feet (1,558 m) above sea level located in Northeast Wyoming.

Shiprock, New Mexico

Description: Yosemite National Park, eastern California.

Bear Butte, Sturgis, SD

Sill

Palisades Sill, NY

MAGMA and Lava Magma: Molten rock formed by partial melting in the crust and uppermost mantle to a depth of ~250 km Lava = Magma that reaches Earth’s surface