Physical Geography

Igneous Rocks

Igneous Rocks

•  Igneous rock forms when magma/lava cools and hardens.

•  Rocks that result when lava solidifies are classified as extrusive igneous rocks.

•  Rocks that result when magma solidifies beneath the earth’s surface are classified as intrusive igneous rocks.

•  The rate of cooling strongly influences the crystallization process, in particular the size of the crystals.

Igneous Rocks

•  In addition to the rate of cooling, the mineral composition of the magma and the amount of volatile material also influence the crystallization process.

•  Consequently, igneous rocks are most often classified by their texture and mineral composition.

Slow cooling ! Formation of large crystals

Rapid cooling ! Formation of small crystals

Instant cooling ! Formation of glass

Igneous Rock Textures

•  The term texture is used to describe the overall appearance of the rock based on the size and arrangement of its interlocking crystals.

•  Texture is an important characteristic because it reveals about the environment in which the rock formed.

•  The most important factor affecting the texture of a rock is the rate at which the magma/lava cooled.

Igneous Rock Textures

•  Igneous rock textures include the following:

" Phaneritic Texture

" Aphanitic Texture

" Porphyritic Texture

" Glassy Texture

" Vesicular Texture

" Pyroclastic Texture

Phaneritic Texture

•  Phaneritic textured rocks are comprised of large crystals that are clearly visible to the eye with or without a hand lens or binocular microscope. This texture forms by slow cooling of magma deep underground

Aphaneritic Texture

•  Aphanitic texture consists of small crystals that cannot be seen by the eye with or hand lens. Aphanitic rocks are characterized by textures in which the mineral grains are not visible to the eye so they generally look rather like a blank slate.

Porphritic Texture

•  T. Porphyritic rocks are composed of at least two minerals having a conspicuous (large) difference in grain size. The larger grains are termed phenocrysts and the finer grains either matrix or groundmass. Porphyritic rocks are thought to have undergone two stages of cooling; one at depth where the larger phenocrysts formed and a second at or near the surface where the matrix grains crystallized.

Glassy Texture

•  Glassy textured igneous rocks are non-crystalline meaning the rock contains no mineral grains. Glass results from cooling that is so fast that minerals do not have a chance to crystallize. This may happen when magma or lava comes into quick contact with much cooler materials near the Earth's surface. Pure volcanic glass is known as obsidian

Vesicular Texture

•  This term refers to vesicles (holes, pores, or cavities) within the igneous rock. Vesicles are the result of gas expansion (bubbles), which often occurs during volcanic eruptions. Pumice and scoria are common types of vesicular rocks. The image to the left shows a basalt with vesicles, hence the name “vesicular basalt”.

Pyroclastic Texture

•  Pyroclastic rocks are blown out into the atmosphere during violent volcanic eruptions. These rocks are collectively termed fragmental. If you examine a fragmental volcanic rock closely you can see why. You will note that it is comprised of numerous grains or fragments that have been welded together by the heat of volcanic eruption. If you run your fingers over the rock it will often feel grainy like sandpaper or a sedimentary rock. You might also spot shards of glass embedded in the rock.

Minerals of Igneous Rock

Plagioclase •  Most common mineral in igneous rock. •  Appears frosty white to grey-white in colour.

Quartz •  A very common mineral in igneous rock. •  Grains appear dark grey and glassy.

Potassium Feldspar •  Has slightly pinkish grains.

Minerals of Igneous Rock

Muscovite •  Not a common mineral. •  Appears as small shiny grains. •  Can be flaked off using a nail.

Biotite •  Appears as small black shiny grains. •  Crystals are hexagonal.

Amphibole •  Occurs as slender needle-like crystals that have a blackish colour •  Commonly found in diorite.

Minerals of Igneous Rock

Pyroxene •  Commonly found in mafic igneous rock. •  Occurs as small, dark green crystals.

Olivine •  Commonly found in ultramafic igneous rock. •  Occurs as small, light green, glassy crystals.

Classification of Igneous Rock

Classification of Igneous Rock