Ausgeology. Senior secondary lessons

SACE Stage 1 Earth and Environmental Science · Topic 2: Composition of the Geosphere

Minerals, soil and dating.

The geosphere is made of minerals, which make rocks, which weather into soil. Rocks and fossils also keep time. Work through eight short units, and prove you have mastered each one before you move on.

How the lesson works

1 · Minerals and rocks

Screens 2 to 4. What counts as a mineral, how to identify minerals at a test bench and what they are used for, and how rocks form.

2 · Soil

Screen 5. What soil is made of, how weathering and the spheres make it, the horizons of a soil profile, and why its origin matters.

3 · Dating

Screens 6 to 8. Ordering events in a cross-section, using half-life to date rocks and the Earth, and carbon-14 at Australian sites.

4 · Put it together

Screens 9 and 10. Choose the right dating method, take the final mastery check, then show what you know.

How mastery works

Each unit ends with a five-question mastery check. Get four or more right and the unit is mastered.

If you get fewer than four, you see a short summary of what to fix, then take a second version of the check. Your mastery map below shows where you are up to; your teacher may ask the class to take each check together.

Your mastery map

What we are learning

What minerals are and how they are identified and used, how rocks and soil form, and how geologists work out the order and the age of events, from carbon-14 in a midden to the age of the Earth.

I can

  • 1explain what a mineral is and the limitations of the criteria.
  • 2classify minerals by their chemical composition.
  • 3identify common rock-forming minerals from their properties.
  • 4explain how the uses of minerals depend on their properties.
  • 5classify rocks as igneous, sedimentary or metamorphic and describe how they form.
  • 6describe what soil is made of, how it forms and how its origin affects its uses.
  • 7use superposition, cross-cutting relationships, inclusions and correlation to order events.
  • 8use half-life to date rocks and explain how the age of the Earth was established.
  • 9explain how carbon-14 is used to date middens and megafauna up to about 50,000 years old.

SACE Stage 1 Earth and Environmental Science, Topic 2: Composition of the Geosphere. This lesson covers all seven of the topic’s Science Understanding statements. The teacher notes at the end quote them in full.

My notebook

Printable activity

Mastery record and test bench sheet

Three pages: a mastery record for the eight units (date, form, score, what to fix), a test bench table for real mineral specimens, and a half-life record sheet for a coin or dice practical, with a grid to plot the decay curve.

Mastery record (PDF) Editable (Word)

1Minerals and rocks

Unit 1: what is a mineral?.

Quartz is a mineral and coal is not. Where exactly is the line, and why is it sometimes blurry?

Minerals are the building blocks of rocks. This unit sets out the criteria for a mineral, the cases that test them, and how minerals are grouped by their chemistry.

This unit · about 35 minutes

  1. Read the definition and its exceptions.
  2. Sort eight substances: mineral, not a mineral, or a borderline case.
  3. Read how minerals are classified, then sort eight minerals into classes.
  4. Take the mastery check. If you score less than 4 of 5, read the help and take the other form.

The definition

The International Mineralogical Association defines a mineral as “an element or chemical compound that is normally crystalline” and that “has been formed as a result of geological processes” (Nickel, 1995). Britannica adds “a definite chemical composition and a highly ordered atomic arrangement”. Geoscience Australia puts it simply: “Minerals are naturally occurring, inorganic, solids. They are the ingredients of rocks.” There are “several thousand known mineral species”, but only about 100 make up most rocks (Britannica).

Each criterion has awkward cases. Liquid mercury is accepted as a mineral; ice is a mineral but liquid water is not; lab-made crystals are “synthetic equivalents”, not minerals; and substances “produced entirely by biological processes”, such as the shells of molluscs, are excluded (Nickel, 1995). Many minerals also vary in composition within a range: in olivine, (Mg,Fe)2SiO4, iron and magnesium substitute for each other, and “nearly all” specimens contain both (Peck). So “definite composition” has to allow a range, called a solid solution.

Mineral or not?

Opal is Australia’s national gemstone, and Australia produces 95% of the world’s precious opal (Geoscience Australia). It is made of “small spheres of silica arranged in a regular pattern, with water between the spheres”, but its atoms are not arranged in crystals, so many textbooks call it a mineraloid.

Classes of minerals

Minerals are grouped by chemistry: native elements such as gold and diamond, sulfides such as galena (PbS), oxides such as hematite (Fe2O3), halides such as halite (NaCl), carbonates such as calcite (CaCO3), sulfates such as gypsum, phosphates such as apatite, and silicates such as quartz (SiO2) and feldspar (Earle, 2015). Silicates “make up about 95 percent of Earth’s crust and upper mantle”, and feldspars alone “make up more than half of Earth’s crust” (Britannica). Quartz “is the most common mineral found on the surface of the Earth” (Mindat).

Which class?

1Minerals and rocks

Unit 2: identifying and using minerals.

How does a geologist name a mineral in minutes, and why do we use each mineral for the jobs we do?

Every mineral has a set of properties you can test with simple tools. Learn the tests, identify nine specimens on the bench, then link each mineral’s uses to its properties.

This unit · about 45 minutes

  1. Read about the properties and the Mohs scale.
  2. Identify the nine specimens on the test bench.
  3. Sort seven uses by the property that makes them work.
  4. Take the mastery check.

The tests

Colour is easy to see but unreliable: hematite can be steel-grey or red, but its streak, the colour of its powder, is always “Cherry-red or reddish brown” (Handbook of Mineralogy). Lustre describes how a surface shines: metallic, vitreous (glassy), pearly or earthy. Cleavage is breaking along flat planes; quartz has none and breaks with a curved, conchoidal fracture. Specific gravity tells you how heavy a mineral is for its size. Two minerals have special tests: calcite fizzes in dilute hydrochloric acid (Perkins and others), and magnetite is “Strongly magnetic” (Handbook of Mineralogy).

The Mohs hardness scale, devised by Friedrich Mohs in 1812, ranks resistance to scratching (Britannica): 1 talc, 2 gypsum, 3 calcite, 4 fluorite, 5 apatite, 6 orthoclase, 7 quartz, 8 topaz, 9 corundum, 10 diamond. It is a ranking, not a measurement: “each increment of one in the scale does not indicate a proportional increase in hardness” (Britannica). A fingernail is about 2.5, copper about 3.5, and a steel knife blade or glass about 5.5 (Branciforte and Haddad).

Uses follow properties

Diamond is “the hardest natural mineral on Earth” (Geoscience Australia), so it is used for cutting, drilling and grinding. Today “Synthetic diamond accounted for more than 99% of global industrial diamond production”, and US users prefer it “because its quality can be controlled” (USGS, 2026). Australia is a major producer of the ore minerals: it is “one of the largest producers of iron ore in the world”, mostly from the Pilbara, and a large lead producer, with mines including McArthur River in the Northern Territory (Geoscience Australia); in 2025 it mined the second-most lead in the world, after China (USGS, 2026).

Which property makes it useful?

1Minerals and rocks

Unit 3: rocks.

How can the minerals and texture of a rock tell you whether it cooled from magma, settled as sediment or was changed by heat and pressure?

“The term rock is used to describe a naturally occurring solid aggregate of one or more minerals” (Geoscience Australia). The way the minerals fit together records how the rock formed.

This unit · about 30 minutes

  1. Read how the three rock types form.
  2. Classify eight rocks from their descriptions.
  3. Take the mastery check.

Three ways to make a rock

Igneous rock crystallises from magma. Magma that cools slowly underground “over many thousands or millions of years” forms coarse-grained intrusive rock; lava that cools quickly at the surface forms fine-grained or glassy extrusive rock, because “mineral crystals don’t have much time to grow” (USGS). Granite is coarse-grained and “rich in quartz and feldspar”, with some mica; basalt is a fine-grained extrusive rock (USGS) that is “low in silica content” (Britannica).

Sedimentary rock forms from fragments that are “eroded, transported, and then deposited as sediments”, then “compressed and cemented” (Earle, 2019). Clastic rocks are named by grain size: conglomerate has clasts larger than 2 mm, sandstone has sand-sized grains, and mudrock is made of silt and clay (Panchuk, 2019). Limestone, made of calcite, fizzes in dilute acid (King, Geology.com).

Metamorphic rock forms when rock is “subjected to high heat, high pressure, hot mineral-rich fluids” without melting (USGS). Pressure lines up flat minerals to give foliation. Shale becomes slate, limestone becomes marble, sandstone becomes quartzite, and in gneiss the minerals separate into bands of different colours (Earle, 2019; Panchuk, 2019). All three types are linked in the rock cycle; The rock cycle and Rock identification go further.

Igneous, sedimentary or metamorphic?

South Australian rocks

Encounter Bay Granites

Granites around Victor Harbor with “a distinctive opalescent blue quartz” and large crystals of potash feldspar (Milnes, 1973), dated to about 510 million years ago, in the Cambrian (Milnes and others, 1977).

Building stone

Parliament House in Adelaide is “built of Kapunda marble on a plinth of granite quarried from West Island near Victor Harbor”. Mintaro slate is used for paving and slate work, and Willunga’s quarries produced almost 30 million roofing shingles (Department for Energy and Mining SA).

A trade name

Black Hill “granite”, quarried near Mannum, is geologically a gabbro (Department for Energy and Mining SA). The trade uses “marble” loosely too: it “covers all carbonate rocks capable of taking a good polish”, including limestones, though Angaston Marble is “a true marble” (Young, 1993). Geologists name a rock by its minerals and texture, not its sales name.

2Soil

Unit 4: soil.

How do rock, air, water and living things together make soil, and why does it matter what a soil formed from?

Soil is where all four spheres meet: weathered rock from the geosphere, water and air in its pores, and the organisms that live in it and add organic matter.

This unit · about 40 minutes

  1. Read what soil is made of and how it forms.
  2. Sort seven weathering processes.
  3. Explore the soil profile, the ribbon test and how soils are used.
  4. Take the mastery check.

What soil is made of

“A typical soil in good condition is composed of approximately 45% mineral matter, 25% air, 25% water and 5% organic matter” (Jones and Goode, 2020). Air and water share the pore space, so their balance changes as soil wets and dries (Earle, 2019). Soil is also alive: “A teaspoon of productive soil generally contains between 100 million and 1 billion bacteria” (Ingham, USDA), and “There are more organisms in 1 gram of healthy soil than there are people on Earth” (FAO, 2020).

Show the numbers as a table

How soil forms

Soil starts with weathering. Physical weathering breaks rock apart: water freezing in cracks, salt crystals growing in pores, roots forcing cracks open, and rock cracking in sheets as the load above it is removed. Chemical weathering changes the minerals: feldspar turns to clay such as kaolinite, weak carbonic acid dissolves calcite, and iron is converted to red hematite (Earle, 2019). Organic matter builds up and decays as the soil develops (Earle, 2019).

Hans Jenny (1941) named five soil-forming factors: climate, organisms, relief, parent material and time (Canadian Society of Soil Science). Soil forms slowly. The FAO says “it can take up to one thousand years to form just one centimeter of soil” (FAO, 2012), and on dated basalt flows in north Queensland soils form at about 0.3 mm per 1,000 years in the semi-arid inland, and up to about 4 mm per 1,000 years on the wet Atherton Tableland (Australian National University).

Physical or chemical weathering?

A soil profile

Horizon descriptions after Earle (2019) and the FAO. The drawing is schematic; real profiles vary, and not every soil has every horizon.

A soil’s origin and its uses

“South Australian soils are largely formed from ancient, highly weathered materials” (Department for Environment and Water, 2021). Australian soils are grouped into orders in the Australian Soil Classification: Vertosols, for example, are clay soils that “crack open when dry”, and Calcarosols are “lime-rich soils” (Queensland Government; CSIRO).

Clay soils matter for houses. About 20 to 30% of Australian surface soils are reactive, and “repeated drying and wetting can cause the ground to shrink and swell beneath foundations”; “South Australia is particularly exposed” (Adelaide University, 2026). Building sites are rated under the Australian Standard AS 2870 from Class A, “Mostly sand and rock with little or no ground movement”, to Class E, extremely reactive clay that can move more than 75 mm (Queensland Building and Construction Commission). Sandy soils, by contrast, hold little water and few nutrients (Queensland Government).

At Coonawarra, red terra rossa soil formed over limestone. The soil scientist Robert White describes quartz and clay left behind as the limestone dissolved (quoted by Edis); Wine Australia (2016) describes winds that “blew silt from nearby plains”, making a clay that “oxidised into the red-coloured ‘terra rossa’ soil”, with a hard calcrete layer beneath that “curbs deep root growth and controls the vines vigour” (Wine Australia, 2016). The Cabernet Sauvignon vines there “produce low yields of grapes that are highly concentrated in chemical compounds called flavonoids” (Edis).

3Dating

Unit 5: relative ages.

Without knowing any dates, how can you tell which rock is older?

Long before radioactivity was discovered, geologists worked out the order of events in rocks from a few simple principles and from fossils. That order is the relative geological time-scale.

This unit · about 35 minutes

  1. Read the principles of relative dating.
  2. Step through the cross-section, then put its seven events in order.
  3. Take the mastery check.

The principles

Superposition: sedimentary layers are deposited in sequence, and “the layers at the bottom are older than those at the top” (Earle, 2019). Original horizontality: “As a consequence of gravity, most sediments are deposited in horizontal layers” (Lumen Learning), so tilted layers were tilted later. Nicolaus Steno set out these principles in 1669 (Columbia University).

Cross-cutting relationships: “any geological feature that cuts across, or disrupts another feature must be younger than the feature that is disrupted”. Inclusions: “any rock fragments that are included in rock must be older than the rock in which they are included” (Earle, 2019).

Fossils add a fifth tool: “fossils succeed one another in a definite and recognizable order” (Penn State). Index fossils, from life forms “which existed during limited periods of geologic time”, are used “as guides to the age of the rocks” (USGS), and correlation is “the technique of piecing together the informational content of separated outcrops” (Britannica). William Smith used index fossils as markers and mapped the rocks of England, Wales and southern Scotland (USGS; Britannica).

Put the events in order

3Dating

Unit 6: absolute ages.

How do radioactive atoms act as a clock, and how did they show that the Earth is 4.5 billion years old?

Relative dating gives an order. Radioactive decay turns that order into ages in years, and it changed the estimated age of the Earth from millions of years to billions.

This unit · about 40 minutes

  1. Read how radioactive decay works.
  2. Run the decay simulation for at least four half-lives, then use the age calculator.
  3. Follow how the age of the Earth was found.
  4. Take the mastery check.

A radioactive clock

In radioactive decay “an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter)” (USGS). The half-life is “the time it takes for one-half of a particular radioactive isotope in a sample to decay” (USGS). After one half-life 50% of the parent is left, after two 25%, after three 12.5%. Measure the ratio of parent to daughter in a mineral, and you can work out how many half-lives have passed since it formed.

Zircon is the favourite mineral for this: it takes in uranium but “excludes with great efficiency the incorporation of lead”, and it “is highly resistant to change” (Britannica). Almost all the lead in a zircon came from decay.

Show the numbers as a table

How old is the Earth?

1650s

Archbishop James Ussher dated the creation to 4004 BCE from biblical chronology (Britannica).

1899

Lord Kelvin, assuming the Earth had been cooling since it formed, “concluded in 1899 that the Earth was between 20 and 40 million years old” (Britannica). John Joly, from the salt in the sea, estimated that the oceans had formed 80 to 90 million years ago.

1905–1911

Bertram Boltwood used uranium–lead decay to work out the ages of 43 minerals; the oldest came out at 2.2 billion years (Britannica; APS News, which dates the work to 1907). Arthur Holmes gave the first numerical ages for several Palaeozoic periods (Britannica).

1956

Clair Patterson dated meteorites by lead isotopes at 4.55 ± 0.07 billion years, in a paper titled “Age of meteorites and the earth” (Patterson, 1956). The USGS now gives the Earth’s age as 4.54 billion years, “with an uncertainty of less than 1 percent”.

The oldest known piece of Earth is a zircon from the Jack Hills in Western Australia, 4,404 ± 8 million years old (Wilde and others, 2001); in 2014 another study confirmed a 4.4-billion-year age for a Jack Hills zircon (Valley and others). The oldest dated rock is the Acasta Gneiss in Canada, about 4.03 billion years old (USGS; Bowring and Williams, 1999). Among the youngest rocks are new lava flows: by mid-June 2018 lava from Kīlauea had added about 100 hectares of new land to Hawaiʻi (USGS). Partly because there are so few usable fossils in very old rocks, the time before the Ediacaran Period is divided by fixed ages in years rather than by fossils (International Commission on Stratigraphy; Van Kranendonk and others, 2012).

3Dating

Unit 7: carbon-14.

How can a shell from a midden be dated, and why does the method stop at about 50,000 years?

Carbon-14 has a short half-life, so it dates the recent past: the remains of living things from the last few tens of thousands of years, including the long history of Aboriginal people in Australia.

This unit · about 30 minutes

  1. Read how carbon-14 dating works and why it has a limit.
  2. Use the age calculator to see how little carbon-14 is left in old samples, and read the Australian examples.
  3. Take the mastery check.

How it works

“Carbon-14 is continually formed in nature by the interaction of neutrons with nitrogen-14 in the Earth’s atmosphere”, and “it is absorbed from the air by green plants and then passed on to animals through the food chain. Once the organism dies, however, it ceases to absorb carbon-14” (Britannica). From then on, carbon-14 halves every 5,730 years. The method was developed by Willard Libby about 1946 (Britannica).

Radiocarbon dating works for material “from 500 to 50,000 years old” (Britannica). After 50,000 years, about 8.7 half-lives, only about a quarter of one percent of the original carbon-14 remains, and in old samples “the fraction of contaminant needed to have measurable effect is quite small” (Britannica). Ages also need calibration, because atmospheric carbon-14 “has varied through time due to cosmic ray fluctuations” (University of Waikato); Australian samples use the Southern Hemisphere calibration curve, SHCal20 (Hogg and others, 2020).

Dating Australia’s past

Riverland middens, SA

Radiocarbon dating at ANSTO of river mussel shells from middens overlooking the Pike River floodplain, downstream of Renmark, in work with the River Murray and Mallee Aboriginal Corporation, extended the record of Aboriginal occupation “to around 29,000 years” (ANSTO, 2020).

Lake Mungo, NSW

Mungo Lady was first given radiocarbon ages of about 20,000 to 26,000 years. In 2003, 25 optical ages showed that both Mungo Lady and Mungo Man were buried 40,000 ± 2,000 years ago (Bowler and others, 2003), near the limit of carbon-14. Mungo Lady was returned to Country in 1992 and Mungo Man in 2017 (SBS NITV, 2022).

Megafauna

Burial ages from 28 sites put the extinction of Australia’s megafauna at about 46,400 years ago (Roberts and others, 2001), close to the limit of carbon-14. Life through deep time weighs the evidence on what caused it.

Madjedbebe, NT

Carbon-14 ages of charcoal reach only about 34,000 years at this rock shelter on the western edge of the Arnhem Land plateau, in Mirarr Country; optical dating of the sediments showed that people were there about 65,000 years ago (Clarkson and others, 2017).

4Put it together

Unit 8: putting it together.

Can you choose the right tool for each question about the geosphere, and combine them?

A geologist in the field uses all of this at once: minerals to name a rock, the rock to read how it formed, relative dating for the order of events and radioactive decay for the ages.

This unit · about 25 minutes

  1. Check your mastery map on screen 1, and go back to any unit you have not mastered.
  2. Choose the dating method for six samples.
  3. Take the final mastery check, which mixes all the units.

Your mastery so far

Which dating method?

In your own words

5Show what you know

Show what you know.

Nine questions, one for each of the I can statements on screen 1. You get one try at each, so think before you tap.

Why is opal a problem for the definition of a mineral?

Calcite has the formula CaCO₃. Which class of minerals is it in?

A black, glassy mineral is about as hard as a steel blade and breaks along two cleavages at nearly right angles. What is it most likely to be?

Why is diamond grit used on drill bits and saws?

A rock from beside a granite intrusion is made of sugary, interlocking calcite crystals, fizzes in dilute acid, and has no fossils or layers. Which type is it, and how did it form?

A soil forms from a granite. Which list best describes what that soil is made of?

A fault breaks layers 1, 2 and 3, but layer 4 above lies across it unbroken. What is the order of events?

A mineral has 25% of its original potassium-40 left. The half-life of potassium-40 is 1.25 billion years. How old is the mineral?

Which sample could be dated by carbon-14?

Printable activity

Mastery record and test bench sheet

Three pages: a mastery record for the eight units (date, form, score, what to fix), a test bench table for real mineral specimens, and a half-life record sheet for a coin or dice practical, with a grid to plot the decay curve.

Mastery record (PDF) Editable (Word)
For teachers

Curriculum

SACE Board of South Australia, Stage 1 Earth and Environmental Science Subject Outline (reissued for 2025), Topic 2: Composition of the Geosphere. NT students take the subject as part of the NTCET. The topic outline begins:

This topic introduces students to the geosphere, the solid part of the Earth. Students investigate how minerals are classified and used according to their properties, while rocks are classified according to their origins through igneous, sedimentary, and metamorphic processes, as well as according to their compositions and textures. Students also investigate the formation of soil resulting from interactions between the spheres, and how the development of the geological time-scale based on fossil evidence and geological principles has led to revisions of the age of the Earth.

Students explore the multidisciplinary nature of earth and environmental science, with physical, chemical, and geological aspects relevant in concepts such as radioactive decay, the formulae of minerals, and the evidence of fossils. Students develop their numeracy skills by representing the half-life of radioactive elements graphically and identifying the trend in radioactivity levels as elements decay.

The topic’s Science Understanding statements and dot points, all of which this lesson covers:

Minerals are constituents of the geosphere and are classified according to their chemical composition.

  • Discuss the limitations of using criteria to categorise substances as minerals.

Minerals may be identified by their characteristic properties.

  • Identify common rock-forming minerals by observing their properties.
  • Discuss how the uses of minerals are related to their properties.

Rocks are composed of characteristic assemblages of mineral crystals or grains that are formed through igneous, sedimentary, and metamorphic processes, as part of the rock cycle.

  • Classify rocks as one of igneous, sedimentary, or metamorphic by identification of their characteristic minerals and texture.
  • Describe the processes that form igneous, sedimentary, and metamorphic rocks.

Interactions between the geosphere, atmosphere, hydrosphere, and biosphere lead to the formation of soil.

  • Describe the composition of soil as rock and mineral particles, organic material, water, gases, and living organisms
  • Discuss the relationship between a soil, its origin, and its possible uses.

Radioisotopes have been used to establish an absolute time-scale and place the age of the Earth at 4.5 billion years old.

  • Explain how the half-life of radioactive elements is used to date Earth materials.

Radioisotopes have been used to date organic remains.

  • Describe how carbon-14 is used to date middens and megafauna up to 50 000 years old.

The relative geological time-scale has been constructed using fossils and geological principles.

  • Explain, with the aid of diagrams, how the relative ages of rock strata can be determined using fossil evidence and the principles of superposition, cross-cutting relationships, inclusions, and correlation.

Where the lesson covers them:

  • Minerals and their classification by chemical composition; the limitations of the criteria: screen 2 (the mineral sort and the class sort), x1 and x2.
  • Identifying common rock-forming minerals by their properties; how uses depend on properties: screen 3 (the test bench with quartz, feldspar, biotite, muscovite, calcite, augite, kaolinite, magnetite and galena; the uses sort), x3 and x4.
  • Classifying rocks by minerals and texture; the processes that form them: screen 4 (the rock sort, the rock cycle, South Australian examples), x5.
  • The composition of soil, its formation through interactions of the spheres, and how its origin affects its uses: screen 5 (soil make-up, the weathering sort, the soil profile, the ribbon test, foundations and Coonawarra), x6.
  • Relative dating by superposition, cross-cutting relationships, inclusions, fossils and correlation, with a diagram: screen 6 (the cross-section stepper and the event order), x7.
  • Half-life, absolute dating and the age of the Earth: screen 7 (the half-life chart, the decay simulation and its decay curve, the age calculator, the history from Ussher to Patterson), x8.
  • Carbon-14 dating of middens and megafauna up to about 50,000 years: screen 8 (Riverland middens, Lake Mungo, megafauna, Madjedbebe), x9.

The lesson uses these of the outline’s possible contexts: properties such as colour, streak, cleavage, hardness, lustre, density, magnetism and reaction to dilute hydrochloric acid; quartz, feldspar, biotite, muscovite, calcite, augite and kaolinite, with two metallic ores; diamond in industry; a simple rock cycle; local rocks and building stone; the life of a grain of quartz (the notebook task on screen 9); physical and chemical weathering; soil suitability for building and growing plants; why time before the Ediacaran is defined by absolute age; radioisotopic dating as evidence of Aboriginal occupation; some of the oldest and youngest rocks; decay curves from a half-life simulation; and how dating techniques changed the estimated age of the Earth.

Stage 1 is assessed through an investigations folio and skills and applications tasks. The test bench and the ribbon test can start a practical with real specimens and soils; the decay simulation and the age calculator suit a data-analysis task.

How the lesson is taught

The lesson uses mastery learning. Each of the eight units ends with a five-question check. A student who gets four or more right has mastered the unit. A student who does not sees a short corrective summary of the unit, then takes a second, parallel form of the check. The mastery map on screen 1 and screen 9 shows each unit’s status.

The approach goes back to Bloom, who argued that “Most students (perhaps over 90 percent) can master what we have to teach them”, given enough time (Bloom, 1968). Guskey describes the cycle the lesson follows: a formative check, then correctives, then a second chance, noting that “High-quality corrective instruction is not the same as ‘reteaching’” and that “corrective activities typically add about 10–20 percent more time to initial learning units” (Guskey, 2010).

The evidence is positive but disputed. A meta-analysis of mastery programs found “The average effect size in the 103 studies was 0.52”, with effects “stronger on the weaker students in a class” (Kulik and others, 1990). Slavin, looking only at group-based programs of at least four weeks, found “essentially no evidence” of effects on standardised tests (median effect size +0.04), and moderate effects on experimenter-made tests, with “little evidence that they were maintained over time” (Slavin, 1987). Part of the difference is the tests: both reviews found larger effects on tests written for the program than on standardised tests. The Education Endowment Foundation rates mastery learning at “+5 months” but says “The security of the evidence around mastery learning is rated as low”, and warns that “the approach appears to be much less effective when pupils work at their own pace” (EEF).

That last point matters for an interactive lesson, which students could otherwise race through alone. The suggestions below keep the class moving together, with the checks as the formative assessment and the teacher deciding the pace.

  • Timing. Just under six hours with correctives: seven 50-minute lessons. Lesson 1: screens 1 and 2. Lesson 2: screen 3. Lesson 3: screen 4. Lesson 4: screen 5. Lesson 5: screen 6, then start screen 7. Lesson 6: finish screen 7, then screen 8. Lesson 7: screens 9 and 10.
  • The mastery bar. Four out of five (80%) is the pass mark, at the bottom of the 80 to 90% the EEF describes. Students who pass on form A can take form B as extra practice, or move to the enrichment ideas below.
  • Correctives. The box that appears after a missed check is a summary, not a full reteach. Use it as a starting point: pair a student who has mastered the unit with one who has not, or take the group who missed it for ten minutes while others do enrichment.
  • Keeping the class together. Run each unit as a lesson segment: teach, then everyone takes the check at the same time. Correctives and form B follow, while those who have mastered it do enrichment.
  • Enrichment. Test real specimens or soil samples; find the rock types used in local buildings or headstones; date the strata in a local cutting; or research one of the disagreements listed below.
  • Saved work. Answers and notes save in the browser on the device being used. Students who change devices should keep a record on the printable sheet.

Handle with care

  • Screen 8 discusses the dating of Aboriginal burials and occupation sites. Mungo Lady was returned to Country in 1992 and Mungo Man in 2017, and in 2022 the federal government approved the reburial of the remains of 108 Aboriginal people from Lake Mungo, including both, with the timing and method left to the Aboriginal Advisory Group (SBS NITV, 2022). Keep the focus on what dating shows about a very long history of occupation, and consider inviting a local Aboriginal community member or education officer to speak about Country in your area.
  • Some students hold religious beliefs about the age of the Earth. Screen 7 presents Ussher’s chronology as a historical estimate, alongside Kelvin’s and Joly’s, before radioactive dating. Present the evidence and the method; the lesson does not ask students to give up a belief.
  • Dilute hydrochloric acid, if you add a real acid test, needs the usual safety precautions. The test bench on screen 3 lets students see the result without acid.

Misconceptions the screens target

  • Anything dug up is a mineral (screen 2, x1). Coal, lab-grown diamond and liquid water are not; ice and mercury are.
  • Colour identifies a mineral (screen 3, x3). Colour varies; streak, hardness and cleavage are more reliable.
  • Rocks and minerals are the same thing (screens 2 and 4). A rock is an assemblage of minerals.
  • Metamorphic rocks have melted (screen 4, x5). Metamorphism changes rock without melting it.
  • Soil is just broken rock (screen 5, x6). It is also water, air, organic matter and living things.
  • After two half-lives the parent is all gone (screen 7, x8). Each half-life halves what is left: 50%, 25%, 12.5%.
  • Carbon-14 dates rocks, or dinosaurs (screen 8, screen 9, x9). It dates organic remains up to about 50,000 years old.
  • Relative dating is guesswork (screen 6, x7). It follows rules that give a definite order, even without ages in years.

About the data and the models

  • The test bench (screen 3) uses property values from the Handbook of Mineralogy and Webmineral, simplified for the lesson. Real specimens vary; biotite and feldspar are groups with a range of compositions.
  • The cross-section (screen 6) is invented for practice and is not to scale.
  • The decay simulation (screen 7) gives each of 100 atoms a 50% chance of decaying in each half-life, using a fixed random sequence so that every student sees the same run. Its curve wobbles around the expected one, as a dice or coin practical does.
  • The age calculator (screen 7) uses the USGS half-lives (uranium-238 4.5 billion years, potassium-40 1.25 billion years, carbon-14 5,730 years) and ignores the starting amount of daughter, which real methods must account for.
  • The half-life chart (screen 7) uses a logarithmic scale; the bars would otherwise be invisible beside uranium-238.
  • The soil chart (screen 5) shows a typical soil in good condition (Oregon State University Extension); real soils vary widely.
  • The soil profile (screen 5) is schematic. Not every soil has every horizon.
  • Quotations in the screens are short excerpts checked against the source pages; where a sentence continues, the lesson quotes only the part shown.

Where sources disagree

  • Half-lives: potassium-40 1.25 billion years (USGS, Britannica) or 1.3 billion years (Earle); rubidium-87 48.8 billion years (USGS) or 47 billion years (Earle). The lesson uses the USGS figures.
  • The limit of carbon-14: 50,000 years (Britannica, “Carbon-14 dating”), about 60,000 years (Britannica’s section on cosmogenic methods; Earle), or useful to 40,000 years (other laboratories). “About 50,000 years” follows the subject outline.
  • Why the limit exists: no source we checked says in its own words that too little carbon-14 is left to measure. Britannica gives contamination as the problem in old samples; the figure of about a quarter of one percent after 50,000 years is our own arithmetic.
  • The oldest zircon: up to 4.3 billion years (an older USGS page) or 4,404 ± 8 million years (Wilde and others, 2001), confirmed by Valley and others (2014).
  • The oldest rock: the Acasta Gneiss at 4.03 billion years (USGS; Bowring and Williams, 1999). Rocks at Nuvvuagittuq in Quebec have been dated at 4.28 billion years by one method (O’Neil and others, 2008) and about 3.8 billion years by another (Zimmer, 2014), and the question is unresolved.
  • The age of the Earth: 4.54 billion years (USGS) and 4.55 ± 0.07 billion years (Patterson, 1956). The oldest solids in the Solar System, in meteorites, are 4,567.30 ± 0.16 million years old (Connelly and others, 2012); that is the age of the Solar System, not a competing age for the Earth.
  • Boltwood’s ages: 1905 (Britannica) or 1907 (APS News). Kelvin: an early estimate of 100 million years (Britannica) or 20 to 400 million years (APS News), later narrowed to 20 to 40 million years.
  • Lake Mungo: 40,000 ± 2,000 years by optical dating (Bowler and others, 2003) or between 40,000 and 42,000 years (National Museum of Australia). Mungo Lady was found in 1968 (National Museum of Australia). Some accounts say she was carbon-dated to 42,000 years; the 2003 ages were optical, not carbon-14.
  • Megafauna: Roberts and others (2001) give extinction across the continent about 46,400 years ago; a 2005 study at Cuddie Springs argued for survival to about 30,000 years, and Grün and others (2010) argue the fauna there is older than those ages.
  • Steno’s principles: 1669 (Columbia University) or “mid-1600’s” (Lumen Learning). William Smith worked around 1800; one university page calls him “17th-century”, which does not fit.
  • Opal: most opal, including precious opal, is amorphous or only poorly crystalline (Webmineral; Mindat), and Mindat calls it “not a true mineral in the accepted sense of the word”, yet it keeps its old status as an approved species (Mindat; Webmineral). The lesson uses it to show the limits of the definition.
  • Building stone names: the stone trade calls all polishable carbonate rocks “marble” and all plutonic igneous rocks “granite” (Young, 1993), so Kapunda Marble and Black Hill granite are trade names. Building marbles are “almost wholly composed of the single mineral calcite” (Young, 1993); the technical note does not say whether Kapunda Marble is a true metamorphic marble.
  • Gypsum: Earle’s table (2015) gives CaSO₄·H₂O, which looks like a typo, as his own text and the Handbook of Mineralogy give CaSO₄·2H₂O. The lesson uses CaSO₄·2H₂O.

Answers at a glance

  • Mineral sort (screen 2): quartz and ice are minerals; liquid water, lab-grown diamond and coal are not; mercury, opal and a mollusc shell are exceptions or borderline cases.
  • Class sort (screen 2): gold native element; galena sulfide; hematite oxide; halite halide; calcite carbonate; gypsum sulfate; quartz and orthoclase silicates.
  • Test bench (screen 3): A quartz, B feldspar (orthoclase), C biotite, D muscovite, E calcite, F augite, G kaolinite, H magnetite, I galena.
  • Uses sort (screen 3): diamond grit, hardness; mica, thin sheets; quartz, an electrical property; galena and iron ores, a useful metal; kaolin and talc, soft, fine and white.
  • Rock sort (screen 4): granite and basalt igneous; conglomerate, sandstone and limestone sedimentary; slate, gneiss and marble metamorphic.
  • Weathering sort (screen 5): frost wedging, salt crystals, roots and unloading are physical; feldspar to clay, limestone dissolving and iron to hematite are chemical.
  • Event order (screen 6): A, B and C laid down; tilted; eroded; D laid down with pebbles of B; the fault; E and F laid down; the dyke G.
  • Method sort (screen 9): the campfire charcoal and the midden shells, carbon-14; the granite and the Jack Hills zircon, uranium–lead; the two quarry layers, relative dating only; the dinosaur bone, cannot be dated by carbon-14.
  • Mastery checks (form A, then form B, questions 1 to 5): what is a mineral? CADBA, CADAB; identifying and using minerals AABDD, BACCA; rocks DBAAD, CCCAB; soil AABCA, CDAAA; relative ages CABDD, DBACD; absolute ages ABDBA, BABCA; carbon-14 BABAB, AAACC; putting it together AAABD, DCACA. Options are lettered A to D in the order they appear.
  • Exit check: x1 opal is not truly crystalline; x2 carbonate; x3 augite; x4 the hardest natural mineral; x5 metamorphic, from limestone; x6 sand, clay, organic matter, water, air and organisms; x7 layers 1 to 3, the fault, layer 4; x8 2.5 billion years; x9 the midden shells.

Saved progress

Answers, mastery results and notes are saved only in this browser on this device. Nothing is sent anywhere.

References

  • Adelaide University (2026, 9 September). Stronger home foundations needed to withstand climate change and cracking soils.
  • Agriculture Victoria (2025). Determining soil texture using ribboning.
  • ANSTO (2020, 15 July). Radiocarbon dating supports Aboriginal occupation of South Australia for 29,000 years.
  • Anthony, J. W., Bideaux, R. A., Bladh, K. W., & Nichols, M. C. (Eds.) (2001–2005). Handbook of Mineralogy. Mineralogical Society of America. Sheets for quartz, orthoclase, albite, muscovite, calcite, augite, kaolinite, magnetite, hematite, galena, pyrite, halite and gypsum.
  • ANU Research School of Earth Sciences (n.d.). How fast do soils form?
  • APS News: Joosse, T. (2024, 11 January). February 1907: Bertram Boltwood estimates Earth is at least 2.2 billion years old.
  • Bloom, B. S. (1968). Learning for mastery. Evaluation Comment, 1(2). UCLA Center for the Study of Evaluation of Instructional Programs. ERIC ED053419.
  • Bowler, J. M., Johnston, H., Olley, J. M., Prescott, J. R., Roberts, R. G., Shawcross, W., & Spooner, N. A. (2003). New ages for human occupation and climatic change at Lake Mungo, Australia. Nature, 421, 837–840. https://doi.org/10.1038/nature01383
  • Bowring, S. A., & Williams, I. S. (1999). Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology, 134, 3–16. https://doi.org/10.1007/s004100050465
  • Branciforte, C., & Haddad, E. (n.d.). GEOS: A Physical Geology Lab Manual for California Community Colleges, 3.2 Assessing hardness values of minerals. Geosciences LibreTexts.
  • Britannica (n.d.). Mineral (chemical compound); Silicate mineral; Feldspar; Mohs hardness; Synthetic diamond; Obsidian; Amber; Granite; Basalt; Weathering; James Ussher; Carbon-14 dating; Geochronology: development of radioactive dating methods; Dating: principles of isotopic dating, importance of zircon in uranium–lead dating, correlation, and carbon-14 dating and other cosmogenic methods.
  • Canadian Society of Soil Science (n.d.). Factors. Soils of Canada.
  • Clarkson, C., Jacobs, Z., Marwick, B., and others (2017). Human occupation of northern Australia by 65,000 years ago. Nature, 547, 306–310. https://doi.org/10.1038/nature22968
  • Columbia University, Lamont–Doherty Earth Observatory (n.d.). Steno (course V1001).
  • Connelly, J. N., Bizzarro, M., Krot, A. N., Nordlund, Å., Wielandt, D., & Ivanova, M. A. (2012). The absolute chronology and thermal processing of solids in the solar protoplanetary disk. Science, 338, 651–655. https://doi.org/10.1126/science.1226919
  • Department for Energy and Mining SA (n.d.). Dimension stone.
  • Department for Environment and Water SA (2021). The Soils of Southern South Australia: Summary of Major Soil Features.
  • Earle, S. (2015). Physical Geology, 2.3 Mineral groups. BCcampus. CC BY 4.0.
  • Earle, S. (2019). Physical Geology (2nd ed.), 3.1, 5.1, 5.2, 5.3, 5.4, 7.2, 8.1, 8.2 and 8.4. BCcampus. CC BY 4.0.
  • Edis, R. (n.d.). The good earth: Coonawarra red dermosol and cabernet sauvignon. The Conversation.
  • Education Endowment Foundation (n.d.). Teaching and Learning Toolkit: Mastery learning.
  • FAO (2012). Securing healthy soils: statement by the FAO Director-General, 145th Session of the FAO Council.
  • FAO (n.d.). Appendix 1: Soil horizon designations (W8594E).
  • FAO Global Soil Partnership (2020). Soil Is Alive! Protect Soil Biodiversity (infographic).
  • Geoscience Australia (n.d.). Australian mineral facts: opal, iron, lead, diamond; Rocks classroom resources.
  • Grün, R., Eggins, S., Aubert, M., Spooner, N., Pike, A. W. G., & Müller, W. (2010). ESR and U-series analyses of faunal material from Cuddie Springs, NSW, Australia: implications for the timing of the extinction of the Australian megafauna. Quaternary Science Reviews, 29(5–6), 596–610. https://doi.org/10.1016/j.quascirev.2009.11.004
  • Guskey, T. R. (2010). Lessons of mastery learning. Educational Leadership, 68(2). ASCD.
  • Hogg, A. G., and others (2020). SHCal20 Southern Hemisphere calibration, 0–55,000 years cal BP. Radiocarbon, 62(4), 759–778. https://doi.org/10.1017/RDC.2020.59
  • Ingham, E. R. (n.d.). Chapter 3: Bacteria. In Soil Biology Primer. USDA Natural Resources Conservation Service.
  • International Commission on Stratigraphy (2022). International Chronostratigraphic Chart v2022/10.
  • Jones, G. B., & Goode, S. (2020). Soil: The Dirty Secrets of a Living Landscape (EM 9304). Oregon State University Extension Service.
  • King, H. M. (n.d.). The “acid test” for carbonate minerals and carbonate rocks. Geology.com.
  • Kulik, C.-L. C., Kulik, J. A., & Bangert-Drowns, R. L. (1990). Effectiveness of mastery learning programs: a meta-analysis. Review of Educational Research, 60(2), 265–299. https://doi.org/10.3102/00346543060002265
  • Lumen Learning (n.d.). Relative dating of earth materials. Geology.
  • Milnes, A. R. (1973). The Encounter Bay Granites, South Australia, and their Environment (PhD thesis). University of Adelaide.
  • Milnes, A. R., Compston, W., & Daily, B. (1977). Pre- to syn-tectonic emplacement of early Palaeozoic granites in southeastern South Australia. Journal of the Geological Society of Australia, 24(2), 87–106.
  • Mindat.org (n.d.). Opal; Hydroxylapatite; Quartz; Plagioclase.
  • National Museum of Australia (n.d.). Defining Moments: Mungo Lady.
  • Nickel, E. H. (1995). The definition of a mineral. The Canadian Mineralogist, 33, 689–690.
  • O’Neil, J., Carlson, R. W., Francis, D., & Stevenson, R. K. (2008). Neodymium-142 evidence for Hadean mafic crust. Science, 321, 1828–1831. https://doi.org/10.1126/science.1161925
  • Panchuk, K. (2019). Physical Geology (First University of Saskatchewan Edition), 9.1 Clastic sedimentary rocks; 10.3 Classification of metamorphic rocks.
  • Patterson, C. (1956). Age of meteorites and the earth. Geochimica et Cosmochimica Acta, 10(4), 230–237. https://doi.org/10.1016/0016-7037(56)90036-9
  • Peck, D. (n.d.). What is a mineral? Mindat.org, with edits by mindat.org members.
  • Penn State (n.d.). Fossils and relative time: the law of faunal succession. GEOSC 10.
  • Perkins, D., and others (n.d.). Mineralogy, 3.9 Reaction to dilute hydrochloric acid. Geosciences LibreTexts.
  • Queensland Building and Construction Commission (n.d.). A guide to preventing structural damage.
  • Queensland Government (2020). Understand and improve your soil; (2025) Common soil types.
  • Roberts, R. G., Flannery, T. F., and others (2001). New ages for the last Australian megafauna: continent-wide extinction about 46,000 years ago. Science, 292, 1888–1892. https://doi.org/10.1126/science.1060264
  • SBS NITV (2022, 11 April). Ancient remains to be reburied at Lake Mungo, ending decades-long campaign.
  • Slavin, R. E. (1987). Mastery learning reconsidered. Review of Educational Research, 57(2), 175–213. https://doi.org/10.3102/00346543057002175
  • University of Waikato Radiocarbon Dating Laboratory (n.d.). Southern Hemisphere radiocarbon calibration.
  • USGS (2026). Mineral Commodity Summaries 2026: diamond (industrial), quartz, sand and gravel (industrial), feldspar, clays, mica, gypsum, salt, iron ore, talc, lead.
  • USGS (n.d.). What is the difference between a rock and a mineral?; What are igneous rocks?; What are metamorphic rocks?; Geologic Time: Age of the Earth, Radiometric time scale, Relative time scale, Fossil succession.
  • USGS Hawaiian Volcano Observatory (2018, 13 June). Photo and video chronology: Kīlauea, June 13, 2018.
  • Valley, J. W., and others (2014). Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography. Nature Geoscience, 7, 219–223. https://doi.org/10.1038/ngeo2075
  • Van Kranendonk, M. J., and others (2012). A chronostratigraphic division of the Precambrian. In The Geologic Time Scale 2012, ch. 16. Elsevier. https://doi.org/10.1016/B978-0-444-59425-9.00016-0
  • Webmineral (n.d.). Opal; Biotite; Plagioclase; Kaolinite; Hematite.
  • Wilde, S. A., Valley, J. W., Peck, W. H., & Graham, C. M. (2001). Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature, 409, 175–178. https://doi.org/10.1038/35051550
  • Wine Australia (2016). Viticulture in Coonawarra.
  • Young, D. (1993). Stone Masonry in South Australia (Heritage Conservation Practice Notes, Technical Note 3.6). Department of Environment and Natural Resources, South Australia; published online 2008 by the Department for Environment and Heritage.
  • Zimmer, C. (2014, March). The oldest rocks on Earth. Scientific American; reprinted on carlzimmer.com.

Sources by screen

Word list