Department of Civil Engineering

H24GGY Geology for Civil Engineers

Lecture 3: Rocks

Formation, types and characteristics

Prof Stuart Marsh Professor of Geospatial Engineering

Department of Civil Engineering 3rd Rock from the Sun

Formed from the solar nebula 4600 Million Years Ago

All rocks that we have today came from this material

Department of Civil Engineering From Rock to Earth

An active planet

- Geology is dynamic

- Rock is constantly

being recycled...

created, changed

and destroyed by

geologic processes

- Geologic timescales

much greater than

human lifetimes, so

make it seem static

Department of Civil Engineering

Differentiation of Crust, Atmosphere & Oceans

Department of Civil Engineering Earliest Crustal Rock

Department of Civil Engineering Elements and Minerals

geocaching.com

Department of Civil Engineering The Geological Cycle

Department of Civil Engineering Igneous Rocks

Department of Civil Engineering Classification

Department of Civil Engineering Granite

- Feldspar, Quartz

- Coarse, crystalline

- Massive, uniform

structure: strong

- Weathers to clays

and quartz sands

- Lack of internal

structure leads to

distinctive Tors

Department of Civil Engineering Basalt

- 50% Feldspar, mica

and Fe-Mg silicates

- Fine, interlocking

crystals, some voids

- Strength depends on

associated deposits

- Iron content rusts;

weathers to clays

- Structures (columns,

pillows) from cooling

Department of Civil Engineering Sedimentary Rocks

Finer sediments transported further

The process of laying down sediments results in structures such as bedding, channel infill, grading of grain size and dune formation

Lithification occurs on burial via increased temperature and pressure by three processes: cementation, re-crystallisation and compaction

Department of Civil Engineering Classification

Department of Civil Engineering Sandstone

- Desert, river, marine

- Quartz grains in a

quartz/clay/calcite

cement, pore space

- Medium grained

- Strength depends on

cementation strength

- Crumbles to sand

- Deposition structures

preserved like beds

Department of Civil Engineering Clay Rocks

- Lakes and marine

- Structure-less mass

of clay minerals like

illite + quartz grains

- Fine grained

- Strength depends on

lithification (older are

strong), weathering

- Reverts to mud

- Mostly structure-less

Department of Civil Engineering Limestone

- Lakes and marine

- 95% calcite, some

clay, dolomite, flint

- Variable grain size,

often with shells etc.

- Strength depends on

lithification (older are

strong) and structure

- Soluble in rainwater

- Massive or bedded

Department of Civil Engineering Evaporites

- Lakes and marine

- Halite, gypsum

- Crystaline, fine or

coarse depending on

mode of deposition

- Often weak, subject

to water dissolution

by rain, groundwater

- Can be massive or

bedded

Department of Civil Engineering Metamorphic Rocks

Metamorphism happens through re-crystalisation (e.g. in Marble), growth of new minerals from old

(e.g. in schist clay becomes mica) and by the development of fabric through the alignment of minerals

Heating by igneous rock leads to thermal metamorphism;

high pressure during tectonics to dynamic metamorphism;

high temperature and pressure to regional metamorphism

Department of Civil Engineering Classification

Department of Civil Engineering Hornfels

- Thermal metamorphic

- Quartzite, if created

from sandstone, but

marble if limestone

- Crystaline, fine grain

- Generally very strong,

hence often used as

building stone

- Very slow weathering

to clays

Department of Civil Engineering Schist

- Regional/dynamic

metamorphic

- Coarse grained mica,

chlorite, and quartz

- Alignment of mica is

cause of schistosity

- Generally very weak,

also anisotropic due

to schistosity; shears

- Slow alteration to clay

Department of Civil Engineering Further Reading

Foundations of Engineering Geology A C Waltham

Chapter 1: Geology and Civil Engineering

Chapter 2: Igneous Rocks

Chapter 3: Surface Processes

Chapter 4: Sedimentary Rocks

Chapter 5: Metamorphic Rocks