Korean, Edit

Chapter 11. Petrology

Recommended article: 【Earth Science】 Earth Science Table of Contents

1. Properties of Rocks

2. Igneous Rocks

3. Sedimentary Rocks

4. Metamorphic Rocks

5. Subterranean Mining and Subsurface Resources

1. Properties of Rocks

⑴ Surface Area

① Method 1. Divide into several parts and assume each as a cube, cone, or frustum of a cone for calculation.

② Method 2. If the structure has a shell, separate the shell and measure the area by spreading them on a plane.

③ Method 3. Apply metal powder to the surface, then measure the surface area based on weight changes.

④ Method 4. Image processing system.

⑵ Volume

① Method 1. Archimedes’ principle, water displacement method: Archimedes’ principle

○ V: Volume of the object (m³)

○ W_a: Weight of the object in air (N)

○ W_w: Weight of the object in water (N)

○ γ_w: Specific weight of water (9810 N/m³)

② Method 2. Air comparison pycnometer: Utilizing Boyle’s law

Figure 1. Structure of a constant volume gas pycnometer

⑶ Sphericity: A scale indicating how much a rock has been eroded

① Definition 1. A scale indicating how closely the projected area matches a circle

○ R: Sphericity

○ A_p: Projected area when naturally laid on a plane (m²)

○ A_c: Area of the smallest circumscribed circle (m²)

○ L: Maximum length of the object (m)

② Definition 2. Average of the curvature radius of particle edges divided by the maximum radius of the inscribed sphere for the particles

③ Volcanic rocks and metamorphic rocks have similar sizes, but metamorphic rocks are superior in terms of sphericity.

⑷ Sphericity: A scale indicating how round a rock is

① Definition 1. Ratio of the diameter of a sphere with the same volume as the object to the diameter of the smallest circumscribed sphere

○ S: Sphericity

○ L: Length of the object (m), i.e., in the longitudinal direction

○ W: Width (m), i.e., in the direction perpendicular to the longitudinal direction

○ T: Thickness (m), i.e., height perpendicular to the plane formed by length and width

② Definition 2. Represented by (1/6) Dp Sp instead of the volume of the circumscribed sphere

③ Definition 3. Wadell’s definition: Ratio of the surface area of a sphere with the same volume as the rock to the surface area of the rock

⑸ Classification

① Definition: Degree of uniformity in particle size

② Example: Grain size distribution in sedimentary materials

Figure 2. Grain size distribution in sedimentary materials

○ Beach gravel: Most favorable classification. Composed only of gravel.

○ Alluvial gravel: Poor classification. Composed of gravel and sand.

○ Argillaceous silt: Poor classification. Composed of gravel, sand, silt, and clay.

○ Eolian sand: Favorable classification due to transportation by wind. Composed of sand and silt.

○ Loess: Intermediate classification. Composed of sand, silt, and clay.

○ Glacial till: Poor classification.

○ Volcanic ash: Locally poor classification.

○ Particle size decreases with longer transport distance in rivers, leading to increased classification and weathering resistance.

⑹ Porosity

① Definition

○ P: Porosity

○ D_B: Bulk density or matrix density (kg/m³)

○ D_T: True density or particle density (kg/m³)

② Opening Ratio: Ratio of void area to total area

2. Igneous Rocks

⑴ Definition: Rocks formed from molten materials within the Earth’s interior

⑵ Magma

① As temperature increases, melting occurs more readily, and as pressure decreases or the content of volatile substances (e.g., water, sulfur dioxide) increases, the melting point lowers.

② Once magma rises to the surface, the effect of pressure on lowering the melting point becomes more significant than the effect of temperature.

Figure 3. Process of magma formation

③ Residual Solution: A solution that remains even after magma has erupted and many igneous minerals have crystallized. It is rich in silica and may still contain unused metal ions. Residual solutions play a role in recrystallizing minerals.

④ Eruption: The rapid release of volatile gases and other materials as the pressure acting on the magma decreases.

⑤ Plume Tectonics

○ A “hot plume” refers to a phenomenon where basaltic magma with low viscosity, heated by the influence of the outer core, easily convects and rises from the lower mantle, creating a sort of tunnel (plume).

○ There are also “cold plumes” that descend.

⑶ Classification of Lava

① Basaltic Lava

○ Low SiO₂. Forms shield volcanoes and lava plateaus (∵ low viscosity), causing significant lava flow damage.

○ Mainly found at mid-ocean ridges or hotspots.

○ Generated at depths of approximately 70 to 250 km.

○ Among volcanic lava temperatures, basaltic lava ranges from 1150 to 1250°C.

② Andesitic Lava

○ Forms stratovolcanoes.

③ Rhyolitic Lava

○ High SiO₂. Forms domed volcanoes and lava domes (∵ high viscosity).

○ Higher cation content, making it closer to being alkaline.

○ Porphyritic Texture: Due to differences in the cooling temperatures of early- and late-crystallizing minerals, each mineral crystallizes to varying degrees.

④ Granitic Lava

○ Refers to andesitic lava or is used as the opposite of basaltic lava.

○ Primarily develops at the Benioff zone (subduction boundaries of two tectonic plates).

○ Magma is generated at a depth of approximately 40 km.

○ Lava temperatures range from 950 to 1050°C when ejected from a volcano.

⑤ Pahoehoe Lava Flow and Aa Lava Flow

○ Unlike aa lava flow, pahoehoe lava flow has a smooth and wrinkled surface.

⑷ Types of Igneous Rocks

Figure 4. Types of igneous rocks

① Volcanic Rocks, Hypabyssal Rocks, and Plutonic Rocks

○ Volcanic Rocks (Extrusive Rocks): Igneous rocks formed near the surface. Faster cooling → smaller grain size. Fine-grained or glassy texture. Examples include basalt.

○ Pores: Formed in environments where heat is rapidly lost, such as the surface, and when the material contains a high amount of volatile substances.

○ Volcanic rocks are further divided into lava flows and pyroclastic rocks.

○ Hypabyssal Rocks: Minerals crystallize in the order of phenocrysts and groundmass, with earlier-crystallizing minerals having higher melting points. These rocks exhibit a porphyritic texture.

○ Porphyritic texture: As magma ascends, mafic minerals crystallize more slowly within the magma, resulting in larger crystal sizes. In contrast, felsic minerals form rapidly only after reaching the surface, leading to smaller crystal sizes. This creates a rock with varying crystal sizes within the same specimen.

○ Plutonic Rocks (Intrusive Rocks): Igneous rocks formed deep underground. Slower cooling → larger grain size. Coarse-grained texture. Examples include granite.

○ Volcanic neck: A structure where lava solidifies within a volcanic conduit, sometimes exhibiting both volcanic and hypabyssal rock characteristics.

② Phaneritic and Aphanitic Textures

○ When classifying igneous rocks based on crystal size, rocks with visible crystals are referred to as phaneritic, while those without visible crystals are aphanitic.

○ If the cooling time is extremely short, the material becomes glassy, meaning no crystals are formed at all.

③ As SiO₂, Na₂O, CaO content increases; FeO, MgO content decreases

○ Indicates more crystal formation, hence lower crystallization temperature

○ Rich in oxygen, therefore acidic rocks

○ Increased intermolecular forces lead to higher viscosity

○ Greater oxygen emission leads to more volcanic gas and increased explosive behavior

○ More volcanic gas leads to a smaller proportion of magma

④ Characteristics of Rocks

○ Basalt: Volcanic and mafic

○ Gabbro: Plutonic and mafic

○ Granite: Plutonic and felsic

○ Pumice: Volcanic

○ Rhyolite: Exhibits porphyritic texture

○ Albite (Plagioclase Feldspar): The zoning structure reveals that the composition of the magma chamber continuously changes over time. Toward the core of the rock, the composition shifts from sodium (Na) to calcium (Ca).

⑤ Memory Tip: Ultramafic, Mafic, Intermediate, Felsic

⑸ Major Igneous Minerals

① Quartz

② Feldspar

③ Mica

④ Amphibole

⑤ Pyroxene

⑥ Olivine

⑹ Formation

① Cause 1: Divergent Boundary

○ 1^st. As two tectonic plates move apart, the pressure exerted at the boundary decreases.

○ 2^nd. A large amount of magma is generated, forming a magma chamber.

○ 3^rd. When the magma chamber develops, volcanic eruptions occur, leading to the formation of volcanic rocks.

○ Mid-Atlantic Ridge: A representative example of a divergent boundary. Igneous rocks are formed by basaltic magma.

② Cause 2: Convergent Boundary with Subduction of One Plate Under Another: Typically occurs when a denser oceanic plate subducts beneath a less dense continental plate.

○ 1^st. A large amount of water from the oceanic plate moves to the subduction boundary, lowering the melting point.

○ 2^nd. This process generates a large amount of magma.

○ 3^rd. Magma is generated deep within the Earth’s crust, and igneous rocks can form either near the surface or deep underground, depending on various factors.

③ Formation of Joints

○ Joints: Cracks in rocks formed as a result of weathering processes.

○ Columnar Joints (Example: Jeju Island’s Columnar Joints)

○ 1^st. Lava erupts onto the Earth’s surface.

○ 2^nd. Joint formation: As the erupted magma cools rapidly, cracks form in the rock.

○ Sheet Joints (Example: Insubong Peak, Bukhansan Mountain)

○ 1^st. Plutonic rocks form deep underground.

○ 2^nd. Plutonic rocks are exposed to the surface: Since atmospheric pressure is lower than underground pressure, the exposed plutonic rocks expand.

○ 3^rd. Joint formation: Cracks form in the rocks as they expand.

○ 4^th. Weathering occurs primarily in the cracks of the joints, leading to exfoliation: A characteristic where rocks peel off in thin, layered sheets.

⑺ Igneous Rock Regions

① Jeju Island: Sheet jointing is rapidly solidified basalt

② Dokdo: Volcanic island formed by solidified lava erupted from the seafloor

③ Ulleungdo

④ Bukhansan

⑤ Seoraksan: Composed of granite

3. Sedimentary Rocks

⑴ Definition: Rocks formed from consolidated sedimentary materials

⑵ Step 1. Erosion

① Narrow definition: Formation of sedimentary materials from parent rocks

② Broad definition: Includes chemical erosion (dissolution)

○ Example: Involves karst processes that create karst topography

③ More energetic motion of transporting media leads to more active erosion

④ Chemical erosion is active in tropical environments

⑶ Step 2. Transportation: Determines particle size

① Definition: Process in which sedimentary materials move

② Factor 1. Water

③ Factor 2. Wind

④ Factor 3. Particle weight: Heavy particles settle rather than being transported

⑷ Step 3. Deposition: Formation of sedimentary rocks after accumulation of sedimentary materials

① Diagenesis: Process where sedimentary materials bind together

○ Involves heat, chemical reactions, therefore weak metamorphism

○ Classified as cementation and compaction processes

② 3-1. Cementation

○ Process that adheres sediment particles together

○ Water, especially groundwater, plays a crucial role

○ Calcium carbonate, silica, iron oxides act as binding agents

③ 3-2. Compaction: Also known as compression

○ Lower layers of sediment are compressed by overlying sediment, reducing pore space between sediments

⑸ Types of Sedimentary Rocks

① Type 1. Clastic sedimentary rocks

○ Definition: Fragments of original rocks, most sedimentary materials belong to this category

○ Wentworth Scale

○ Φ = -log₂ Particle diameter (mm)

○ Features

○ Particle edges erode due to weathering and erosion

○ Interparticle spaces contain cementing material

○ Bedding

○ Examples

○ Shale: Diameter ＜ 0.001 mm, Φ ＞ 10 ( ∵ Clay)

○ Siltstone: Diameter = 0.001 ~ 0.1 mm, Φ = 3 ~ 10 ( ∵ Silt)

○ Sandstone: Diameter = 0.1 ~ 1 mm, Φ = 0 ~ 3 ( ∵ Sand)

○ Conglomerate: Diameter = 1 mm, Φ ＜ 0 ( ∵ Granule)

○ Tuff: Rock formed by volcanic debris deposition

② Type 2. Clastic Sedimentary Rock

○ Definition: Excludes sediments from weathering and erosion, except for terrigenous sediments. Includes organic clastic rocks, etc.

○ 2-1. Chemical Clastic Rocks

○ Limestone (CaCO₃): Chemically and organically formed clastic rock. Precipitated from solution.

○ Halite (NaCl): Formed by evaporation.

○ Gypsum (CaSiO₄·2H₂O): Formed by evaporation.

○ 2-2. Organic Clastic Rocks: Fossils can be found.

○ Coal: Plant remains deposited.

○ Limestone: Calcareous organisms (corals, mollusks, foraminifera) deposited.

○ Chert (siliceous rock): Derived from harmful secretions of deposited organisms.

③ Type 3. Mixed Clastic Rock

○ Definition: Possesses characteristics of clastic and non-clastic sedimentary rocks.

○ Example 1. Limestone shale

○ Example 2. Carbonaceous shale

○ Example 3. Tuff shale, breccia

⑹ Classification of Sedimentary Strata

① Continental Strata

○ Aeolian Strata: Materials transported by wind and deposited on continents (e.g., Loess layer, Loam layer)

○ Glacial Strata: Deposits of glacial transport (e.g., lateral moraines, terminal moraines, ground moraines)

○ Lacustrine Strata: Sediments within lakes (e.g., clay, sand, gravel, silt)

○ Fluvial Strata: Deposits mainly in river catchments (e.g., floodplains, alluvial fans, deltas, terraces)

② Marine Strata

○ Coastal Strata: Deposits formed by tides and currents

○ Littoral Strata: Sediments deposited up to 200 m from the coast, including continental shelf deposits

○ Sublittoral Strata: Sediments in depths of 200 to 1,500 m on the continental slope

○ Abyssal Strata: Deep-sea deposits in depths exceeding 1,500 m

⑹ Sedimentary Rock Regions

① Geokpo-ri Coast

② Mt. Mai: Terrain primarily composed of metamorphic rock

4. Metamorphic Rock

⑴ Definition: Rock formed through the alteration of pre-existing rocks

⑵ Type 1. Contact Metamorphic Rock

① Definition: Rocks altered due to contact with high-temperature magma

② Characteristics

○ Recrystallization: Process where minerals dissolve and reform as larger particles

○ Dense and hard texture

○ Porphyroblastic texture: Found in igneous and metamorphic rocks. Large grains interlock closely.

○ Hornfelsic texture: Found in hornfels. Fine-grained and hard texture.

③ Types: Remember using the mnemonic “sa-gyu-seok-dae-sae-hon” (quartzite, marble, hornfels, metapelite)

○ Quartzite: Formed from sandstone. Porphyroblastic texture.

○ Marble: Formed from limestone. Porphyroblastic texture.

○ Hornfels: Formed from shale or limestone.

○ Metapelite: Formed from clay-rich rocks.

⑶ Type 2. Regional Metamorphic Rock

① Definition: Rocks formed over a large area due to heat and pressure

② Characteristics: Foliation

○ Foliation is categorized into slate, phyllite, and more based on intensity

○ Slate: Thin, parallel layers of colorless and colored minerals

○ Phyllite: Thick layers of colorless and colored minerals

③ Types

○ Amphibolite: Formed from basalt

○ Metamorphic Grade: Shale → Slate (schist) → Phyllite → Schist → Gneiss

○ Schist: Exhibits foliation

○ Gneiss: Exhibits gneissic banding

○ Gneiss has larger particle sizes compared to schist

⑷ Type 3. Dynamic Metamorphic Rock

① Definition: Rocks formed due to increased pressure accompanying tectonic movements

⑸ Type 4. Shock Metamorphic Rock

① Definition: Rocks formed in localized, high-pressure environments caused by meteorite impact

⑹ Metamorphic Rock Regions

① Baekryeong Island: Composed of metamorphic rock, with developed cliffs and caves due to erosion

② Jirisan (Mount Jiri)

5. Subterranean Mining and Underground Resources

⑴ Subterranean Mining

① Formation of Ore Deposits

○ Vein Differentiation Mining: Hydrothermal, geothermal, contact

○ Precipitation Mining: Supergene, residual, sedimentary veins due to groundwater, organic precipitation

○ Dynamic Metamorphism Mining

② Residual Mining

○ Supergene Mining: Deposition from transported weathered materials

○ Examples: gold, platinum, sphalerite, cassiterite

○ Residual Weathering Deposit: The product of chemical weathering of pre-existing rocks.

○ Examples: Magnetite, Hematite, Ilmenite, Kaolinite, Bauxite, Monazite.

○ Hematite: Formed by the oxidation of iron, resulting in the creation of hematite.

○ Kaolinite: A non-metallic mineral. Formed by the hydrolysis of orthoclase feldspar, serving as a raw material for ceramics.

○ Bauxite:

○ Formed through the hydrolysis of kaolinite.

○ A metallic mineral and a primary source of aluminum.

○ Used as aluminum ore after undergoing a refining process.

○ Sedimentary Mining: Precipitates from evaporating seawater

○ Examples: halite, limestone, chert, manganese nodules, oolitic iron ores

○ Limestone: Raw material for cement

○ Manganese Nodule

○ Mainly composed of manganese

○ Contains nickel, copper, iron, and other metals

○ Occurs mainly in deep-sea sediments

○ Requires refining due to its metallic content

③ Mineral Resources of the Seabed (examples: platinum, iron, tungsten, copper, lead, zinc, gold, silver)

○ Magmatic Deposit: Formation of dense minerals during magma cooling

○ Pegmatite Deposit: Formation from volatile-rich magma invading surrounding rocks during late magma cooling

○ Hydrothermal Deposit: Formation due to interaction of vapor and volatile constituents between magma and rocks

○ Hydrothermal Vein Deposit: Remaining hot water solution forms minerals after magma cooling

④ Metamorphic Mining

○ Regional Metamorphic Mining

○ Formation due to wide-scale regional metamorphism and heated water

○ Examples: graphite, serpentine, asbestos (non-metallic)

○ Remember “black serpentine” for mnemonic

○ Contact Metasomatic Mining

○ Formation at the contact between limestone and intrusive rocks

○ Example: iron, copper, tungsten, lead, zinc

○ Sometimes categorized as hydrothermal mining

⑵ Marine Resources

① Biological Resources

○ Mostly edible

○ Increased reproduction → marine ranching

② Mineral Resources

○ Salt, manganese nodules, etc.

③ Energy Resources

○ Fossil Fuels

○ Coal, petroleum, natural gas, etc.

○ Abundant in continental shelves

○ Contribute to global warming

○ Gas Hydrates

○ Methane + ice

○ Found in deep-sea and permafrost. Present near Dokdo.

○ High-pressure, low-temperature environment

○ Non-renewable resource due to long formation time

○ Emission of greenhouse gases when used

○ Tidal Power Generation

○ Current Power Generation

○ Wave Power Generation

○ Ocean Thermal Energy Conversion

Input: 2016-06-22 20:53

Updated: 2021-02-20 12:23