IGNEOUS ROCKS

IGNEOUS ROCKSIGNEOUS ROCKSform when magma/lava cools and solidifiesform when magma/lava cools and solidifies

Cleveland Volcano, AlaskaCleveland Volcano, Alaska

IGNEOUS ROCKS are “born of fire”

How Igneous Rock Is FormedHow 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.

Mafic LavaMafic Lava

Hot, thin, and Hot, thin, and dark in color dark in color (similar to flowing (similar to flowing water)water)

Rich in iron and Rich in iron and magnesium low magnesium low in silica contentin silica content

Color of Igneous rocks depend on type of lava and mineral composition

Felsic LavaFelsic Lava

Cooler, thicker, Cooler, thicker, and light in colorand light in color

Contains much Contains much silicasilica

Explosive Explosive eruptions!!eruptions!!

Extrusive RocksExtrusive Rocks

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

Ryolite is an exampleRyolite is an example

Intrusive Rocks (Plutonic) Intrusive Rocks (Plutonic)

Slow cooling deep beneath the Earth’s surface allows Slow cooling deep beneath the Earth’s surface allows crystals to grow to large size (1/8” or more). These crystals to grow to large size (1/8” or more). These crystals are easily visible and distinguish this group of crystals are easily visible and distinguish this group of igneous rocks as igneous rocks as INTRUSIVEINTRUSIVE..

GRANITEGRANITE is a coarse to medium-grained rock that forms from the cooling of magma deep within the Earth (intrusive).

Intrusive textures:1. Fine-grained textureFine-grained texture -- due to fast cooling (at or near surface) 2. Coarse-grained textureCoarse-grained texture -- due to slow cooling at depth 3. Porphyritic Porphyritic texture (Combination)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. GlassyGlassy texture texture -- due to very rapid cooling -- magma cools so fast crystals don't have time to form. Obsidian (volcanic glass) forms this way. 5. VesicularVesicular texture (Frothy) 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. PyroclasticPyroclastic texture (Fragmental) texture (Fragmental)- chunks of molten material that fuse together

Igneous rock texturestextures depend on cooling historycooling history

Coarse-grained Fine-grained Combination Coarse-grained Fine-grained Combination

Glassy Frothy Pyroclastic Glassy Frothy Pyroclastic (Fragmental)(Fragmental)

Igneous Rock TexturesIgneous Rock Textures

 Igneous intrusionsintrusions 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:

DikesDikes 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.

SillsSills 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.

BatholithsBatholiths are huge igneous intrusions. Their size is on the scale of an entire mountain range (100’s of miles).

LaccolithsLaccoliths 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  

Types of Igneous Intrusions

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

SillSill

Palisades Sill, NY

MAGMA and MAGMA and LavaLava

MagmaMagma: Molten rock: Molten rock formed formed by by partial meltingpartial melting in the in the crust and uppermost mantle crust and uppermost mantle to a depth of ~250 kmto a depth of ~250 km

LavaLava = Magma that reaches = Magma that reaches Earth’s surfaceEarth’s surface