Lecture 8
Igneous Rocks: Lecture Notes
Game Plan
Focus on concepts related to igneous rocks:
Differences among igneous rocks
Composition and texture
Formation of magmas
Magma formation at plate boundaries
Additional reminders:
Module 3 Activity due on Wed 9/24
Module 3 Quiz due on Thurs 9/25
Lab 3 scheduled for Tues (9/23)
Lab 4 scheduled for Fri (9/26) & Tues (9/30)
Exam 1 is set for after Module 4 on Oct. 10
Learning Outcomes
At the conclusion of this lecture:
Students should be able to:
Classify igneous rocks by composition and texture, distinguishing between:
Felsic
Intermediate
Mafic
Identify rock names based on their texture (fine-grained vs. coarse-grained)
Explain the difference between intrusive and extrusive igneous rocks
Discuss the three mechanisms by which rocks can melt
Understand how magmas can produce minerals/rocks of varying compositions, including partial melting and Bowen’s reaction series
Relate igneous processes to plate tectonics, addressing:
Factors leading to melting at specific plate boundaries
Expected igneous rock types at certain plate boundaries
Overview of Igneous Rocks
Formation:
Igneous rocks originate from liquid rock (magma).
Intrusive igneous rocks form from magma that cools within the Earth.
Extrusive igneous rocks result from lava that cools at or near the Earth’s surface.
Differences Among Igneous Rocks
Texture and Composition
Texture refers to crystal size:
Coarse-grained rocks (large crystals)
Fine-grained rocks (small crystals)
Mixed texture rocks (a combination of large and small crystals)
Texture correlates with cooling rate:
Intrusive igneous rocks cool slowly, leading to larger crystals.
Extrusive igneous rocks cool rapidly, resulting in smaller crystals.
Varieties of textures include fine-grained, glassy, and vesicular textures.
Mineral Size and Location of Cooling
Cooling Locations:
Fine-grained (texture = aphanitic):
Characteristics: Small minerals, visible only under a microscope
Type: Extrusive
Coarse-grained (texture = phaneritic):
Characteristics: Large minerals, visible to the naked eye
Type: Intrusive
Glassy: No minerals formed due to rapid cooling, thus appearing glass-like.
Type: Extrusive
Porphyritic: Exhibits two different mineral sizes, resulting from two stages of cooling (first underground, then at the surface).
Cooling Times for Various Types
Ash: Cooling time = minutes
Lava Flow: Cooling time = days to months
Deep Plutons: Cooling time = centuries to a million years
Shallow Sills: Cooling time = weeks to months
Coarse-Grained Granite: Medium cooling and slow heat escape
Glassy Obsidian: Very fast cooling
Porphyritic Rocks: Slow first, then fast cooling
Classification of Igneous Rocks
Based on Composition and Texture
Silica Content: Determines the classification of igneous rocks into various categories:
High Silica: Felsic
Medium Silica: Intermediate
Low Silica: Mafic
Color Proxy: Color can indicate silica content:
Dark = low silica (Mafic)
Light = high silica (Felsic)
Specific Rock Examples
Felsic: Granitic rocks such as:
Rhyolite (extrusive)
Granite (intrusive)
Intermediate: Diorite (intrusive) and Andesite (extrusive)
Mafic: Basalt (extrusive) and Gabbro (intrusive)
Ultramafic: Peridotite (intrusive)
Identifying Igneous Rocks
Use texture and composition to classify igneous rocks:
Composition indicates which minerals are present (indicated by color).
Texture indicates the size of the minerals present.
Mineral Formation from Magma
As magma cools, minerals form at different temperatures:
Early crystallizing high-temperature minerals will solidify first and lower-temperature minerals will remain molten longer.
Bowen’s Reaction Series: Describes the sequential order of mineral crystallization in cooling magma:
Higher temperature minerals crystallize first while lower temperature minerals crystallize later.
Relevance of Partial Melting: Initially heavy, mafic minerals crystallize, transforming the remaining magma composition, leading to different rock types over time.
Mechanisms Leading to Melting
Increase in Temperature: Necessary to rise above the melting point of the rock, affected by both composition and pressure conditions. However, this is not the primary way most magma forms.
Partial Melting Mechanics: Rock components melt at varying temperatures. As certain minerals melt (melt first), the remaining solid alters the chemical composition of the magma.
Decrease in Pressure: Decompression melting, prevalent at divergent boundaries where solid mantle rock rises and melts due to falling pressure as tectonic plates separate.
The resultant magma is typically mafic in composition, forming basalts and gabbros.
Adding Water: Reduces the melting temperature of rock, facilitating melting even without pressure change and is notably significant in sedimentary rock environments during subduction zones.
Subduction Zone Melting Processes
Fluid-induced Melting: Water released from subducting plates into the hot mantle lowers the melting point, generating silicic (felsic) to mafic magmas.
Composition evolves depending on the material present and how it interacts in the crust.
Summary of Melting Processes
There are three primary ways for rocks to melt:
Increase Temperature: Partial melting near magma chambers.
Decrease Pressure: Critical in rifting zones and beneath hotspots.
Add Water: Facilitates fluid-induced melting primarily at convergent boundaries.
Concept Questions and Examples
Debate on Mineral Size: Discuss how mineral size correlates with cooling times.
Bowen’s Reaction Series: Identify first minerals to melt in a rock and those that remain solid at varying temperatures.
Practical Application: Understanding the melting behavior through the chocolate chip ice cream analogy to clarify concepts in partial melting.
By comprehending these principles, geology students will gain insights into the diverse formation processes and properties of igneous rocks, enhancing their knowledge of geological systems and their settings.