1. Physical Geology

Introduction

Physical geology studies the Earth’s materials and the processes that shape its surface — the foundation on which every later, more specialised chapter in this subject builds.

1. Origin and age of Earth; internal structure

The Earth formed roughly 4.6 billion years ago through the accretion of material within the early solar system’s protoplanetary disk; this age is established primarily through radiometric dating of meteorites and the oldest terrestrial rocks. Internally, the Earth is layered by both composition and physical behaviour: a crust (thin, outermost, oceanic or continental), a mantle (thick, mostly solid but capable of slow convective flow), and a core (an outer liquid iron-nickel layer and an inner solid iron-nickel layer) — this layering was determined largely from how seismic waves (point 2) behave as they pass through the Earth’s interior.

2. Earthquakes and seismic waves

An earthquake results from the sudden release of accumulated stress along a fault (developed further in Structural Geology, Chapter 4), radiating energy as seismic waves. Body waves travel through the Earth’s interior: P-waves (primary, compressional, fastest, able to travel through solids and liquids) and S-waves (secondary, shear, slower, unable to travel through liquids — their absence in certain regions was key evidence for the Earth’s liquid outer core). Surface waves travel along the Earth’s surface, generally slower than body waves but often causing the most severe damage due to their larger amplitude.

3. Volcanoes and igneous activity

Volcanoes form where magma reaches the Earth’s surface, and their eruption style depends heavily on magma composition: low-viscosity, low-silica (basaltic) magma tends to erupt relatively gently and effusively, forming broad shield volcanoes, while high-viscosity, high-silica (rhyolitic/andesitic) magma traps gas, tends to erupt explosively, and forms steeper composite (stratovolcanoes) cones. Volcanic activity is heavily concentrated along plate boundaries (Chapter 4), particularly subduction zones and mid-ocean ridges, connecting this topic directly to the plate tectonics framework developed later in this Paper.

4. Weathering, erosion, mass wasting, and soils

Weathering is the in-place breakdown of rock, either physical/mechanical (e.g., frost wedging, where water freezing in cracks expands and fractures rock) or chemical (e.g., dissolution of minerals by acidic water) — the two often act together, with physical weathering increasing surface area available for chemical attack. Erosion is the removal and transport of weathered material by agents such as water, wind, or ice (point 5), distinct from weathering’s in-place breakdown. Mass wasting is the downslope movement of rock and soil under gravity (landslides, slumps, creep), triggered or worsened by factors like water saturation, slope steepness, and seismic shaking. Soil forms through the combined long-term action of weathering and biological activity on parent rock material.

5. Work of rivers, groundwater, glaciers, wind, and oceans

Each of these agents both erodes and deposits characteristic landforms:

• Rivers erode by abrasion and hydraulic action, transport sediment, and deposit it as their velocity decreases (forming features like floodplains, deltas, and meanders).
• Groundwater dissolves soluble rock (particularly limestone), producing karst landscapes with caves and sinkholes, and is also a critical resource studied further in engineering/environmental geology (Chapter 9).
• Glaciers erode through plucking and abrasion, producing distinctive U-shaped valleys, and deposit unsorted sediment as moraines upon melting.
• Wind erodes and transports fine sediment (particularly significant in arid regions), depositing it as dunes or loess.
• Oceans shape coastlines through wave erosion and longshore sediment transport, producing features like cliffs, beaches, and spits.

6. Basic geomorphology and landform development

Geomorphology studies the origin and evolution of landforms as the product of the interacting processes above, acting over geological time on the underlying rock structure — the same landform-forming agents produce systematically different results depending on climate, rock type, and tectonic setting, which is why a process–landform framework (recognising which agent and process combination produced a given feature) is a more useful study approach than memorising landforms in isolation.

Key Takeaways

• The Earth’s layered internal structure (crust, mantle, core) was established largely through seismic wave behaviour, particularly the absence of S-waves in the liquid outer core.
• Volcanic eruption style depends on magma composition (viscosity and gas content), and both volcanoes and earthquakes concentrate heavily along plate boundaries.
• Weathering (in-place breakdown) is distinct from erosion (removal and transport); mass wasting is gravity-driven downslope movement, distinct from both.
• Rivers, groundwater, glaciers, wind, and oceans each have characteristic erosional and depositional landforms, determined by their specific mode of action.
• Geomorphology is best studied through a process–landform framework, since the same agents produce different results depending on climate, rock type, and tectonic setting.