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Igneous Rocks:-

• Igneous rocks form from molten rock (magma) crystallizing below earth's

surface or from volcanic activity. They commonly form at plateboundaries and are commonly exposed in mountainous areas.

• Igneous rocks form from crystallization of magma at depth (within

the earth's crust) or at the surface (from volcanic eruptions)

There are two (2) basic types or forms of igneous rocks:-

1- Plutonic rocks = intrusive igneous rocks = igneous rocksthat form from cooling magma at depth.

2- Extrusive igneous rocks = igneous rocks that form from

volcanic activity (at or near surface).

In general:Plutonic rocks are usually coarse-grainedExtrusive rocks are usually fine-grained

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These samples represent

igneous rocks which have

formed at depth within theEarth, although each exhibit

different textures.

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What are textures?

• The texture of a rock is how it appears on the surface and how minerals

are connected together.

• Igneous rock textures depend on cooling history .

Igneous Rocks may have one of two (2) types of textures:

Intrusive textures:

• Fine-grained texture (Aphanitic) --due to fast cooling (at or near surface)

• Coarse-grained texture (Phaneritic) --due to slow cooling at depth.

• Porphyritic texture Porphyritic texture -- coarse crystals (phenocrysts)

surrounded by fine-grained matrix (groundmass); forms due to initial slow

cooling, then magma rising to (or close to) surface and the remaining magmacooling quickly.

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• Extrusive textures:-

• Glassy texture -- due to very rapid cooling --magma cools so fast crystals don't

have time to form. Obsidian (volcanic glass) forms this way.

• Vesicular texture -- full of rounded holes (vesicles) --forms due to escape ofgas bubbles during cooling of lava. Pumice is a light-colored rock with this

vesicular texture.

• Pyroclastic texture -chunks of molten material that fuse together

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Igneous Rock Textures

Coarse-grained Fine--grained Porphyritic

GlassyVesicular

Pyroclastic

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Magma composition

• The composition of the magma determines thecomposition of the rock that forms .

• Igneous rock classification scheme shows that rocks

vary in silica content (e.g. quartz-rich rocks have higher

silica content)

• Other darker rocks: gabbros and basalts are low in

silica, with bulk compositions less than about 60% silica

• Granites and rhyolites are high in silica (bulk

compositions with higher than 60% silica)

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What are 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 overallsize.

• They come in a variety of shapes and sizes:

• Discordant : cut across pre-existing fabric of rock layers:-

Dikes are small igneous intrusions that cut across rocks into which the magma

intrudes. They are commonly sheet-like, only a few meters wide, but possibly laterally

extensive. Think of magma invading a vertical or near-vertical fracture in rock.

Igneous rock would fill the crack due to crystallization of magma. One would call the

rock body a dike.

 –Stocks are fairly large (10’s of miles) igneous intrusions that cut across pre -existing

rock layers. In size, they are on the order of an individual mountain peak. – Batholiths are huge igneous intrusions made of many stocks. Their size is on the

scale of an entire mountain range (100’s of miles).

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 –Sillsare also small igneous intrusions. They are sheets of rock that, unlikedikes, are parallelto pre-existing rocks. Think of magma invading

sedimentary rocks by spreading out between rock layers. That magma

would cool to form a sill.

 –Laccoliths are rather large, mushroom-shaped intrusions that “puff up ” in

the center due to gases.

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An Igneous Intrusion as Viewed from Satellite

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