Igneous Rocks Practice

IGNEOUS ROCKS

MAGMA

• Magma: Refers to completely or partially molten rock that is found underground.

• Lava: Molten rock that has reached the surface of the Earth.

NATURE OF MAGMA

• Consists of 3 phases: Solid, Liquid, and Gas.

  • Melt: The liquid portion of magma.

  • Composed primarily of 8 most common elements in the Earth’s crust:

  • Oxygen

  • Silicon

  • Aluminum

  • Iron

  • Calcium

  • Sodium

  • Potassium

  • Magnesium

  • Hint: "Only silly artists in college study past midnight" is a mnemonic for recalling the elements (O, Si, Al, Fe, Ca, Na, K, Mg).

NATURE OF MAGMA CONTINUED

• Solid Component: Crystals of silicate minerals found within.

• Gaseous Component:

  • Known as Volatiles, which include:

  • Water vapor

  • Carbon dioxide

  • Sulfur dioxide

IGNEOUS PROCESSES

• Igneous rocks form in 2 main settings:

  • Underground (within the Earth’s crust)

  • Known as Intrusive rocks or Plutonic rocks.

  • The term originates from Pluto, the Greek god of the underworld.

  • At the Earth’s surface

  • Known as Extrusive rocks or Volcanic rocks.

  • Named after Vulcan, the Roman god of fire.

IGNEOUS COMPOSITION CATEGORIES

• Two main categories exist, each classified based on their mineral content:

  • Felsic:

  • Dominated by light-colored silicate minerals.

  • Mafic:

  • Contains at least 45% dark-colored (ferromagnesian) silicate minerals.

  • Intermediate:

  • Contains a composition between felsic and mafic, with at least 25% dark-colored silicate minerals.

SILICA CONTENT

• Silica: Another term for quartz.

• Relationship between silica content and mineral composition:

  • High silica content = Low ferromagnesian minerals.

  • Low silica content = High ferromagnesian minerals.

• Viscosity: A measure of a liquid's resistance to flow.

  • Magma and lava high in silica are highly viscous, making them thick and resistant to flow.

  • Conversely, low silica content produces runny magma/lava.

• High silica magma* often leads to explosive volcanic eruptions due to its viscosity.

TEXTURE OF IGNEOUS ROCKS

• Texture: Defined as the overall appearance of a rock, based on the size, shape, and arrangement of mineral grains.

• Factors influencing the texture:

  • Rate of cooling of magma (most significant factor).

  • Amount of silica present in magma.

  • Amount of dissolved gases in magma.

TYPES OF TEXTURES

• Aphanitic Texture:

  • Formed at the surface due to rapid cooling.

  • Individual crystals are only noticeable under a microscope.

• Phaneritic Texture:

  • Formed underground; requires thousands to millions of years to cool.

  • Crystals are visible to the naked eye.

• Porphyritic Texture:

  • Characterized by two different groups of crystal sizes.

• Vesicular Texture:

  • Contains air bubbles in the rock, common in extrusive rocks.

• Glassy Texture:

  • Occurs when lava is quenched during eruptions, forming glassy texture.

  • High silica content can also result in volcanic glass (e.g., obsidian).

• Pyroclastic Texture:

  • Composed of rock fragments from volcanic eruptions (e.g., ash, molten blobs, large blocks).

IGNEOUS ROCK CLASSIFICATION CHART

FELSIC ROCKS

• Granite:

  • Very abundant in the continental crust.

  • Phaneritic rock comprised of approximately 25% quartz and around 65% feldspar (primarily potassium and sodium feldspar).

  • Generally white to pink in color due to feldspar components.

  • May also contain dark silicates such as:

  • Biotite

  • Amphibole

  • Feldspar significantly determines the color of granite.

GRANITE

• Often a by-product of mountain-building processes.

• Highly resistant to weathering, typically forming the cores of eroded mountains.

RHYOLITE

• The extrusive equivalent of granite.

• Composed entirely of light-colored silicates.

• Exhibits an aphanitic texture with potential glass fragments and voids due to fast cooling.

• Rhyolite deposits are less widespread compared to granite deposits.

MAFIC ROCKS

• Characterized by a significant amount of dark silicate minerals and calcium-rich plagioclase feldspar.

• Generally lack quartz and feature a high percentage of ferromagnesian minerals, which results in a darker color.

• Denser than felsic rocks.

• Mafic rocks constitute the majority of the ocean floor and many volcanic islands.

BASALT

• Appears dark green to black in color.

• Fine-grained texture (crystals are not visible to the naked eye).

• Composed mainly of pyroxene and calcium plagioclase feldspar, occasionally containing olivine and amphibole.

• Recognized as the most common extrusive rock, prominently found in places like Hawaii, Iceland, and the oceanic crust.

GABBRO

• The intrusive equivalent of basalt.

• Shares a dark green to black coloration.

• Predominantly composed of pyroxene and calcium plagioclase feldspar.

INTERMEDIATE ROCKS

• Present a composition that is between felsic and mafic rocks.

• Contain at least 25% dark silicate minerals including:

  • Amphibole

  • Pyroxene

  • Biotite mica

• Often associated with volcanic activity, particularly along the margins of continents (e.g., Andes Mountains).

ANDESITE

• Medium gray in color.

• Exhibits a fine-grained texture (individual crystals not visible to the naked eye).

• Named after the Andes Mountains, indicating its volcanic origin.

DIORITE

• Served as the plutonic equivalent to andesite.

• Exhibits a coarse-grained texture (intrusive).

• Comparable to granite but contains less quartz and more dark silicate minerals, including sodium-rich plagioclase feldspar and amphibole.

• Sometimes characterized by a salt-and-pepper appearance due to the contrasting mineral colors.

IGNEOUS CATEGORIES SUMMARY

• Felsic:

  • Phaneritic: Granite

  • Aphanitic: Rhyolite

• Intermediate:

  • Phaneritic: Diorite

  • Aphanitic: Andesite

• Mafic:

  • Phaneritic: Gabbro

  • Aphanitic: Basalt

OTHER IGNEOUS ROCKS

• Obsidian: Volcanic glass, pure silica, classified as felsic even though usually dark.

• Pumice: Glassy volcanic rock with a vesicular texture; forms from gas escaping through silica-rich lava, resulting in a gray, frothy rock.

BOWEN’S REACTION SERIES

• N.L. Bowen established that as mafic magma cools, minerals crystallize systematically based on temperature gradients.

• Cooling Sequence:

  • Mafic minerals crystallize at higher temperatures before felsic minerals.

  • The sequence is represented by temperature ranges:

  • $~1200^{ ext{°C}}$: Olivine

  • $~1100^{ ext{°C}}$: Pyroxene

  • $~1000^{ ext{°C}}$: Amphibole

  • $~900^{ ext{°C}}$: Biotite mica

  • $~800^{ ext{°C}}$: Calcium-rich plagioclase feldspar

  • $~700^{ ext{°C}}$: Sodium-rich plagioclase feldspar

  • $~600^{ ext{°C}}$: Potassium feldspar

  • $~500^{ ext{°C}}$: Muscovite mica

  • $~400^{ ext{°C}}$: Quartz

INTRUSIVE IGNEOUS ACTIVITY

• Occurs when magma ascends through the crust and displaces existing rock (referred to as country rock).

• Defines various bodies of rock including:

  • Pluton: The resultant structure from magma emplacement into pre-existing rock.

  • Dikes and Sills: Tabular bodies created by magma intrusions.

  • Dike: A magmatic injection that cuts across the layering of the country rock.

  • Sill: A magmatic injection that runs parallel along the layering of the country rock.

  • Batholith: A large pluton characterized by a surface area exceeding 100 square kilometers - typically comprising larger bodies.

VISUALIZING INTRUSIVE STRUCTURES

• Relationships between volcanism and intrusive igneous activity illustrated through:

  • Cinder Cones

  • Laccolith

  • Conduit

  • Sills

  • Composite Cones

• Basic intrusive structures, some exposed by erosion, include:

  • Plutons, such as Laccoliths and Sills.

  • Volcanic Necks and Fissures are vital structural concepts reflective of intrusive dynamics.

• Example of an extensive uplift and erosion process is seen in structures such as the Sierra Nevada Batholith.