Basic Igneous Rocks

1. What is scoria?

Quartz

~ Mafic / dark~ Glassy / extremely vesicular (no crystals)~ Scoria may contain pyroxene, olivine, magnesium, plagioclase feldspar, biotite mica...

Basalt

Granite

Scoria is a dark, mafic volcanic rock that is glassy and extremely vesicular. It commonly contains minerals like pyroxene, olivine, magnesium, plagioclase feldspar, and biotite mica. Unlike granite, basalt, and quartz, scoria forms as a result of explosive volcanic activity that leads to the rapid cooling and solidification of lava with high gas content, resulting in its vesicular texture.

Explanation

Scoria is a dark, mafic volcanic rock that is glassy and extremely vesicular. It commonly contains minerals like pyroxene, olivine, magnesium, plagioclase feldspar, and biotite mica. Unlike granite, basalt, and quartz, scoria forms as a result of explosive volcanic activity that leads to the rapid cooling and solidification of lava with high gas content, resulting in its vesicular texture.

2. What mineral composition is characteristic of pumice rocks?

Mafic, dark in color, high in iron and magnesium content

~ Felsic, light in color~ Often contains glassy strands~ Extremely vesicular, glassy (no crystals)~ Pumice is composed of rhyolitic, dacitic, or andesitic rock glass, highly silicic

Hardness exceeding that of diamond

Heavy in weight and density, often sinks in water

Pumice is characterized by its felsic mineral composition, light color, glassy strands, and vesicular texture, which are in stark contrast to the characteristics listed in the incorrect answers.

Explanation

Pumice is characterized by its felsic mineral composition, light color, glassy strands, and vesicular texture, which are in stark contrast to the characteristics listed in the incorrect answers.

3. What are the characteristics of obsidian?

~ typically green in color~ porus texture~ INTERMEDIATE

~ commonly pink in color~ has visible crystals~ MAFIC

~ mainly white in color~ metallic luster~ ULTRAMAFIC

~ usually black in color~ extremely glassy (no crystals)~ FELSIC

Obsidian is typically black in color, has a glassy appearance with no visible crystals, and is classified as felsic in terms of composition.

Explanation

Obsidian is typically black in color, has a glassy appearance with no visible crystals, and is classified as felsic in terms of composition.

4. What are the characteristics of Basalt (Porphyritic-Aphanitic example shown)?

Basalt is an intrusive igneous rock formed from the cooling and solidification of lava underground.

Basalt is a felsic rock with light color and visible crystals.

Basalt is commonly composed of quartz, potassium feldspar, and muscovite minerals.

**Aphanitic example shown**
~ Mafic, dark in color~ Can't see crystals, but they exist~ Plagioclase feldspar, pyroxene, olivine, magnetite, and ilmenite are common minerals present in basalt.

Basalt is a mafic rock with a dark color, typically aphanitic, meaning crystals are present but not visible to the naked eye. It contains minerals such as plagioclase feldspar, pyroxene, olivine, magnetite, and ilmenite.

Explanation

Basalt is a mafic rock with a dark color, typically aphanitic, meaning crystals are present but not visible to the naked eye. It contains minerals such as plagioclase feldspar, pyroxene, olivine, magnetite, and ilmenite.

5. What is an Andesite rock made of? (Porphyritic-Aphanitic example shown)

**Aphanitic example shown**
~ Intermediate~ Medium to light grey, brown/purple-ish~ Contains phenocrysts of feldspar (Ca and Na rich varieties), quartz, pyroxene, and biotite mica.

A) Basalt~ Mafic~ Dark grey to black~ Contains phenocrysts of olivine, pyroxene, and plagioclase feldspar.

C) Granite~ Felsic~ Pink to grey~ Contains no phenocrysts, composed mainly of quartz, orthoclase feldspar, plagioclase feldspar, and biotite mica.

B) Rhyolite~ Felsic~ Light grey to pink~ Contains phenocrysts of quartz, potassium feldspar, and biotite mica.

Andesite is classified as an intermediate rock with medium to light grey color, commonly containing phenocrysts such as feldspar, quartz, pyroxene, and biotite mica. Basalt, rhyolite, and granite are different types of rocks with distinct compositions and characteristics compared to andesite.

Explanation

Andesite is classified as an intermediate rock with medium to light grey color, commonly containing phenocrysts such as feldspar, quartz, pyroxene, and biotite mica. Basalt, rhyolite, and granite are different types of rocks with distinct compositions and characteristics compared to andesite.

6. What are the characteristics of Rhyolite (Porphyritic-Aphanitic example shown)?

**Aphanitic example shown**
~ Light color, felsic (tan, pink, white)~ No visible crystals~ sometimes contains phenocrysts of feldspar (Na- rich varieties), quartz, and biotite mica.~ Common mineral types include quartz, feldspar and biotite and are often found in a glassy matrix.

Dark color, mafic (black, dark green)~ Large visible crystals~ Contains primarily olivine, pyroxene, and amphibole minerals.

Heavy color, intermediate (gray, brown)~ Medium-sized visible crystals~ Commonly contains pyroxene, plagioclase feldspar, and amphibole minerals.

Light color, felsic (tan, pink, white)~ No visible crystals~ Tends to contain large amounts of olivine and quartz with minimal feldspar.

Rhyolite typically has a light color, felsic composition, and no visible crystals. It may contain phenocrysts like feldspar, quartz, and biotite mica in a glassy matrix. The incorrect answers describe characteristics of other types of rocks such as mafic, intermediate, and compositions not typically found in Rhyolite.

Explanation

Rhyolite typically has a light color, felsic composition, and no visible crystals. It may contain phenocrysts like feldspar, quartz, and biotite mica in a glassy matrix. The incorrect answers describe characteristics of other types of rocks such as mafic, intermediate, and compositions not typically found in Rhyolite.

7. What is GabbroPhaneritic composed of?

~ Intermediate~ Phaneritic~ Composition can include: plagioclase feldspar, amphibole, and biotite.

~ Felsic~ Phaneritic~ Composition can include: quartz, potassium feldspar, muscovite, and biotite.

~ Ultramafic~ Aphanitic~ Composition can include: serpentine, pyroxene, and olivine.

~ Mafic~ Phaneritic~ Composition can include: pyroxene, plagioclase feldspar, amphibole, and olivine.

GabbroPhaneritic is a mafic, phaneritic rock composed of pyroxene, plagioclase feldspar, amphibole, and olivine.

Explanation

GabbroPhaneritic is a mafic, phaneritic rock composed of pyroxene, plagioclase feldspar, amphibole, and olivine.

8. What is the composition of Diorite?

Ultramafic

Felsic

Mafic

~ Intermediate~ Think "Diorite <=> Dalmation"
~ "Salt and Pepper" Rock: Diorite is of intermediate composition. It has nearly equal proportions of light, felsic minerals (mostly plagioclase and quartz) and dark, mafic minerals (amphibole, pyroxene, and biotite).

Diorite has nearly equal proportions of light, felsic minerals and dark, mafic minerals, making it an intermediate composition.

Explanation

Diorite has nearly equal proportions of light, felsic minerals and dark, mafic minerals, making it an intermediate composition.

9. What are the mineral compositions of GranitePhaneritic?

~ Mafic~ Composed of three minerals. These minerals are pyroxene, olivine, and plagioclase.

~ Ultramafic~ Composed mainly of olivine and pyroxene minerals, with very little feldspar or quartz.

~ Felsic~ Composed of four minerals. These minerals are quartz, feldspar, mica, and usually hornblende.~ Granites can be pink to gray in color, depending on their chemistry and mineralogy.

~ Intermediate~ Contains a mix of dark and light minerals, including biotite, amphibole, and plagioclase, giving it a shade between light and dark colored granites.

GranitePhaneritic is abundant in granite and is characterized by its felsic composition. The correct answer lists the four main minerals found in GranitePhaneritic, while the incorrect answers offer descriptions of other types of rock compositions to create a diverse array of options for the multiple-choice question.

Explanation

GranitePhaneritic is abundant in granite and is characterized by its felsic composition. The correct answer lists the four main minerals found in GranitePhaneritic, while the incorrect answers offer descriptions of other types of rock compositions to create a diverse array of options for the multiple-choice question.