WEEK 3 - Igneous Flashcards
What are Rocks?
ROCKS are solid collections of minerals and mineral-like materials
Three Basic Classes of Rocks
Rocks are classified based on formation process
Three main types:
- Igneous
- Sedimentary
- Metamorphic
Igneous Rocks
Formed from magma (hot, molten material)
The term “igneous” comes from Latin ignis = fire
Magma contains:
- Melt (liquid rock)
- Volatiles (gases like water vapor & CO₂)
- When magma reaches the surface, it is called lava
Where Does Magma Come From? - Igneous
Most magma is formed in the mantle
Heat & pressure increase with depth in Earth’s lithosphere
Temperature melts rocks, but pressure keeps them solid
Balance between heat & pressure determines if rock melts
Melting Rocks: Decompression Melting - Igneous
Occurs where hot rock rises & pressure decreases
Less pressure allows rock to fully melt into magma
Common in plate boundaries where lithosphere is pulling apart
Melting Rocks: Hydration-Related Melting - Igneous
Subduction: One plate sinks under another.
Oceanic crust contains water, which turns into vapor as it sinks
Vapor hydrates mantle rocks, lowering melting point & forming magma
Wet rocks melt at lower temperatures than dry rocks
What Happens to the Magma? - Igneous
Magma is lighter than surrounding rock & rises
Can collect in magma chambers or escape as a volcanic eruption
Some magma remains trapped, forming underground rock bodies
Crystallization Below the Surface - Igneous
As magma cools, minerals crystallize from chemical components in the melt
If trapped in a magma chamber, it cools slowly, forming large crystals
Plutonic (Intrusive) Igneous Rocks
Form when magma cools underground
Named after Pluto, Roman god of the underworld
Intrusive rocks form when magma enters pre-existing crust
Can take various shapes & sizes depending on how they intrude
Igneous Intrusions
Magma from a chamber intrudes into pre-existing layered rocks through fractures
This happens at contacts between sedimentary rock layers
The magma then cools and crystallizes into solid rock
Types of Igneous Intrusions
Batholith
- A very large mass of igneous rock formed by cooling of magma in a major chamber
Dyke
- A vertical or steeply angled igneous body cutting through layers of pre-existing rock
Sill
- A horizontal igneous body intruded parallel to rock layers
Laccolith
- A dome-shaped igneous body that pushes up overlying rock layers
Volcanic Neck
- The solidified remnant of magma that once occupied a volcano’s vent
Igneous Intrusions Exposed at the Surface
Igneous intrusions like batholiths, dykes, sills, laccoliths, and volcanic necks can be exposed at the surface over time due to erosion
Half Dome & Ansel Adams
Half Dome: A granite dome in Yosemite National Park, part of the Sierra Nevada Batholith
Ansel Adams (Photographer): Famous for capturing Half Dome in his iconic 1960 photo, “Moon and Half Dome.”
His photography showcased tonal contrast and natural textures
Plutonic/Intrusive Igneous Rock Texture
Intrusive igneous rocks cool slowly beneath Earth’s surface
Slow cooling allows large crystals to form
This texture is called phaneritic (large, visible crystals)
Extrusive Igneous Rocks
Form when magma cools above the surface
Also called volcanic igneous rocks (named after Vulcan, Roman god of fire)
Formed from lava that erupted from the Earth
Various types form depending on magma composition and surface conditions
Lava
Magma that reaches the surface becomes lava
Lava erupts from volcanic vents due to escaping volatiles (gases) as pressure decreases
How does Lava Differ from Magma?
Lava differs from magma because it has lost much of its gases
Lava Eruptions (type of Eruption + Loudness)
The type of eruption depends on temperature, composition, and gas content
Lava can erupt quietly (flows smoothly) or violently (explosive eruptions)
Volcanic/Extrusive Igneous Textures: Aphanitic (Fine Grained)
Forms when lava cools quickly, preventing large crystals from growing
Mineral crystals are too small to be seen with the naked eye
Example: Andesite
Volcanic/Extrusive Igneous Textures: Porphyritic (Mixed Crystal Sizes)
Forms in two stages: slow cooling underground (large crystals), then fast cooling at the surface (small crystals)
Large visible crystals in a fine-grained matrix
Example: Porphyritic Igneous Rock
Volcanic/Extrusive Igneous Textures: Glassy
Forms when lava cools too quickly for crystals to form
Atoms freeze in place, creating a glassy appearance
Example: Obsidian
Volcanic/Extrusive Igneous Textures: Pyroclastic
Created from violent volcanic eruptions material that cools and solidifies rapidly
Rock fragments, ash, and pulverized debris form a fragmented texture
Pyroclastic = “fire-broken”
Igneous Rock Composition
Most igneous rocks are made of silicate minerals
Silica unit: 1 silicon atom bonded to 4 oxygen atoms (SiO₄⁴⁻)
Silica can form chains, sheets, or frameworks
Ferromagnesian Minerals
Dark-colored silicate minerals containing iron (Fe) and magnesium (Mg)
Can be found WITHIN many igneous rocks
Examples:
- Olivine
- Pyroxene
- Amphibole
- Biotite Mica
Silicate Structure & Bonding
Strong covalent bonds within silica units
Ionic bonds (weaker) hold units together
Bonding differences affect cleavage and fracture in minerals
Why Are Ferromagnesian Minerals Unique?
Silica forms tetrahedral units linked into chains, sheets, or frameworks
Iron (Fe) & magnesium (Mg) bond to silica via ionic bonds
Cleavage & fracture depend on bonding:
- Strong covalent bonds within silica units
- Weak ionic bonds between units create planes of weakness
Quartz
Most common non-ferromagnesian silicate mineral
Composed entirely of silicon (Si) and oxygen (O)
All bonds are equally strong → no cleavage, instead shows conchoidal fracture
Pure quartz = clear & colorless
Feldspar: The Most Common Mineral
Found in a wide range of temperatures & pressures
Structure is a 3D silica framework with Na, Ca, or K
Bonds are less uniform than in quartz
Cleavage: 2 directions at 90°, marking weak bonding
Two Main Types of Feldspar
Plagioclase Feldspar – Na & Ca, white to bluish-gray
Potassium Feldspar – K-rich, creamy white to salmon pink
Why Don’t Igneous Rocks Have Just One Mineral?
Different minerals crystallize at different temperatures
Ferromagnesian minerals crystallize in a discontinuous series
Plagioclase feldspar crystallizes in a continuous sequence
Bowen’s Reaction Series
Explains the sequence of mineral crystallization as magma cools
High-temp: Olivine, Pyroxene form first
Low-temp: Quartz, Muscovite form last
Discontinuous Series: Ferromagnesian minerals change (Olivine → Pyroxene → Amphibole → Biotite)
Continuous Series: Plagioclase shifts from calcium-rich to sodium-rich
Cooling stages create different rock types
Three Types of Igneous Rock Classification (Based on Colour)
- Mafic Rocks – Dark-colored, rich in iron & magnesium. (e.g., Basalt, Gabbro)
- Felsic Rocks – Light-colored, high in feldspar & silica. (e.g., Granite, Rhyolite)
- Intermediate Rocks – Between mafic & felsic. (e.g., Diorite, Andesite)
Importance of Rock Names
“Felsic,” “intermediate,” and “mafic” describe composition, not texture
Rock names simplify communication for geologists
Naming Igneous Rocks (Texture & Composition)
💎 Rocks are classified by composition (color) and texture (crystal size)
🔹 Phaneritic (Large Crystals): Gabbro (Mafic), Diorite (Intermediate), Granite (Felsic)
🔹 Aphanitic (Small Crystals): Basalt (Mafic), Andesite (Intermediate), Rhyolite (Felsic)
Magma Melting Composition and What They Produce:
Magma Composition Varies!
Decompression melting (spreading plates) → produces mafic magma (rich in iron & magnesium): DARK AND DENSE MAGMA
- Extrusive Rock: Basalt
- Intrusive Rock: Gabbro
Hydration melting (subduction zones) → forms felsic to intermediate magma (higher silica): LIGHT AND STICKY MAGMA
- Extrusive Rocks: Rhyolite, Andesite
- Intrusive Rocks: Granite, Diorite
How Do We Use Igneous Rocks?
✔ Durable & Uniform: Used for tombstones, monuments, and commercial buildings
✔ Attractive “salt & pepper” texture makes them ideal for architectural uses
Why Are Igneous Rocks So Durable?
✅ Strong Composition – Made of tough silicate minerals that resist chemical damage
✅ Hardness – Score 6.0+ on Mohs Scale, making them scratch-resistant
✅ Interlocking Crystals – Creates high strength and durability
✅ No Weak Layers – Unlike sedimentary/metamorphic rocks, they don’t split easily
Examples of Durable Igneous Rocks
🪨 Granite vs. Limestone Headstones – Granite resists weathering, limestone erodes
🛁 Granite Countertops – Scratch, heat, and stain-resistant
🔍 Granite vs. Sandstone – Granite is dense and strong (igneous); sandstone is softer and breaks easily (sedimentary)
