INSTRUCTIONS:
- This paper consists of seven (7) questions
- Answer all questions
- Write your answers clearly in the answer booklet provided
1. Classify igneous rocks
Igneous rocks can be classified based on several criteria:
By formation process:
- Intrusive (plutonic) rocks: Formed from magma that cools slowly beneath Earth's surface (e.g., granite, gabbro)
- Extrusive (volcanic) rocks: Formed from lava that cools quickly at the surface (e.g., basalt, rhyolite)
By silica content:
- Felsic rocks: High silica content (>65%), light-colored (e.g., granite)
- Intermediate rocks: 55-65% silica (e.g., diorite)
- Mafic rocks: 45-55% silica, dark-colored (e.g., basalt)
- Ultramafic rocks: <45% silica (e.g., peridotite)
By texture:
- Phaneritic: Coarse-grained (slow cooling)
- Aphanitic: Fine-grained (rapid cooling)
- Porphyritic: Mixed grain sizes
- Glassy: No crystals (very rapid cooling)
- Pyroclastic: Fragmental (volcanic ejecta)
1. Classify igneous rocks
Igneous rocks, formed through the cooling and solidification of magma or lava, can be systematically classified based on three primary characteristics: their formation environment, chemical composition, and textural properties.
By Formation Environment: The cooling rate and location of formation significantly influence igneous rock characteristics. Plutonic (intrusive) rocks like granite and gabbro form when magma cools slowly beneath Earth's surface, allowing large mineral crystals to develop. In contrast, volcanic (extrusive) rocks such as basalt and rhyolite form from rapidly cooling lava at the surface, resulting in fine-grained or glassy textures. Hypabyssal rocks like porphyry represent an intermediate category, forming in shallow intrusions with mixed cooling rates.
By Chemical Composition: The silica (SiO₂) content provides a fundamental classification metric. Felsic rocks (granite, rhyolite) contain over 65% silica and are rich in light-colored minerals like quartz and feldspar. Intermediate rocks (diorite, andesite) contain 55-65% silica. Mafic rocks (gabbro, basalt) with 45-55% silica are dominated by dark ferromagnesian minerals. Ultramafic rocks (peridotite) with less than 45% silica are composed almost entirely of olivine and pyroxene.
By Texture: Phaneritic textures (visible crystals) indicate slow cooling, while aphanitic textures (microscopic crystals) indicate rapid cooling. Porphyritic textures show two distinct crystal sizes, recording changing cooling conditions. Other special textures include glassy (obsidian), vesicular (pumice), and fragmental (tuff) formations.
2. Classify sedimentary rocks and give out their significances
Sedimentary rocks, formed through the accumulation and lithification of sediments, can be classified into three major groups based on their origin and composition, each with distinct geological significance.
Clastic Sedimentary Rocks: These form from mechanical weathering debris. Conglomerates and breccias consist of coarse, rounded or angular fragments respectively, indicating high-energy environments. Sandstones (quartz, arkose, graywacke) reflect different source rocks and transport histories. Shales and mudstones represent quiet water deposition. These rocks serve as important reservoirs for groundwater and hydrocarbons, while their structures (cross-bedding, ripple marks) provide paleoenvironmental clues.
Chemical Sedimentary Rocks: Formed by precipitation from solution. Limestones (including chalk and travertine) dominate marine carbonate platforms and store vast amounts of carbon. Evaporites (rock salt, gypsum) indicate arid environments and are economically important mineral resources. Chert forms from silica precipitation and preserves exceptional microfossils. These rocks are crucial for understanding Earth's chemical history and climate evolution.
Biogenic Sedimentary Rocks: Organic processes dominate their formation. Coal series (peat to anthracite) record ancient swamp environments and provide fossil fuels. Fossiliferous limestones preserve biological records. Diatomite consists of microscopic algae skeletons. These rocks contain the fossil record essential for understanding biological evolution and past ecosystems.
Sedimentary rocks are economically vital (containing coal, oil, groundwater), scientifically invaluable (preserving fossils and environmental indicators), and engineeringly significant (foundation materials). Their layered nature provides the stratigraphic record that forms the basis of geological time scales.
3. Classify rocks according to their modes of formation/origin
The three fundamental rock types in geological classification are distinguished by their distinct formation processes, representing different stages of the rock cycle and Earth's dynamic systems.
Igneous Rocks: These primordial rocks form through the cooling and crystallization of molten material. Plutonic igneous rocks like granite crystallize slowly beneath Earth's surface, developing coarse textures. Volcanic rocks like basalt form from rapid cooling of lava at the surface, producing fine-grained or glassy textures. Unique varieties include pegmatites with exceptionally large crystals and tuffs formed from volcanic ash. These rocks provide direct information about Earth's interior composition and thermal state.
Sedimentary Rocks: Formed through the accumulation and lithification of weathered materials. Detrital types (sandstone, shale) consist of transported mineral fragments. Chemical precipitates (limestone, rock salt) form from solution. Organic accumulations (coal, diatomite) derive from biological activity. Characterized by layering (stratification) and often containing fossils, these rocks preserve Earth's surface history and past environments.
Metamorphic Rocks: Result from the transformation of existing rocks under increased temperature and pressure. Foliated types (schist, gneiss) show mineral alignment from directed pressure. Non-foliated rocks (marble, quartzite) form under uniform pressure. Metasomatic rocks result from chemical alteration. These rocks provide evidence of Earth's tectonic processes and deep crustal conditions, with index minerals indicating specific pressure-temperature regimes.
This genetic classification system reflects fundamental Earth processes: igneous (magmatic), sedimentary (surface), and metamorphic (transformational), demonstrating how rocks record Earth's dynamic history.
4. Explain the rock cycle processes and its evidence
The rock cycle describes the dynamic transitions between igneous, sedimentary, and metamorphic rocks through geological processes, demonstrating Earth's continuous material recycling over time.
Key Processes: The cycle begins with magma generation through partial melting in the mantle or crust. Upon cooling, this forms igneous rocks. Surface exposure leads to weathering - mechanical disintegration and chemical decomposition. Erosion transports the resulting sediments, which undergo deposition, compaction, and cementation to form sedimentary rocks. When buried deeply or subjected to tectonic forces, rocks experience metamorphism through heat, pressure, and chemically active fluids. At sufficient depths, melting occurs, completing the cycle. Human observations confirm all stages: volcanic eruptions create new igneous rocks, rivers deposit sediments, and mountain belts expose metamorphic cores.
Supporting Evidence: Intermediary rock types demonstrate transitions - sedimentary rocks with volcanic ash layers show igneous inputs to sedimentary systems. Metamorphosed sediments (e.g., marble from limestone) prove sedimentary-metamorphic links. Partial melting textures in migmatites illustrate metamorphic-igneous transitions. Radiometric dating shows recycled components - zircon grains in sedimentary rocks older than their matrix reveal prior igneous histories. Geochemical signatures demonstrate material continuity - similar elemental ratios appear across rock types. Global distribution patterns match theoretical expectations - sedimentary basins flanking mountain belts with metamorphic cores.
The rock cycle's universality explains Earth's surface diversity while demonstrating matter conservation over geological time. Its operation regulates global geochemical cycles, influences climate through weathering feedbacks, and controls the distribution of geological resources essential for civilization.
5. "The mother igneous rock". How does this statement help to show the relationship between igneous rock and other rocks
The concept of igneous rock as the "mother" of all rocks reflects its fundamental position in the rock cycle and Earth's geological evolution, establishing genetic relationships between all rock types.
Primary Origin: Igneous rocks form directly from magma, Earth's primordial material. Early Earth's surface consisted entirely of igneous rocks, with other types developing later as surface processes diversified. All subsequent rocks derive from this igneous precursor through various transformation processes. Even today, new igneous material continues to form at divergent boundaries and hotspots, replenishing Earth's surface.
Sedimentary Rock Genesis: Most sedimentary particles originate from weathered igneous rocks. Granite weathering produces quartz grains for sandstones, feldspar clays for shales, and dissolved ions for chemical sediments. Volcanic ash contributes to many sedimentary deposits. The mineralogical and chemical diversity of sediments directly reflects their igneous sources, with sedimentary basins essentially being collections of redistributed igneous material.
Metamorphic Rock Formation: Both sedimentary and existing metamorphic rocks can undergo metamorphism, but the ultimate protolith for most metamorphic rocks was originally igneous. Regional metamorphic belts often reveal igneous ancestries through relict textures or geochemical signatures. Even when metamorphism affects sediments, those sediments typically derived from igneous sources, making igneous rocks the "grandmother" in this lineage.
This metaphor highlights Earth's material continuity - while rocks undergo dramatic transformations, their substance ultimately traces back to igneous origins. The concept underpins petrogenetic studies and reminds us that Earth remains geologically active, with igneous processes continually renewing the planetary surface.
6. Explain ten (10) properties of minerals
Minerals possess distinctive physical and chemical properties that enable their identification and determine their geological and technological applications.
Crystal Form: The geometric shape reflecting internal atomic arrangement. Well-formed crystals like quartz's hexagonal prisms indicate slow growth in open spaces. Crystal symmetry classifies minerals into six systems (cubic, tetragonal, etc.).
Hardness: Resistance to scratching measured on Mohs scale (1=talc to 10=diamond). Controlled by bond strength, it determines mineral durability and industrial uses. Quartz (7) abrades softer minerals.
Cleavage: Tendency to break along planes of weak atomic bonding. Mica's perfect basal cleavage yields thin sheets, while quartz lacks cleavage, fracturing conchoidally.
Luster: Quality of reflected light. Metallic (pyrite), vitreous (quartz), pearly (talc), or dull (kaolinite). Related to refractive index and surface properties.
Color: While sometimes diagnostic (malachite's green), often variable due to impurities. Streak (powder color) is more reliable - hematite's red streak differs from its metallic gray bulk.
Specific Gravity: Density relative to water. Gold's high SG (19.3) allows placer concentration, while pumice floats due to vesicularity.
Magnetism: Strong in magnetite, weak in pyrrhotite. Important in geophysical prospecting and plate tectonic studies of paleomagnetism.
Optical Properties: Birefringence in calcite, fluorescence in fluorite. Used in mineral identification under polarizing microscopes.
Tenacity: Response to stress. Sectile (cuttable) like gypsum, elastic (mica), or brittle (quartz). Important for industrial applications.
Chemical Composition: Defines mineral groups. Carbonates effervesce in acid, sulfides have metallic bonding. X-ray diffraction reveals precise atomic structures.
These interrelated properties stem from minerals' specific chemical compositions and crystalline structures, making each mineral species unique. Understanding these characteristics enables mineral identification without sophisticated equipment and predicts mineral behavior in geological and technological contexts.
7. Explain the importance and weakness of geological time scale
The geological time scale provides Earth's chronological framework, but like all scientific models, it has both tremendous utility and inherent limitations.
Importance: This temporal classification system organizes Earth's 4.6-billion-year history into eons, eras, periods, and epochs marked by significant biological or geological events. It enables global correlation of rock strata through index fossils and radiometric dating. The scale reveals evolutionary patterns - the Cambrian explosion of life, mass extinctions, and hominid development. It frames tectonic events like supercontinent cycles and orogenies. Economically, it guides resource exploration by correlating hydrocarbon source rocks and mineral deposit ages. The Anthropocene concept demonstrates its continuing relevance in classifying current Earth changes.
Weaknesses: Resolution decreases with age - Precambrian divisions are less detailed than Phanerozoic. Boundaries often represent incomplete records due to unconformities. Radiometric dating has technical limitations and error margins. The scale is Eurocentric - many divisions based on European stratotypes. Biostratigraphic markers can be ambiguous due to migration lags or preservation biases. Absolute dates occasionally require revision as techniques improve. Terrestrial events may not align perfectly with global standards.
Despite these challenges, the geological time scale remains indispensable for Earth sciences. Ongoing refinements through stratigraphic research and improved dating methods continue to enhance its accuracy and global applicability, while new subdivisions like the Anthropocene demonstrate its capacity to incorporate contemporary geological understanding.
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