Rocks are a record of Earth’s history. Geologists use layers, fossils and radioactive decay to put events in order and to give them ages.
📚 Relative dating
The law of superposition: in undisturbed sedimentary layers, the oldest are at the bottom and the youngest at the top. An intrusion or a fault is younger than the rock it cuts. Index fossils (widespread species that lived a short time, such as trilobites) match layers in different places. Fossils form when remains are buried quickly and minerals replace or fill the hard parts.
layers from top: A, B, C, D
oldest = D (bottom), youngest = A (top)
fault cuts layers B, C and D but not A
→ fault is younger than B, C, D and older than A
🖼️ See it
The first events are not drawn to scale. Humans appear in the last fraction of a percent of Earth’s history.
☢️ Radiometric dating
A radioactive isotope decays at a steady rate. The half-life is the time for half of the atoms to decay. By measuring how much parent isotope is left, we find the age. Earth is about 4.6 billion years old; life appeared more than 3.5 billion years ago.
half-life 1000 years
after 1 half-life: 1/2 left
after 2: 1/4 left
after 3: 1/8 left → 3 · 1000 = 3000 years
80 g, 3 half-lives: 80 → 40 → 20 → 10 g
Carbon-14 has a half-life of about 5700 years and is used to date organic remains up to about 50 000 years old. Older rocks need isotopes like uranium-238 (4.5 billion years).
✏️ Quick check · 1 of 7
Superposition
In undisturbed layers of sedimentary rock, the oldest layer is:
Bottom.
✏️ Quick check · 2 of 7
Index fossil
A good index fossil lived:
Widespread and brief.
✏️ Quick check · 3 of 7
Fault age
A fault that cuts through rock layers is:
Cut after.
✏️ Quick check · 4 of 7
Half-life
A half-life is the time for:
Half.
✍️ Type your answer · 5 of 7
An isotope has a half-life of 1000 years. A sample has 1/8 of its original amount left.
How old is it, in years? (1/8 = 3 half-lives)
Enter an exact answer, without rounding.
years
Count half-lives.
✍️ Type your answer · 6 of 7
A sample starts with 80 g of a radioactive isotope.
How many grams remain after 3 half-lives?
Enter an exact answer, without rounding.
g
Halve three times.
✍️ Type your answer · 7 of 7
Earth is 4600 million years old. A fossil is 460 million years old.
What percent of Earth’s history has passed since that fossil?
Enter an exact answer, without rounding.
%
Divide, ×100.
📝 Lesson quiz · 5 questions
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Re-read the lesson, then give it another go.
Question 1 of 5
Fossils are most often found in:
Question 2 of 5
Relative dating tells us:
Question 3 of 5
Radiometric dating relies on:
Question 4 of 5
About how old is Earth?
Question 5 of 5
Carbon-14 dating is used for:
Lesson 2 · Atmosphere
🌫️ The Atmosphere
The atmosphere is a thin envelope of gases that gives us air to breathe, protects us from radiation and keeps the planet warm enough for life.
🧪 Composition and layers
Dry air is about 78% nitrogen, 21% oxygen, about 1% argon, and a tiny amount of carbon dioxide (about 0.04%). The layers, from the ground up, are the troposphere (weather, to about 12 km), the stratosphere (ozone layer, to about 50 km), the mesosphere (to about 85 km) and the thermosphere (auroras, very high temperatures). The ozone layer absorbs harmful UV radiation.
78 + 21 = 99% → 1% other gases
1000 L of air: oxygen = 0.21 · 1000 = 210 L
stratosphere thickness = 50 − 12 = 38 km
🖼️ See it
Layers are named for how temperature changes with height. The sketch is not to scale.
🌡️ Energy balance and the greenhouse effect
About 30% of incoming sunlight is reflected (albedo) by clouds, ice and the surface. The rest warms the Earth, which emits infrared radiation. Greenhouse gases (water vapour, CO₂, methane) absorb some of this and re-emit it, warming the surface from −18 °C to about +15 °C. Adding more greenhouse gases strengthens the effect.
incoming sunlight = 100 units
reflected 30 → absorbed = 100 − 30 = 70 units
natural greenhouse warming = 15 − (−18) = 33 °C
The greenhouse effect is natural and essential. The problem is the extra warming from added CO₂ and other gases.
✏️ Quick check · 1 of 7
Most abundant
The most abundant gas in the atmosphere is:
More than three quarters.
✏️ Quick check · 2 of 7
Weather layer
Weather happens in the:
Lowest.
✏️ Quick check · 3 of 7
Ozone
The ozone layer is in the:
Second layer.
✏️ Quick check · 4 of 7
Greenhouse gas
Which is a greenhouse gas?
CO₂.
✍️ Type your answer · 5 of 7
Air is 21% oxygen.
How many litres of oxygen are in 1000 L of air?
Enter an exact answer, without rounding.
L
Multiply.
✍️ Type your answer · 6 of 7
Of 100 units of sunlight, 30 are reflected.
How many units are absorbed?
Enter an exact answer, without rounding.
Subtract.
✍️ Type your answer · 7 of 7
The Earth would be −18 °C without greenhouse gases and is +15 °C with them.
How much warming do they cause, in °C?
Enter an exact answer, without rounding.
°C
Subtract a negative.
📝 Lesson quiz · 5 questions
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Question 1 of 5
The ozone layer protects life by absorbing:
Question 2 of 5
Albedo is the:
Question 3 of 5
Which layer is closest to space?
Question 4 of 5
CFCs damaged the ozone layer, and they were:
Question 5 of 5
The gas most responsible for recent warming is:
Lesson 3 · Resources
⚡ Energy Resources
Modern societies run on energy. Where it comes from affects cost, the environment and the future.
⛽ Non-renewable and renewable
Non-renewable resources (coal, oil, natural gas, uranium) form over millions of years and run out. Burning fossil fuels releases CO₂. Renewable resources (solar, wind, hydro, geothermal, biomass) are replaced naturally and release little or no CO₂ in use. Each has trade-offs: wind and solar are intermittent; hydro changes rivers; nuclear leaves waste.
fossil fuels: coal, oil, gas → CO₂ when burned
renewables: sun, wind, water, heat from Earth
energy = power · time
2 kW for 5 h = 10 kWh
🖼️ See it
Fossil fuels release many times more greenhouse gas per kWh than renewables or nuclear power (typical published figures).
📊 Efficiency and cost
No device turns all its input into useful output; the rest is wasted, usually as heat. Efficiency = useful output ÷ total input × 100%. Electricity is sold by the kilowatt-hour (kWh).
input 200 J, useful 50 J → 50/200 · 100 = 25%
10 kWh at 0.15 per kWh = 1.5
wind turbine 2 MW, 30% of the time:
2 · 0.3 = 0.6 MW average
Saving energy is the cheapest way to cut emissions. Insulation and efficient devices reduce the amount we need.
✏️ Quick check · 1 of 7
Renewable
Which is a renewable resource?
Fossil fuels are not.
✏️ Quick check · 2 of 7
Fossil fuel
Fossil fuels formed from:
Once alive.
✏️ Quick check · 3 of 7
Hydro trade-off
A disadvantage of large hydroelectric dams is that they:
River effects.
✏️ Quick check · 4 of 7
Waste energy
Most wasted energy is lost as:
Thermal.
✍️ Type your answer · 5 of 7
A device takes in 200 J and gives 50 J of useful energy.
What is its efficiency, in %?
Enter an exact answer, without rounding.
%
Useful ÷ total.
✍️ Type your answer · 6 of 7
A 2 kW heater runs for 5 hours.
How many kWh does it use?
Enter an exact answer, without rounding.
kWh
power · time.
✍️ Type your answer · 7 of 7
Electricity costs 0.15 per kWh. A house uses 10 kWh.
What is the cost?
Enter an exact answer, without rounding.
Multiply.
📝 Lesson quiz · 5 questions
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Re-read the lesson, then give it another go.
Question 1 of 5
Burning fossil fuels mainly releases:
Question 2 of 5
Which source is geothermal?
Question 3 of 5
A solar panel converts:
Question 4 of 5
Which is NOT renewable?
Question 5 of 5
A drawback of wind and solar is that they are:
Lesson 4 · Land
🌱 Soil
Soil is a mixture of weathered rock, organic matter, water and air. It takes thousands of years to form and feeds the world.
🪱 Making and layering soil
Soil forms when rock is weathered and organic matter (humus) from dead plants and animals mixes in. Climate, parent rock, living things, slope and time all control it. A soil profile has horizons: O (organic), A (topsoil, rich in humus), B (subsoil, collects minerals washed down), C (broken rock) and then bedrock.
soil forms about 1 cm per 100 years
30 cm of topsoil: 30 · 100 = 3000 years
horizons from top: O, A, B, C, bedrock
🖼️ See it
Soil forms slowly from weathered rock and organic matter. Topsoil is the layer plants depend on and the first to be lost to erosion.
🏜️ Texture, fertility and erosion
Soil texture is the mix of sand (large grains, drains fast), silt and clay (tiny grains, holds water). Loam is a balanced mix, ideal for crops. Bare soil is easily eroded by wind and rain; roots, cover crops, terracing and no-till farming protect it.
sample: 40% sand, 40% silt, clay = 100 − 40 − 40 = 20%
erosion 5 mm a year → in 20 years: 5 · 20 = 100 mm = 10 cm
Healthy soil is alive. A spoonful holds more microorganisms than there are people on Earth.
✏️ Quick check · 1 of 7
Topsoil
The most fertile soil horizon is the:
Humus-rich.
✏️ Quick check · 2 of 7
Humus
Humus is:
Decayed plants.
✏️ Quick check · 3 of 7
Sandy soil
Sandy soil has:
Coarse.
✏️ Quick check · 4 of 7
Erosion protection
Which practice reduces soil erosion?
Roots.
✍️ Type your answer · 5 of 7
A soil sample is 40% sand and 40% silt.
What percent is clay?
Enter an exact answer, without rounding.
%
Subtract.
✍️ Type your answer · 6 of 7
Soil forms about 1 cm per 100 years.
How many years to form 30 cm?
Enter an exact answer, without rounding.
years
Multiply.
✍️ Type your answer · 7 of 7
A field loses 5 mm of soil each year.
How many mm are lost in 20 years?
Enter an exact answer, without rounding.
mm
Multiply.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Soil forms from:
Question 2 of 5
Loam is a:
Question 3 of 5
Terracing helps by:
Question 4 of 5
The C horizon is:
Question 5 of 5
Overgrazing can cause:
Lesson 5 · Hydrosphere
💧 Groundwater & Watersheds
Beneath our feet is the largest store of liquid fresh water. Understanding how it moves helps us protect supplies.
🕳️ Groundwater
Rain infiltrates the soil and fills the pores and cracks in rock. The water table is the top of the saturated zone. An aquifer is rock that holds and transmits useful amounts of water (sand, gravel, sandstone). Porosity is the fraction of empty space; permeability is how easily water flows through. Clay has high porosity but low permeability.
rock volume 200 L with 30% pores: 0.3 · 200 = 60 L of water
groundwater flows about 1 m a day
in 30 days: 30 m
well pumping 50 L/min for 60 min = 3000 L
🖼️ See it
A watershed is the land that drains into one river. Pumping groundwater faster than rain refills it lowers the water table.
🏞️ Watersheds and pollution
A watershed (drainage basin) is the land that drains into a river or lake. Whatever happens upstream affects downstream: fertiliser runoff causes algal blooms and dead zones, and leaking tanks and over-pumping can contaminate or deplete aquifers. Wells that are pumped faster than the aquifer recharges lower the water table.
recharge 100 units per year, pumping 150 per year
net change = 100 − 150 = −50 units/year → water table falls
We all live in a watershed. What goes down a storm drain usually flows untreated to a river.
✏️ Quick check · 1 of 7
Water table
The water table is the:
Saturated.
✏️ Quick check · 2 of 7
Aquifer
An aquifer is:
Holds water.
✏️ Quick check · 3 of 7
Watershed
A watershed is:
Drainage area.
✏️ Quick check · 4 of 7
Over-pumping
Pumping groundwater faster than it recharges:
Falls.
✍️ Type your answer · 5 of 7
A rock sample of 200 L has 30% pore space.
How many litres of water can its pores hold?
Enter an exact answer, without rounding.
L
Multiply.
✍️ Type your answer · 6 of 7
Groundwater flows at 1 m per day.
How far, in m, in 30 days?
Enter an exact answer, without rounding.
m
Multiply.
✍️ Type your answer · 7 of 7
An aquifer gets 100 units of recharge a year and 150 units are pumped out.
What is the net change in units a year? (Use a negative for a loss.)
Enter an exact answer, without rounding.
units
Recharge minus pumping.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Porosity is the:
Question 2 of 5
Which would make a good aquifer?
Question 3 of 5
Fertiliser runoff in a watershed can cause:
Question 4 of 5
Impermeable rock:
Question 5 of 5
A spring is where:
Lesson 6 · Space
🪐 The Solar System
Our Solar System has a star, eight planets and countless smaller bodies, all bound by the Sun’s gravity.
☀️ Planets and distances
The four terrestrial planets (Mercury, Venus, Earth, Mars) are small and rocky. The four giants (Jupiter, Saturn, Uranus, Neptune) are large and gaseous or icy. Between Mars and Jupiter lies the asteroid belt. Distances are measured in astronomical units (AU): 1 AU is the Earth–Sun distance, about 150 million km.
Jupiter is 5.2 AU from the Sun
5.2 · 150 million = 780 million km
light speed 300 000 km/s
Sun → Earth: 150 000 000 / 300 000 = 500 s ≈ 8.3 min
🖼️ See it
1 AU is the Earth–Sun distance. The inner planets are packed close together; the outer ones are far apart.
🎯 Orbits and gravity
Planets move in elliptical orbits. Kepler’s third law: the square of the orbital period (years) equals the cube of the average distance (AU): T² = a³. Gravity depends on mass: weight = mass × g. On the Moon g is about 1.6 N/kg, compared with 10 N/kg on Earth.
a = 4 AU: T² = 4³ = 64 → T = 8 years
mass 60 kg:
Earth weight = 60 · 10 = 600 N
Moon weight = 60 · 1.6 = 96 N
Mass does not change from place to place. Weight does, because gravity does.
✏️ Quick check · 1 of 7
Terrestrial
Which is a terrestrial planet?
Inner four.
✏️ Quick check · 2 of 7
AU
One astronomical unit is the distance from:
Earth–Sun.
✏️ Quick check · 3 of 7
Asteroid belt
The asteroid belt lies between:
Inner and outer.
✏️ Quick check · 4 of 7
Gravity
Gravity at the Moon’s surface is weaker than on Earth because the Moon has:
Smaller.
✍️ Type your answer · 5 of 7
Light travels 300 000 km/s. The Sun is 150 000 000 km from Earth.
How many seconds does sunlight take to reach Earth?
Enter an exact answer, without rounding.
s
time = distance ÷ speed.
✍️ Type your answer · 6 of 7
A planet orbits at 4 AU, so T² = 4³ = 64.
What is its orbital period, in years?
Enter an exact answer, without rounding.
years
Square root.
✍️ Type your answer · 7 of 7
An astronaut has a mass of 60 kg. On the Moon g = 1.6 N/kg.
What is the weight, in N, on the Moon?
Enter an exact answer, without rounding.
N
weight = mass · g.
📝 Lesson quiz · 5 questions
0 / 5
Re-read the lesson, then give it another go.
Question 1 of 5
Which planet is largest?
Question 2 of 5
The Sun is mainly powered by:
Question 3 of 5
The gas giants are:
Question 4 of 5
A planet’s year is longer if it is:
Question 5 of 5
Comets have tails because:
Final Challenge
🏆 12-Question Challenge Quiz
Questions from every topic in this course. Pick an answer for each one, then hit Check My Answers.
0 / 12
💪 Keep practicing — go back through the lessons and retake the quiz!
Q1 · Geologic Time & Fossils
After 2 half-lives, what fraction of the original isotope is left?
Q2 · Geologic Time & Fossils
Carbon-14 dating is used for:
Q3 · The Atmosphere
The atmosphere layer where auroras occur is the:
Q4 · The Atmosphere
The gas most responsible for recent warming is:
Q5 · Energy Resources
Efficiency is:
Q6 · Energy Resources
A drawback of wind and solar is that they are:
Q7 · Soil
Soil takes much longer to form than it takes to lose, so it is:
Q8 · Soil
Overgrazing can cause:
Q9 · Groundwater & Watersheds
Which material has high porosity but low permeability?