Concepts [Lecture 2]

Rocks are

Heterogenous at local and regional scale
Still forming today

Tectonic plates

Active rock system

Mid Atlantic Ridge: Atlantic moves apart = new rocks
Ring of fire: rock subduction

Asthenosphere

hot, under high pressure but not yet melted

Rock cycle

Magma (molten rocks) under asthenosphere = liquid

⇒ Igneous rock

• Extrusive = solidification at surface

• Intrusive = cooling and crystallisation underground

⇒ Soils

• Come from weathering

• Residual soils

• Transported soils

⇒ Sediments

• Weathering, erosion, transportation and deposition

⇒ Sedimentary rocks

• Lithification = compacting and cementing ⇒ leads to development of tensile strength + frictional strength

• Vertical cracks form due to overpressure of fluids once tensile strength is gained

⇒ Metamorphic rocks

• Heat and pressure (metamorphism)

• Transformation of existing rock, do not melt

Rock

any naturally occurring aggregate of one or more mineral or mineraloid matter

nature and properties of a rock are determined by the minerals in it & by the manner in which the minerals are arranged relative to each other

Igneous rock (origin, environment, texture, structure, strength)

• Crystallized from molten magma

• Underground, as lava flows

• Massive structure = structureless

• Uniform high strength

Sedimentary rock (origin, environment, texture, structure, strength)

• Erosional debris on earth's surface

• Deposition basins (mainly seas)

• Mostly granular and cemented

• Layered, bedded, bedding planes

• Variable low strength, planar weakness

Igneous rock examples

Granite, granodiorite, basalt

Sedimentary rock examples

Sandstone, limestone, clay, chalk

Metamorphic rocks (origin, environment, texture, structure, strength)

• Altered by heat and pressure

• Mostly deep inside mountain chains

• Mosaic of interlocking crystals

• Crystal orientated due to pressure

• High variability due to pressure

Detailed rock cycle

Rockhead

line between soil and rock (hard to place, more of a gradual transition)

UCS

unconfirmed compressive strength

Rock compressive strength

> 0.6 MPa
Igneous rocks = strong
Chalk = weak

Soil shear strength

< 0.3 MPa or 300KPa

= weak

Fresh rock

UCS up for 250 MPa, not at earth surface

Slightly weathered

joints may be stained, looks fresh
several hammer blows to break sample

Moderately weathered

discoloured, weakened but still rock
needs hammer to break

Highly weathered

becoming soil like but doesn’t disintegrate in water
micro-cracks
broken by hand

Completely weathered

feldspar soft, grooved with pin and disintegrates in water
micro-cracks can be sealed with clay

Residual soil

original texture lost
most quartz is dissolved
typically red, clay rich

Rock tensile strength

weak

Soil & rock cementation

cementation from pore fluids and plastic migration from stresses grain contact
calcium carbonate cement or silicate cement (hardest)

bonding + cementation → tensile strength, cohesion & frictional strength

Today’s period

Interglacial

Cretaceous

Asteroid shot up sediments into atmosphere
Chalk formation (deposition in seas)

Deep weathering

happens during colder periods, ground freezes and breaks up rock

Mineral

naturally occurring inorganic substance which has a definite chemical composition and presents an ordered atomic arrangement

Mineral characteristics

1. Colour

2. Lustre = reflectance

3. Form = shape, grouping clustering

4. Hardness = resistance to abrasion

5. Cleavage = the way minerals break along planes of weakness, exhibiting smooth, flat surfaces.

6. Fracture = nature of broken surfaces, lack of cleavage

7. Tenacity = malleable vs brittle under sudden impact

8. Specific gravity

Metamorphic rock examples

Gneiss, schist, slate

Rock forming minerals & susceptibility to weathering

Olivine - Very

Pyroxene - High

Amphibole - High

Mica - Medium/High

Feldspar - Medium/High

Silica - Low

Silicate structure

silicate minerals make up most rock forming minerals
SiO4

Non silicate minerals

• Carbonates (chalk)

• Oxides

• Halides

• Sulphides

• Sulphates

• Phosphates

Silicate structural linkages

Slowly cooled rock linkages

crystalline (regular and repeating arrangement of atoms or molecules in a solid substance, forming a well-defined crystal lattice)

polycrystalline (structure lacks this regular arrangement, with atoms or molecules randomly distributed, resulting in a non-crystalline, often glassy appearance)

Quickly cooled rock linkages

Amorphous

Granite composition

• Plagioclase K feldspars = white

• Micca = black

• Quartz = transparent

Hydrolisis

reaction between mineral and H+ and OH- of water

Orthoclase feldspar hydrolysis

kaoline produced
only happens with moving water

Forms of igneous rocks & magma formation

Magma formation and composition

Volcanoes formation

Mafic to felsic classification

Felsic = high silicate %

Intrusive rocks

slow cool down & compression → big crystals

Extrusive rocks

fissure eruptions → low viscosity, far flowing basaltic lava = fine grained

central eruptions (volcanoes) → ejection of lava, gases, ash, broken rock (explosive or effusive)

Texture / structure

• Equigranular = minerals same size (intrusive)

• Aphanitic = can't see crystals (extrusive)

• Inequigranular = some crystals larger than others

• Porphyritic = large crystals surrounded by much smaller ones

Shape

nice crystal shape = euhedral

other = anhedral

Bowen’s reaction series

Feldspar + mica cool first → biggest

Quartz = interstitial mineral → last mineral to harden, squeezes into spaces → smallest

Goldich’s dissolution series

Vesicular texture

fine-grained (basalt) + gas holes (from being shot out)

Glassy texture

obsidian = cooled quickly

Mica weathering

complex reaction where metal ions are removed from mica
→ results in soluble metal ions and clay minerals

Soil types

• residual soils

• transported soils

• chemical & biological soils

• volcanic soils