Every fact in the three rock lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the lessons simplify: the rock cycle is drawn as three boxes, but real rocks can take many routes and some rocks are hard to place in one group; the sorting games give each rock one answer. The sugar-cube jar for ages 10–11 is only a model of rocks being knocked and worn while they are moved; sugar is far softer than rock. The density values for ages 15–16 are averages from Britannica: real samples vary, especially porous rocks such as sandstone and basalt with gas bubbles. The porosity method (soaking up water) only measures the pores water can reach, and some water may be absorbed while the volume is measured; the lesson asks pupils to discuss this.
All sources
[1] Rocks and minerals
British Geological Survey (BGS) · Reliability: High
Type: National geological survey education page
A rock is a solid collection of minerals; there are three main types: sedimentary, igneous and metamorphic; granite is mostly quartz, feldspar and mica in interlocked crystals, while sandstone can contain the same minerals as cemented grains; sedimentary rocks are recycled rocks formed by the deposition of fragments, usually in seas and lakes but also in deserts; shells made largely of calcium carbonate are compressed to form limestone; dead plant material can over millions of years turn into coal; igneous rocks form by the cooling of magma; extrusive (fast-cooling) rocks have small crystals, such as the basalt of the Giant's Causeway; intrusive rocks such as granite cool slowly, maybe over thousands or millions of years, and have larger crystals; deeply buried rocks can be subject to very high temperatures and intense pressures; metamorphosed limestone becomes harder and more crystalline and changes into marble; shale may change into slate; contact metamorphism happens when rock is 'baked' by nearby hot igneous material without melting.
British Geological Survey (BGS) · Reliability: High
Type: National geological survey education page
Planet Earth is dynamic with a surface that is always changing; rocks can be converted into another type of rock, for example igneous to metamorphic, and are slowly recycled over millions of years; weathering is the wearing down or breaking of rocks while they are in place; erosion is the movement of rock fragments through gravity, wind, rain, rivers, oceans and glaciers; deposition is the laying down of sediment carried by wind, water or ice.
British Geological Survey (BGS) · Reliability: High
Type: National geological survey education page
Water can get into cracks in a rock and, when it freezes, the ice expands and pushes the cracks apart; chemical weathering is chemicals in rainwater changing the minerals in a rock: carbon dioxide from the air dissolves in rain, making it slightly acidic; biological weathering is caused by plants and animals, for example roots growing in a crack or a rabbit burrowing into one; footwear walking over the same rock can wear it down.
[4] The Rock Cycle (Physical Geography and Natural Disasters, section 3.5)
R. Adam Dastrup, open textbook (Salt Lake Community College Pressbooks) · Reliability: Medium-high
Type: Open college textbook
Intrusive igneous rocks cool and solidify beneath the surface and have larger crystals; extrusive igneous rocks cool much more rapidly; lavas that cool exceptionally rapidly may have a glassy texture, and gas bubbles give a vesicular texture; physical weathering breaks rock apart and chemical weathering dissolves less stable minerals; sediment is carried by water, wind, ice or gravity and comes to rest by deposition; sediments are squeezed together by the weight of sediments above (compaction), and fluids crystallise in the spaces between grains and bind them (cementation); each layer is younger than the layer beneath it; metamorphism changes rock through heat and pressure without melting it entirely; regional metamorphism comes from high pressure over a wide area, contact metamorphism from the heat of magma.
[5] Chapter 5: The Rock Cycle (Investigating the Earth: Exercises for Physical Geology)
D. Hauptvogel, V. Sisson and M. Comas, open lab textbook (University of Houston Libraries Pressbooks) · Reliability: Medium-high
Type: Open university lab textbook
Plate tectonics and other natural processes slowly change rocks from one type to another; igneous rocks form when magma or lava cools and solidifies; sedimentary rocks form when pieces of weathered and eroded rock are cemented together; metamorphic rocks form when pre-existing rocks are altered by heat and/or pressure; igneous and metamorphic rocks have a crystalline texture of interlocking minerals; sedimentary rocks mainly have a clastic texture in which grains are separated by cement; grains in igneous rocks are randomly oriented, while most metamorphic rocks have grains with a preferred orientation; texture is often what distinguishes rocks.
Mean dry bulk densities: granite 2.66 g/cm³, basalt 2.74 g/cm³, sandstone 2.22 g/cm³; typical ranges: limestone 1.55–2.75, marble 2.67–2.75, shale 2.06–2.67, slate 2.72–2.84 g/cm³; the bulk volume is the volume of the grains or crystals plus the pore space, so bulk density is lower than grain density and would equal it if porosity were zero; granite has low porosity; basalt can contain gas-bubble holes (vesicles); sandstone porosity can vary from low to more than 30 percent.