Question

Name the following rocks: a. An extrusive rock with \(40 \%\) Ca-rich plagioclase and \(60 \%\) pyroxene b. An intrusive rock with \(65 \%\) plagioclase, \(25 \%\) amphibole, and \(10 \%\) pyroxene c. An intrusive rock with \(25 \%\) quartz, \(20 \%\) potassium feldspar, \(50 \%\) plagioclase feldspar, and minor amounts of biotite

Short answer

a) Basalt b) Diorite c) Granodiorite

Step-by-step solution

Identify rock type by composition for part (a)

The rock has 40% Ca-rich plagioclase and 60% pyroxene. Since it is an extrusive rock and the dominant minerals are plagioclase and pyroxene, it is most likely a basalt.

Identify rock type by composition for part (b)

The rock has 65% plagioclase, 25% amphibole, and 10% pyroxene. As it is an intrusive rock with these compositions, it is classified as a diorite.

Identify rock type by composition for part (c)

The rock has 25% quartz, 20% potassium feldspar, 50% plagioclase feldspar, and minor amounts of biotite. These mineral percentages indicate that it is an intrusive rock, specifically a granodiorite.

Key concepts

Igneous Rocks

Igneous rocks are formed from the solidification of molten magma or lava. Depending on where they solidify, they are categorized as either intrusive or extrusive. Intrusive igneous rocks form below the Earth’s surface and typically have larger crystals due to the slower cooling process. Examples include granite and diorite.
Extrusive igneous rocks, on the other hand, form at the Earth's surface and cool quickly, resulting in smaller crystals. Basalt and pumice are common examples. The texture and mineral composition of these rocks are critical in identifying them.
Here are some key points:

• Intrusive rocks cool slowly, forming large crystals.
• Extrusive rocks cool quickly, forming small crystals.
• The mineral composition determines the specific type of igneous rock.

Mineral Composition

Mineral composition is a key factor in identifying different types of igneous rocks. Each mineral has unique properties and forms in specific conditions.
For example, plagioclase feldspar can vary in composition from calcium-rich to sodium-rich, and this can significantly influence a rock's classification. Pyroxene and amphibole are dark-colored, high-density minerals often found in both intrusive and extrusive rocks. Quartz and potassium feldspar are light-colored minerals commonly found in intrusive rocks like granite.
Understanding the mineral composition helps to determine the rock type:

• Ca-rich plagioclase and pyroxene in an extrusive rock: Basalt.
• Plagioclase, amphibole, and pyroxene in an intrusive rock: Diorite.
• Quartz, potassium feldspar, plagioclase feldspar, and biotite in an intrusive rock: Granodiorite.

Rock Textures

Rock texture is essential in classifying igneous rocks and understanding their history. It refers to the size, shape, and arrangement of minerals within the rock. Large, well-formed crystals indicate slow cooling (intrusive), while small, fine-grained crystals suggest rapid cooling (extrusive).
Common textures include:

• Phaneritic: Coarse-grained texture, with crystals visible to the naked eye, typical of intrusive rocks such as granite.
• Aphanitic: Fine-grained texture, with crystals too small to see without magnification, common in extrusive rocks like basalt.
• Porphyritic: A mix of coarse and fine-grained textures, indicating varied cooling rates.

Understanding these textures helps to identify the rock and its formation environment.