GeologyQ&A by dot point

Geochronological principles let geologists order events and estimate ages: the law of superposition (in undisturbed strata the oldest is at the base), the principle of cross-cutting relationships (a feature that cuts another is younger), the use of fossils to correlate rocks of the same age, and the idea of half-life, which gives the absolute age of a rock in years from radioactive decay; relative dating gives the order of events, absolute dating gives the age in years.

The rock record preserves evidence of past climate and sea-level change: rock types act as climate indicators (coal for warm wet swamps, evaporites for hot arid conditions, tillite for cold glacial conditions, reef limestone for warm shallow seas); rising sea level (transgression) gives a fining-upward, deepening sequence and falling sea level (regression) a coarsening-upward sequence; these changes are driven by ice ages, plate movement and changes in the volume of the ocean basins.

The fossil record shows that life began early and became more complex and diverse over geological time; major milestones include the first simple cells, the Cambrian appearance of abundant shelly animals, the move of life onto land, and the rise and fall of major groups; evolution by natural selection explains the changes, fossils provide the evidence, and mass extinctions repeatedly reset the course of life (for example the end-Permian and end-Cretaceous events).

The Earth's outer layer is divided into tectonic plates that move slowly over the mantle, driven by convection; the evidence for plate tectonics includes the fit of the continents, matching fossils and rock sequences across oceans, and the symmetrical magnetic stripes of the sea floor; plates meet at constructive (divergent), destructive (convergent) and conservative (transform) margins, each with characteristic earthquakes, volcanoes and landforms.

The rock cycle links igneous, sedimentary and metamorphic rocks through the processes of weathering, erosion, transport, deposition, burial and lithification, melting and crystallisation, and metamorphism; the cycle is driven by energy from the Sun (at the surface) and from the Earth's interior (at depth), and any rock can be changed into any other given time and the right conditions.

Dip is the angle a bed makes with the horizontal, measured in the direction of steepest slope; strike is the compass direction of a horizontal line on the bed, at right angles to the dip; dip and strike are measured with a compass-clinometer and recorded with the dip and strike symbol on geological maps, and the apparent dip seen in a cross-section can differ from the true dip.

Rocks deform when stressed: compression produces folds (anticlines arch upwards, synclines sag downwards) and reverse faults, while tension produces normal faults; the type and orientation of folds and faults are evidence of the direction of past Earth movements and are shown on geological maps and cross-sections.

Joints are fractures with no movement, formed by cooling, drying or pressure release; an unconformity is a buried erosion surface separating older rocks below from younger rocks above, recording a gap in time during which deposition stopped and erosion occurred; unconformities and joints are interpreted from cross-sections to reconstruct geological history.

Geological history is reconstructed from a cross-section using the principles of superposition (younger beds lie above older), original horizontality, cross-cutting relationships (a fault or intrusion is younger than the rocks it cuts) and included fragments; the order of deposition, deformation, intrusion, erosion (unconformities) and faulting is deduced to give a relative sequence of events.

Earthquakes are caused by the sudden release of stress along faults, mainly at plate margins; they radiate seismic waves (P-waves and S-waves) whose arrival times locate the epicentre and whose amplitude measures magnitude; the hazards include ground shaking, building collapse, tsunamis, fires and landslides; the risk is reduced by hazard mapping, building design and emergency planning, but precise short-term prediction remains impossible, so forecasting relies on probability from past records.

Hydrocarbons (oil and gas) form from buried organic matter, then migrate from a source rock into a porous, permeable reservoir rock where an impermeable cap rock and a trap (for example an anticline or fault) hold them; groundwater is stored in a porous, permeable aquifer beneath the water table and supplied to wells; both depend on the rock properties porosity (storage) and permeability (flow), and both can be over-exploited or polluted.

Engineering geology assesses the ground before construction: foundations, tunnels, dams and reservoirs must suit the rock and soil present; geologists check the strength and stability of rock, the presence of faults, the slope stability, the permeability of the ground (for a reservoir to hold water or a tunnel to stay dry), and the hazards of weak, soluble or swelling materials; poor ground investigation can lead to subsidence, leakage, collapse or failure, so a site investigation is carried out first.

Mass movement is the downslope movement of rock and soil under gravity; it includes slow creep, slides, slumps, flows and rockfalls; failure is triggered when the driving force (gravity, increased by steep slopes, heavy rain, loading and undercutting) exceeds the resisting force (friction and cohesion, reduced by water and weak or weathered rock); the risk is reduced by improving drainage, reducing slope angle, building retaining structures and avoiding building on unstable ground.

An ore is a rock from which a metal can be extracted economically; ore minerals (for example galena for lead, haematite for iron) are concentrated by geological processes such as hydrothermal veins, magmatic settling and weathering and deposition; whether a deposit is worked depends on its grade, size, depth and the metal price; extraction by surface or underground mining has environmental costs, so it is balanced against the need for the metal and is followed by site restoration.

Volcanic activity ranges from gentle effusive eruptions of runny basaltic lava to violent explosive eruptions of viscous silica-rich magma; the hazards include lava flows, ash falls, pyroclastic flows, lahars (mudflows) and toxic gases; volcanoes can be monitored using seismometers (earthquake swarms), ground deformation (tilt and bulging), gas emissions and rising temperatures, so eruptions are more predictable than earthquakes, and risk is reduced by monitoring, hazard mapping, exclusion zones and evacuation.

A geological cross-section is a vertical slice through the ground constructed from a map by transferring the topography and the boundaries of the rock units onto a profile and drawing the beds at their measured dip; a graphic (sedimentary) log records a vertical sequence of beds to scale, showing thickness, grain size, rock type and structures; both turn observations into a diagram from which the order of beds, the structures and the geological history can be read.

Fieldwork involves recording observations systematically: making annotated field sketches, recording rock type, colour, grain size, texture, structures and fossils, measuring features such as dip and bed thickness, and identifying hand specimens of minerals and rocks using their physical properties; observations must be objective, located on a map or grid reference, and recorded safely and accurately so they can be interpreted later.

A simplified geological map shows the distribution of rock units at the surface using colours and a key, with a scale, a north arrow and grid lines; features are located using grid references (four-figure for a square, six-figure for a precise point), and the map is read together with topography to identify the rock units present, the order of the beds, and structures such as folds and faults shown by the outcrop pattern.

A directed field investigation answers a geological problem or question through a planned enquiry: forming a question or hypothesis, choosing a suitable site and methods, collecting data safely and systematically (measurements, samples, logs and sketches), recording it accurately and located, then analysing the data and drawing a justified conclusion while evaluating the reliability and limitations of the method; a minimum of two days of fieldwork, including such an investigation, is required.

Geological investigations use quantitative skills: converting between map distance and real distance using the scale, calculating rates (of deposition, erosion or plate movement) from an amount and a time, reading and plotting graphs and gradients, and handling data with means, ranges and percentages; the distance to an earthquake epicentre can be estimated from the gap between P-wave and S-wave arrivals, and rates and ages are calculated using simple formulae and the half-life idea.

Minerals are identified using physical properties: colour, crystal size, hardness (tested against fingernail, copper coin, steel and glass), cleavage and fracture, lustre, streak, and the reaction of carbonates with dilute hydrochloric acid; common minerals include quartz, feldspar, mica, calcite, halite, galena and haematite.

Igneous rocks form by the crystallisation of magma or lava; cooling rate controls crystal size (slow cooling at depth gives coarse-grained intrusive rocks such as granite, fast cooling at the surface gives fine-grained extrusive rocks such as basalt); rocks are classified by crystal size and by silica content (felsic, intermediate, mafic); minerals also crystallise from hydrothermal fluids to form veins.

Metamorphic rocks form by recrystallisation of existing rocks in the solid state under heat and pressure, without melting; contact metamorphism (heat from an intrusion) produces non-foliated rocks such as metaquartzite and marble; regional metamorphism (heat and directed pressure over a wide area) produces foliated rocks such as slate and schist; protolith and conditions determine the product.

Sedimentary rocks form by weathering, erosion, transport, deposition, and lithification (compaction and cementation); they are classified as clastic (conglomerate, breccia, sandstone, shale), biological (limestone) or chemical (evaporites); grain size, shape, sorting, sedimentary structures and fossil content are used to interpret the depositional environment; fossils form by preservation of hard parts and record past life.

The Earth can be compared with its planetary neighbours (the other rocky planets and the Moon) in terms of their rocks and surface materials, surface landforms, atmosphere, surface temperature, pressure and gravity; differences in size, distance from the Sun and the presence of an atmosphere and liquid water explain why the Earth is geologically active and habitable while the Moon and Mars are not, and why some bodies preserve an ancient cratered surface.

The surfaces of the Moon and Mars record their geological histories: the Moon has heavily cratered highlands and smoother dark maria (ancient basalt plains), showing impact and volcanism on a body that is now dead; Mars shows giant volcanoes, a vast canyon system, dried-up channels and polar ice, showing past volcanism and flowing water; the relative density of impact craters is used to work out the relative ages of different surfaces (more craters means an older surface).

Meteorites are fragments of asteroids and other bodies that fall to Earth; the main types (stony, iron and stony-iron) are thought to sample the interiors of broken-up rocky bodies, so they give evidence for the composition of the Earth's deep interior; iron meteorites support an iron-rich core and stony meteorites a silicate mantle, because we cannot sample the deep Earth directly.

Uniformitarianism (the principle that the present is the key to the past, so the same physical processes operate everywhere and at all times) lets geologists interpret the surfaces of other planetary bodies; by comparing landforms seen in space imagery (craters, volcanoes, channels, dunes) with landforms made by known processes on Earth, geologists infer the processes (impact, volcanism, flowing water, wind) that shaped other worlds, even though we have never seen them happen there.