Igneous Rocks Lecture

Geological Classification of Rocks

Introduction

• The discussion centers around mineral composition with a focus on color and types of igneous rocks.

• Key concepts include distinguishing between different rock types based on the presence of dark-colored minerals.

Mineral Types

• Dark Colored Minerals: Generally known as ferromagnesian minerals, which include biotite and amphibole.

• Olivine: Ranges from green to dark black, depending on iron vs. magnesium composition.

• Color Index of Dark Minerals:

  • Importance of color in classifying rocks.

  • The Leaflet Color Index (MCI) used to quantify the percentage of dark minerals.

Rock Classifications

Felsic Rocks

• Definition: Light-colored rocks with high amounts of quartz and feldspar.

• MCI Range: 0-15% dark minerals.

• Characteristics:

  • Often contain >50% quartz and potassium feldspar.

  • Appear white, light gray, or pink; pink indicates the presence of potassium feldspar.

• Examples: Rhyolite (aphanitic), granite (phaneritic).

Intermediate Rocks

• Definition: Rocks with a mix of light and dark minerals.

• MCI Range: 16-50% dark minerals; often contains quartz and plagioclase.

• Characteristics:

  • Black and white minerals are visible in coarse grains.

  • Aphanitic variants appear dark gray, e.g., andesite.

• Examples: Diorite (phaneritic), andesite (aphanitic).

Mafic Rocks

• Definition: Dark-colored rocks rich in iron and magnesium.

• MCI Range: 50-85% dark minerals.

• Characteristics:

  • Typically lack quartz; contain dark-colored minerals such as pyroxene and amphiboles.

  • Dark green to black in appearance.

• Examples: Basalt (aphanitic), gabbro (phaneritic).

Ultramafic Rocks

• Definition: Extremely high in magnesium and iron content.

• MCI Range: 85% or higher dark minerals.

• Characteristics:

  • Predominantly contain olivine, often green in color.

  • Primarily found in the mantle.

• Examples: Peridotite, commonly referred to as mantle rocks.

Color and Classification in Rocks

• Color plays a crucial role in identifying rock types, aiding in understanding rock composition and origins.

Crystallization of Minerals

• Process Discussion: Minerals crystallize from magma at different temperatures, forming a hierarchy known as Bowen's Reaction Series.

• Thermodynamics: Temperature effects on crystallization and how minerals change from solid to liquid states with heat.

• Bohm’s Experiments:

  • Conducted to ascertain crystallization temperatures, leading to findings on mineral formation order in solidifying magma.

  • Result: Each mineral has a specific temperature at which it crystallizes, creating a diverse mineralogical profile in the resulting rock.

Weathering Processes

Physical Weathering

• Definition: Breaking down of rocks into smaller pieces without changing their chemical composition.

• Types of Physical Weathering:

  • Pressure Release Fracturing: Occurs when overburden pressure is removed, leading to rock expansion and fractures.

  • Wedge Weathering: Expansion of ice from freezing water widening fractures.

• Grand Examples: Talus slopes, granite exfoliation as observed in natural features like Stone Mountain.

Chemical Weathering

• Definition: Breakdown of rocks through chemical reactions, typically involving water, leading to alteration of minerals.

• Types of Chemical Weathering:

  • Dissolution: Rocks like limestone dissolve in acidic water, leading to cave formations.

  • Oxidation: Reaction of oxygen with minerals, particularly iron, resulting in rust.

  • Hydrolysis: The process where water breaks down minerals into clay and ions, notably with feldspar turning into clay minerals.

Conclusion

• Understanding the classification of rocks based on mineral content and color leads to greater comprehension of geological processes, both in formation and weathering.

• Each group of igneous rocks offers insight into the earth's geological composition and the conditions under which they are formed.