Geology Notes: Igneous, Sedimentary, and Metamorphic Rocks (NZ emphasis)

Introduction and course logistics

• Lecture context: continuation on geology, focus on minerals and rocks, rock types and their formation, plus engineering implications.
• Sample calculations: not annotated in slides; to see solutions, attend lecture or watch recording.
• Aim: connect material to real-world examples (e.g., New Zealand geology map) and relate to engineering properties.
• Emphasis for engineers: mineral composition and rock texture influence properties; rock type affects strength, weathering, and groundwater behavior.

Igneous rocks: origin, types, and textures

• Definition: igneous rocks form by cooling of magma (molten rock) inside the Earth or at the surface as lava.
• Magma vs lava: magma is molten rock within the Earth; when it erupts to the ground surface it is called lava. Cooling forms igneous rocks.
• Where they form: deep in the crust (intrusive/plutonic) or at/near the surface (extrusive/volcanic).
• Intrusive (plutonic) igneous rocks
  • Form inside the Earth under high pressure and slow cooling (hundreds of thousands to millions of years).
  • Typical textures: coarse-grained (phaneritic) because crystals have time to grow.
  • Common examples: diorite, granite, gabbro.
  • Characteristics: non-vesicular (few or no air bubbles); strong, intact rock with few voids; often uplifted and exposed by erosion.
• Extrusive (volcanic) igneous rocks
  • Form when magma reaches the ground surface, in a low-pressure environment, with rapid cooling.
  • Textures: fine-grained (aphanitic) due to rapid cooling; sometimes glassy (obsidian) or vesicular (pumice basalts with air pockets).
  • Common examples: basalt, andesite, rhyolite, pumice, obsidian, etc.
  • Vesicularity and viscosity: magma viscosity influences vesicle formation and flow; high-viscosity magmas trap gas and form vesicles; low-viscosity magmas allow gas to escape and form fewer vesicles.
• Specific rock examples and everyday connections from the lecture
  • Basalt: common in Auckland; used in bricks; associated with volcanoes in Auckland (e.g., Albert Park, Mount Eden, Harbour Volcano).
  • Granite: formed deep in the crust; used for countertops and other countertops in homes.
  • Andesite: volcanic rock with fewer vesicles mentioned relative to basalt.
  • Pumice: highly porous volcanic rock; floats on water; used for exfoliation; forms from explosive eruptions of gas-rich magma; important for New Zealand volcano contexts as pumice-rich deposits are widespread.
  • Obsidian: volcanic glass, historically used as a tool/knife material.
• Textures and crystal growth in igneous rocks
  • Geologists use terms like fine-grained (aphanitic) and coarse-grained (phaneritic).
  • Porphyry: texture with very large crystals (phenocrysts) embedded in a fine-grained matrix; indicates two stages of cooling (slow crystallization of large crystals, followed by rapid cooling of the remainder).
  • Crystal formation during cooling: early crystals form from minerals with lower melting temperatures; as cooling proceeds, more minerals crystallize.
  • Bowen’s Reaction Series (mentioned as an interesting topic to discuss with a geologist): explains the order of mineral crystallization from a cooling melt (not covered in depth here).
• Volcanic processes and rock types within volcanism
  • Extrusive rocks form from lava eruptions at/near the surface.
  • Magma rising through cracks can form dikes and sills:
  • Dike: magma that cuts across existing rock layers (perpendicular to bedding).
  • Sill: magma that intrudes parallel to existing layers.
  • Pyroclastic materials and volcanism
  • Tephra: a collective term for volcanic ejecta of various sizes (ash, lapilli, bombs).
  • Tephra classifications:
    • Ash: very small particles; can travel far with wind.
    • Lapilli: pebble-sized particles (roughly angular fragments related to Hawaiian term “lapilli”).
    • Volcanic bombs: large, often up to tens of centimeters; can fuse with surrounding material when hot enough.
  • Aerofoil/ash deposits: ash fallout can cover areas hundreds of kilometers from eruption (historical example Mount Saint Helens).
  • Pyroclastic flows and pumice tuff
  • Pyroclastic flow: hot rock fragments and gas flow along the ground; can form welded or poorly welded deposits.
  • Thickness variability: flow deposits can be as thin as 10–20 cm or meters thick; welding increases with depth due to heat and pressure.
  • Poorly welded top, welded bottom: bottom layers weld and fuse due to high heat; top layers remain vesicular if cooling and gas escape are limited.
  • Lava flow behavior and rock properties
  • Lava viscosity influences the shape and behavior of flows and final rock textures; high-viscosity lava tends to trap more gas and form vesicles; low-viscosity lava allows gas to escape, resulting in fewer vesicles.
• Additional volcanic rocks and related terms
  • Tephra-related lithologies: pumice tuff, volcanic ash deposits, tephra-rich sediments.
  • Lahars: volcanic mudflows consisting of ash and water-saturated material, a major hazard.
• Volcanism in New Zealand context
  • New Zealand’s volcanic activity produces basalt, andesite, rhyolite, pumice, tephra, and pyroclastic deposits; weathering of volcanic materials can create problematic clays in Auckland.
  • The viscosity and fragmentation of lava influence soil properties and hazard potential.
• Dikes, sills, and ages
  • Dikes and sills are useful for relative dating of geological events; their orientation helps interpret the sequence of events.

Sedimentary rocks: formation, types, and engineering relevance

• Core concept: sedimentary rocks form from sediments that originate as soil, derived from weathering and erosion of pre-existing rocks, transported and deposited, then lithified (turned into rock) by compaction and cementation.
• Sediment definition and sources
  • Sediment is essentially soil (grains such as gravel, sand, silt, clay).
  • Sedimentary rocks are derived from weathered and transported material that becomes compacted and cemented over time.
  • Sedimentary rocks can form from: pre-existing rocks (igneous, metamorphic), or biologic/biogenic material (e.g., shells, skeletons).
• Sedimentary rock groups (classification by process and composition)
  • Clastic (siliciclastic): formed from grains of weathered rock (gravel, sand, silt, clay).
  • Biogenic (biogenic sedimentary): formed from biological material (e.g., limestone from calcite in shells and skeletons; coal from plant material).
  • Chemical (inorganic precipitates): formed from mineral precipitation out of solution (e.g., evaporites like halite; some forms of limestone can be biogenic or chemical).
  • Volcanic/clastic: tephra-derived sediments and rocks (dust, ash, pumice-derived sediments) can become sedimentary rocks over time.
• Lithification processes: how loose sediments become rock
  • Compaction: burial under additional sediment increases pressure, reducing pore space and compacting grains.
  • Cementation: minerals precipitate from pore water and cement grains together (e.g., calcite cement in sandstone).
  • Both processes can occur together; concrete is a practical analogy for lithification (aggregates bound by cement).
  • The deeper the burial, the greater the pressure, typically increasing strength and compactness.
• Sedimentary rock architecture and textures
  • Clasts and matrix: larger grains (clasts) may touch each other (clast-supported) or may be separated by a cementing matrix (matrix-supported).
  • Breccia vs conglomerate
  • Breccia: angular clasts with little transport; clasts are sharp and need cement to hold together due to lack of rounding.
  • Conglomerate: rounded clasts formed by long transport in water; cement holds them together.
  • Grain size and sorting influence rock name and properties:
  • Sandstone: sand-sized clasts
  • Mudstone: clay-sized clasts
  • Siltstone: silt-sized clasts
  • Gravel: gravels; if rounded, conglomerate; if angular, breccia
  • Greywacke: a dirty sandstone and often baseline bedrock in New Zealand; lightly metamorphosed in some contexts.
  • Mudstone and sandstone textures: mudstone feels smooth to touch; sandstone/siltstone feels gritty (grainy tactile sense).
  • Limestone and chalk (biogenic): composed largely of calcite from shells and skeletons; chalk is fine-grained limestone with microfossils.
  • Coal: biogenic sedimentary rock formed from plant material accumulated and transformed under pressure.
  • Fossil-rich and ooidal limestones (as background examples in NZ and globally, e.g., Waitomo caves as limestone exposure; White Cliffs of Dover as chalk example).
• Engineering implications of sedimentary rocks
  • Generally weaker than crystalline igneous rocks; grain size, shape, and sorting influence strength and deformation behavior.
  • Matrix strength matters: weak matrix reduces overall rock strength; clast-contact strength can make some rocks stronger.
  • Weathering susceptibility: weathering is common; more exposed rocks weather faster.
  • Limestone dissolution with acids creates voids and potential sinkholes; aquifer behavior becomes a concern under structures; Florida sinkhole example illustrates risk.
  • Sedimentary rocks can host complex ground conditions due to layering and varying cementation and porosity.
• Sedimentary rock exemplars and NZ context
  • Greywacke as NZ basement bedrock; important in South Island's Southern Alps region.
  • Limestone presence and associated karst features (caves, sinkholes) discussed in NZ context and globally.
  • The lithification of ash and tephra can contribute to sedimentary sequences in volcanic regions, including NZ.

Metamorphic rocks: transformation by heat and pressure

• Core idea: metamorphic rocks arise from pre-existing rocks (igneous, sedimentary, or metamorphic) that have been altered by heat and/or pressure, changing mineralogy, texture, and structure.
• Driving forces and settings
  • Heat and pressure can come from deep burial, proximity to igneous intrusions, regional tectonic forces, or dynamic faulting.
  • Regional metamorphism (e.g., Taupo Volcanic Zone region) can alter vast rock volumes; contact metamorphism occurs near magma bodies; dynamic metamorphism around faults.
• Common metamorphic rocks and textures
  • Granular (non-foliated) metamorphic rocks: formed when deformation is minimal or crystals stay largely intact; examples include certain granitoids that have been metamorphosed with little foliation.
  • Foliated metamorphic rocks: develop distinct planar layering (foliation) due to aligned mineral grains under directed pressure.
  • Slate: fine foliation from low-grade metamorphism of shale; can produce flat, smooth surfaces (example: slate used for billiard tables and chalk writing historically on slate boards).
  • Phyllite: slightly higher grade than slate with glossy sheen.
  • Schist: higher-grade metamorphism with noticeable foliated texture and larger mineral crystals.
  • Gneiss: high-grade metamorphism with pronounced banding, alternating mineral layers.
  • Non-foliated/metamorphic rocks: marble (from limestone), hornfels (from mudstone/shale), quartzite (from sandstone—in general terms).
• Examples and how they relate to parent rocks
  • Slate from shale/mudstone; marble from limestone; hornfels from mudstone; schist and gneiss from various originals with stronger foliation or mineral segregation.
  • Granular metamorphic rocks like metamorphosed granitoid rocks generally retain stronger, more uniform texture when foliation is weak.
• Textures and structural implications
  • Foliation indicates direction of pressure; rocks with strong foliation exhibit planes of weakness along which shear is easier.
  • Non-foliated rocks tend to be mechanically stronger in their plane and can be favorable for foundations, depending on mineralogy and cementation.
• Engineering implications of metamorphic rocks
  • Metamorphic rocks with strong foliation may present planes of weakness and potential shear failure under load.
  • Granular metamorphic rocks without foliation can provide strong foundations.

New Zealand context: geologic map, rocks beneath Auckland, and engineering relevance

• Geologic map and how it informs practice
  • NZ geologic maps (GNS) show rock types beneath surfaces; the same area can have multiple rock types layered or faulted at shallow depths.
  • In Auckland, beneath the campus area, you can encounter Auckland basalt or tuff near the surface, with other formations nearby (e.g., East Coast bas e/delta formations) showing weak sedimentary substrates.
  • The map highlights spatial variation: basalt/tuff near volcanic centers with weaker sedimentary layers in surrounding zones; thickness and proximity to volcanic sources influence ground conditions.
• Auckland-specific notes on volcanism and soils
  • Multiple volcanoes (e.g., Albert Park area) have produced basalt, tuff, and other volcanic materials in the shallow subsurface.
  • Weathering of volcanic deposits in Auckland can produce problematic clays; this is a recurring theme in local geotechnical discussions and will be explored further in future lectures on soil formation from rock.
  • The depth and distribution of volcanic deposits influence groundwater flow, geotechnical strength, and foundation design.
• General NZ geology and broader implications
  • NZ’s geology includes a mix of igneous, sedimentary, and metamorphic rocks across the country; the local rock type determines engineering behavior (strength, stiffness, permeability, and weathering susceptibility).
  • NZ-specific examples (Waitomo limestone, Dover chalk) illustrate biogenic sedimentary rocks used as classic references elsewhere; similar processes occur in NZ with local limestone exposures and karst features, including sinkhole risks.

Key concepts and takeaways for engineering practice

• Rock classification and its practical significance
  • Igneous rocks: strong, often low-porosity, good foundations when crystalline and intact; pyroclastic layers can be weak and variable.
  • Sedimentary rocks: properties depend on grain size, sorting, cementation, and whether the rock is clast- or matrix-supported; weathering and dissolution (especially in limestone) can create voids and hazards.
  • Metamorphic rocks: texture (foliated vs non-foliated) and mineralogy determine strength and potential planes of weakness.
• How rock history affects behavior under load
  • Cooling history (slow intrusive vs rapid extrusive) governs grain size and porosity, which in turn influence strength and deformability.
  • Deposition and lithification processes in sedimentary rocks influence stiffness, permeability, and strength; for example, well-cemented sandstone vs weakly cemented mudstone.
  • Metamorphism alters mineralogy and fabric, creating rocks with directional strength or weakness due to foliation.
• Practical NZ-specific considerations
  • Basalt and tuff near Auckland imply relatively strong, dense rocks in some zones, but overlying or interbedded loose sediments can create complex ground conditions.
  • Weathering of volcanic deposits can generate clays that weaken soils beneath structures; Christchurch’s experiences with sediments and volcanic soils highlight similar risks in Auckland.
  • Understanding the geologic map helps engineers anticipate soil-structure interaction, groundwater flow, and potential for sinkholes or differential settlement.

Quick reference points and prompts from the lecture

• Common igneous rock examples: granite, diorite, gabbro, basalt, andesite, rhyolite, pumice, obsidian.
• Textures to know (engineer-friendly): aphanitic (fine-grained), phaneritic (coarse-grained), porphyritic (mixed grain sizes).
• Key volcanic features: vesicularity related to lava viscosity; pumice as a low-density vesicular rock; pyroclastic deposits; dikes vs sills.
• Sedimentary rock naming based on clast: sandstone (sand), mudstone (clay), limestone (biogenic calcite), conglomerate vs breccia (rounded vs angular gravel).
• Metamorphic textures and terms: foliated vs non-foliated; slate, phyllite, schist, gneiss, hornfels, marble.
• Engineering implications: rock strength, weathering, water permeability, and soil-rock boundary behavior; limestone and karst hazards; basalt and tuff layers near volcanic centers.
• NZ-specific context: use the GNS geologic map to understand subsurface rock distribution around Auckland; expect volcanic rocks near major centers and weaker sedimentary layers in some basins.

Summary

• Rocks are classified into igneous, sedimentary, and metamorphic, with distinctive formation processes that strongly influence their engineering behavior.
• Igneous rocks exhibit a wide range of textures depending on cooling rates and emplacement (intrusive vs extrusive); their mineralogy and porosity determine strength and permeability.
• Sedimentary rocks form through weathering, transport, deposition, and lithification; their properties are controlled by grain size, sorting, cementation, and whether they are clast- or matrix-supported.
• Metamorphic rocks arise from the alteration of existing rocks by heat and pressure, producing foliation and changing mineralogy, which affects mechanical properties.
• New Zealand’s geology (and Auckland in particular) presents a mix of volcanic rocks, sedimentary formations, and metamorphic units, with notable practical implications for ground conditions, soil strength, and hazard assessment.