EVSC10001 · The Global Environment
Origins: Solar System, Earth & Rocks
This opening chapter traces how the Solar System condensed from a collapsing molecular cloud into a spinning disk, how planetesimals accreted into planets around 4.6 billion years ago, and how heating then differentiated the young Earth into a metallic core, a silicate mantle and a thin crust. It also fixes the core vocabulary of the solid Earth — mineral vs rock, the three rock families, and the rock cycle that links them. These are classic Part A short-answer topics, where a clearly labelled diagram earns marks just as much as the written explanation.
What this chapter covers
- 01The nebular hypothesis: cloud collapse → disk → planetesimals → accretion
- 02Meteorites as Solar-System samples (iron / stony-iron / stony; chondrites, chondrules, CAIs)
- 03Condensation / temperature sorting and why composition varies with distance from the Sun
- 04Planetary differentiation: heat sources, density sorting, and the molten-stage requirement
- 05Earth's layered structure: core, mantle, crust
- 06Minerals (naturally occurring inorganic crystalline solids; silicates dominate) vs rocks (aggregates)
- 07The three rock families: igneous, sedimentary, metamorphic
- 08The rock cycle and its endogenic (tectonic) + exogenic (Sun/climate) drivers
Part-A short-answer: sketch and explain the rock cycle
- +1Draw the four nodes as boxes and join them with labelled arrows: MAGMA, IGNEOUS rock, SEDIMENTARY rock, METAMORPHIC rock. The diagram is required and is worth a mark on its own.
- +1Magma → Igneous: as magma cools it undergoes crystallisation, locking minerals into an igneous rock. Label this arrow 'cooling + crystallisation'.
- +1Igneous → Sedimentary: surface exposure drives weathering (in-situ breakdown), then erosion and transport, deposition, and finally compaction + cementation (lithification). Label the arrow with this surface pathway.
- +1Sedimentary → Metamorphic: deeper burial applies heat + pressure, recrystallising the rock in the solid state (no melting). Label the arrow 'heat + pressure (solid state)'.
- +1Metamorphic → Magma: further heating causes melting back to magma, closing the loop; annotate the diagram that endogenic (internal heat / tectonics) drives the deep limbs and exogenic (Sun / climate) energy drives the surface limbs, and that any rock can convert to any other.
Key terms
- Nebular hypothesis
- The leading model for Solar-System formation: a cold, dense molecular cloud collapses under gravity into a spinning proto-planetary disk, dust clumps into planetesimals, and these accrete into the planets (~4.6 Ga).
- Planetary differentiation
- The density-driven sorting of a once-homogeneous molten planet into layers — dense metallic iron sinks to form the core while light silicates rise to form the mantle and crust. It separates pre-existing material and requires an at-least-partly-molten interior.
- Chondrite
- A stony meteorite containing chondrules (quenched melt droplets) and rare Ca–Al-rich inclusions (CAIs, the earliest Solar-System solids). Type-1 carbonaceous chondrites are essentially unaltered nebula material, matching the Sun's surface chemistry minus volatiles.
- Mineral
- A naturally occurring, inorganic, crystalline solid with a regular atomic structure and a definite chemical composition (e.g. quartz SiO2, halite NaCl). Most crustal minerals are silicates (built from Si + O).
- Rock
- An aggregate of one or more minerals (e.g. granite = quartz + feldspar + biotite). Rocks are grouped into three families — igneous, sedimentary, metamorphic — by how they form.
- Rock cycle
- The set of processes that converts any rock type into any other, driven by endogenic energy (internal heat / tectonics) on the deep limbs and exogenic energy (Sun / climate) on the surface limbs.
Origins: Solar System, Earth & Rocks FAQ
How did the Solar System and Earth form?
A collapsing molecular cloud (possibly triggered by a nearby supernova) flattened into a spinning disk; dust clumped into planetesimals that accreted into planets about 4.6 billion years ago. The hot inner disk could only condense metals and silicates, giving small rocky planets, while the cool outer disk also condensed volatiles, giving the gas and ice giants.
Why does Earth have a layered core, mantle and crust?
Because of planetary differentiation. Impact heating during accretion plus radioactive decay melted the young Earth; in the molten interior dense iron sank to form the core while lighter silicates floated to form the mantle and crust. The layers mirror the iron / stony-iron / stony meteorite classes.
What is the difference between a mineral and a rock?
A mineral is a single naturally occurring inorganic crystalline solid with a definite composition (like quartz). A rock is an aggregate of one or more minerals (granite, for example, is quartz + feldspar + biotite). A mineral is the building block; a rock is the assembly.
How are the three rock families different?
Igneous rocks crystallise from molten magma or lava; sedimentary rocks form by lithification of weathered grains or chemical precipitates; metamorphic rocks recrystallise from existing rock under heat and pressure in the solid state (no melting). The rock cycle shows any one can transform into any other.
What does the rock cycle exam question usually ask for?
A Part-A short-answer asking you to draw a labelled diagram of the cycle and trace one full loop, naming each process. You earn marks by labelling every arrow (cool + crystallise, weather/erode/deposit/lithify, heat + pressure, melt) and noting that internal heat and Sun-driven energy are the two drivers.
Exam move
Treat this chapter as a sequence of three reliable Part-A diagrams and rehearse drawing each one from a blank page under time pressure: (1) the formation-and-differentiation cartoon (cloud → disk → planetesimals → accretion → molten Earth → concentric core/mantle/crust shells, with arrows for sinking iron), (2) the meteorite-to-layers analogy (iron = core, stony-iron = boundary, stony = mantle/crust, chondrite = unaltered nebula), and (3) the four-node rock cycle. The single biggest mark-grabber is labelling every arrow with its process, not just naming the boxes — Part A explicitly requires a labelled sketch, so practise the diagram and the caption together. Lock the two definitions the exam tests verbatim (mineral = naturally occurring inorganic crystalline solid; rock = aggregate of minerals) and keep the two trap distinctions sharp: weathering (in-situ breakdown) is not erosion (removal/transport), and metamorphism is solid-state (once it melts you are in the magma node).
