Igneous Rocks Quiz

What is the primary factor that determines the texture of an igneous rock?

• Mineral composition

• Cooling rate

• Color index

• Chemical weathering

Answer explanation

The texture of an igneous rock is primarily determined by its cooling rate. Faster cooling results in smaller crystals, while slower cooling allows larger crystals to form, affecting the rock's overall texture.

Which of the following is an intrusive igneous rock?

• Basalt

• Rhyolite

• Granite

• Pumice

Answer explanation

Granite is an intrusive igneous rock formed from magma that cools slowly beneath the Earth's surface. In contrast, basalt and rhyolite are extrusive, and pumice is a volcanic rock that forms from rapidly cooled lava.

Which mineral is NOT typically found in granite?

• Quartz

• Plagioclase

• Olivine

• Alkali feldspar

Answer explanation

Granite is primarily composed of quartz, plagioclase, and alkali feldspar. Olivine is not typically found in granite; it is more common in mafic rocks like basalt.

Aphanitic texture indicates:

• Slow cooling

• Fast cooling

• Deep-seated crystallization

• Metamorphism

Answer explanation

Aphanitic texture indicates fast cooling of magma, resulting in small crystals that are not visible to the naked eye. This typically occurs when lava cools quickly on the Earth's surface.

What is the main difference between granite and rhyolite?

• Mineral composition

• Cooling rate

• Chemical composition

• Presence of vesicles

Answer explanation

The main difference between granite and rhyolite is their cooling rate. Granite forms from slow cooling of magma beneath the Earth's surface, resulting in large crystals, while rhyolite cools quickly on the surface, leading to smaller crystals.

Which of the following is a glassy volcanic rock?

• Gabbro

• Obsidian

• Diorite

• Pegmatite

Answer explanation

Obsidian is a glassy volcanic rock formed from rapidly cooled lava, giving it a smooth, glass-like texture. In contrast, Gabbro, Diorite, and Pegmatite are all crystalline igneous rocks, not glassy.

The term 'felsic' refers to rocks that are rich in:

• Iron and magnesium

• Calcium and sodium

• Silica

• Aluminum

Answer explanation

The term 'felsic' describes rocks that are high in silica content, typically over 65%. This distinguishes them from mafic rocks, which are richer in iron and magnesium. Therefore, the correct answer is silica.

Which mineral group is most abundant in basalt?

• Olivine

• Pyroxene

• Quartz

• Muscovite

Answer explanation

Basalt is primarily composed of pyroxene, which is the most abundant mineral group in this volcanic rock. While olivine is also present, pyroxene dominates the mineral composition.

Which intrusive body is the largest in size?

• Sill

• Dike

• Laccolith

• Batholith

Answer explanation

A batholith is the largest type of intrusive igneous body, typically exceeding 100 square kilometers in area and formed from the cooling of magma deep within the Earth's crust. In contrast, sills, dikes, and laccoliths are smaller.

Which of the following textures is characteristic of volcanic rocks?

• Phaneritic

• Porphyritic

• Glassy

• Granular

Answer explanation

Volcanic rocks often have a glassy texture due to rapid cooling of lava, which prevents crystal formation. This distinguishes them from phaneritic and porphyritic textures, which are typical of slower-cooled igneous rocks.