Earth is not solid all the way through. It is built in layers, each with a different temperature, state and composition.
🧅 Four main layers
From the outside in: the thin, rocky crust, the thick mantle of hot, slowly flowing rock, the liquid outer core (iron and nickel) and the solid inner core. The inner core is solid because the pressure is so high, even though it is the hottest layer.
Each layer differs in state and temperature; the planet gets hotter towards the centre.
🌡️ Temperature and the lithosphere
Temperature and pressure rise with depth. The crust and the rigid top of the mantle make up the lithosphere, which is broken into tectonic plates and floats on the softer asthenosphere.
temperature rises about 25 °C every km in the crust
4 km down: 4 · 25 = 100 °C hotter than the surface
Earth’s diameter = 2 · radius = 2 · 6400 = 12 800 km
inner core diameter = 2 · 1270 = 2540 km
Scale: the crust is thinner compared with the whole Earth than an apple’s skin is compared with the apple.
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Liquid layer
Which layer of the Earth is liquid?
It surrounds the solid centre.
✏️ Quick check · 2 of 7
Crust
The layer we live on is the:
Outside.
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Inner core
The inner core is solid because of:
Pressure.
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Lithosphere
The lithosphere includes:
Rigid shell.
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Earth’s radius is 6400 km.
What is Earth’s diameter, in km?
Enter an exact answer, without rounding.
km
Double the radius.
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The inner core has a radius of 1270 km.
What is its diameter, in km?
Enter an exact answer, without rounding.
km
Double the radius.
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Temperature rises about 25 °C for every km below the surface.
How much hotter is it 4 km down, in °C?
Enter an exact answer, without rounding.
°C
Multiply.
📝 Lesson quiz · 5 questions
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Re-read the lesson, then give it another go.
Question 1 of 5
The thickest layer of the Earth is the:
Question 2 of 5
The outer core is made mostly of:
Question 3 of 5
Which layer is the hottest?
Question 4 of 5
Scientists learn about the inside of the Earth mainly using:
Question 5 of 5
The mantle is best described as:
Lesson 2 · Materials
🪨 Rocks & the Rock Cycle
Rocks are made of minerals and are always changing. The rock cycle shows how one type can turn into another over millions of years.
🔥 Three rock types
Igneous rocks form when magma or lava cools and hardens (granite, basalt). Slow cooling underground gives large crystals; fast cooling at the surface gives small crystals. Sedimentary rocks form when sediments are pressed and cemented together in layers (sandstone, limestone) and often hold fossils. Metamorphic rocks are changed by heat and pressure (marble from limestone, slate from shale).
Cooling, weathering, compaction, heat and pressure, then melting: the cycle never stops.
🔄 The rock cycle
Any rock can become any other. Weathering and erosion break rock into sediment, burial and pressure create sedimentary rock, heat and pressure create metamorphic rock, and melting makes magma, which cools into igneous rock.
sediment layers build up 2 mm per year
in 500 years: 500 · 2 = 1000 mm = 1 m
3 rock types, 4 processes: weathering, burial, heat/pressure, melting
Fossils are found almost only in sedimentary rock, because heat would destroy them in the other two types.
✏️ Quick check · 1 of 7
Igneous
Which rock is igneous?
Cooled magma.
✏️ Quick check · 2 of 7
Metamorphic
Marble is formed when limestone is changed by:
Metamorphic means changed.
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Fossils
Fossils are most likely to be found in:
Layers of sediment.
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Crystal size
Large crystals in an igneous rock show that it:
Slow = large.
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How many main types of rock are there?
Give the number.
Enter an exact answer, without rounding.
Count them.
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Sediment layers build up at 2 mm per year.
How thick is the layer after 500 years, in mm?
Enter an exact answer, without rounding.
mm
Multiply.
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A layer of rock is 1000 mm thick. How many metres is that?
Give the number.
Enter an exact answer, without rounding.
m
Divide by 1000.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Sedimentary rock forms by:
Question 2 of 5
Basalt has tiny crystals because it:
Question 3 of 5
Which process starts the formation of sediment?
Question 4 of 5
Slate is formed from:
Question 5 of 5
The rock cycle shows that:
Lesson 3 · Materials
💎 Minerals
Minerals are the building blocks of rocks. Each has properties that scientists use to identify it.
🔬 What is a mineral?
A mineral is a naturally occurring, solid, inorganic substance with a definite chemical composition and a regular crystal structure. Rocks are mixtures of minerals. Common examples: quartz, feldspar, mica, calcite and halite.
mineral: quartz (SiO₂)
rock: granite = quartz + feldspar + mica
NOT minerals: coal (organic), glass (no crystal structure), water (liquid)
🖼️ See it
A harder mineral scratches a softer one. A fingernail (about 2.5) scratches talc and gypsum but not calcite.
🧪 Identifying minerals
Tests include colour, streak (colour of powder), lustre (shiny or dull), hardness on the Mohs scale (1 talc to 10 diamond), cleavage or fracture, and density.
Mohs scale: 1 talc, 2 gypsum, 3 calcite, 4 fluorite, 5 apatite,
6 orthoclase, 7 quartz, 8 topaz, 9 corundum, 10 diamond
density = mass / volume
270 g in 100 cm³ → 2.7 g/cm³
150 g in 50 cm³ → 3 g/cm³
Colour can mislead. Streak and hardness are more reliable tests.
✏️ Quick check · 1 of 7
Mineral
Which of these is a mineral?
Has a crystal structure.
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Hardest
Which is the hardest mineral on the Mohs scale?
Top of the scale.
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Streak
The streak of a mineral is:
Powder.
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Granite
Granite is a:
A mixture.
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A mineral sample has a mass of 270 g and a volume of 100 cm³.
What is its density, in g/cm³?
Enter an exact answer, without rounding.
g/cm³
density = mass ÷ volume.
✍️ Type your answer · 6 of 7
A sample has a mass of 150 g and a volume of 50 cm³.
What is its density, in g/cm³?
Enter an exact answer, without rounding.
g/cm³
density = mass ÷ volume.
✍️ Type your answer · 7 of 7
Diamond is 10 on the Mohs scale and talc is 1.
What is the difference between them?
Enter an exact answer, without rounding.
Subtract.
📝 Lesson quiz · 5 questions
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Question 1 of 5
A mineral must be:
Question 2 of 5
Quartz can scratch glass because it is:
Question 3 of 5
Cleavage is the way a mineral:
Question 4 of 5
Lustre describes:
Question 5 of 5
Rocks are made of:
Lesson 4 · Surface processes
🏞️ Weathering, Erosion & Deposition
The surface of the Earth is slowly being worn down and rebuilt. Weathering breaks rock, erosion moves it and deposition drops it somewhere new.
🔨 Weathering
Physical (mechanical) weathering breaks rock into pieces without changing its chemistry: frost wedging (water freezes and expands in cracks), plant roots and abrasion. Chemical weathering changes the minerals: acid rain dissolves limestone, and oxygen rusts iron minerals.
frost wedging: water freezes → expands about 9% → cracks widen
acid rain + limestone → dissolves (chemical)
roots split rock (physical)
smaller pieces = more surface area = faster weathering
🖼️ See it
Weathering breaks rock where it is; erosion moves the pieces; deposition lays them down in layers that may become sedimentary rock.
🚚 Erosion and deposition
Erosion is the movement of weathered material by water, wind, ice or gravity. Deposition happens when the moving agent slows and drops its load, forming features such as deltas, dunes and moraines.
a river wears down a surface 2 mm per year
1 m = 1000 mm → 1000 / 2 = 500 years
fast river: carries big pebbles
slow river: drops them → sediment sorted by size
Fast water carries more. When a river slows at its mouth, it drops sediment and builds a delta.
✏️ Quick check · 1 of 7
Frost wedging
Frost wedging happens because water:
Ice takes more room.
✏️ Quick check · 2 of 7
Chemical weathering
Which is chemical weathering?
Changing chemistry.
✏️ Quick check · 3 of 7
Erosion agents
Which is NOT an agent of erosion?
Not at the surface.
✏️ Quick check · 4 of 7
Delta
A delta forms where a river:
Slow = drop.
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A river wears down rock at 2 mm each year.
How many years to wear down 1 m (1000 mm)?
Enter an exact answer, without rounding.
years
Divide.
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A rock cracks into 4 pieces and each of those cracks into 2 pieces.
How many pieces are there now?
Enter an exact answer, without rounding.
Multiply.
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Wind moves sand 3 cm a day.
How far, in cm, in 20 days?
Enter an exact answer, without rounding.
cm
Multiply.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Weathering is:
Question 2 of 5
Which agent carved the Grand Canyon?
Question 3 of 5
A glacier erodes by:
Question 4 of 5
Which would weather fastest?
Question 5 of 5
Sand dunes are formed by:
Lesson 5 · Atmosphere
☔ Water & Weather
Weather is the condition of the atmosphere at a place and time. It is driven by the Sun’s heat and the movement of water.
💧 The water cycle
The Sun’s heat causes evaporation; water vapour rises, cools and condenses into clouds; precipitation (rain, snow, hail) falls; water then runs off over land or soaks in (infiltration) and returns to the oceans. About 97% of Earth’s water is salty and only about 3% is fresh, most of it frozen.
evaporation → condensation → precipitation → collection
Total water 1000 units:
salt 97% = 970, fresh 3% = 30
rain gauge: 5 mm a day for 6 days = 30 mm
🖼️ See it
The Sun’s energy drives evaporation; gravity brings the water back down and back to the sea.
🌤️ Measuring weather
Instruments: thermometer (temperature), barometer (air pressure), anemometer (wind speed), rain gauge (rainfall) and hygrometer (humidity). Falling pressure often means stormy weather, and rising pressure means fair weather.
max 24 °C, min 12 °C → range = 24 − 12 = 12 °C
mean of 10, 14 and 12 = 36/3 = 12 °C
wind 36 km/h for 2.5 h → 90 km
Weather is short-term; climate is the average over 30 or more years.
✏️ Quick check · 1 of 7
Condensation
Condensation is when water vapour:
Vapour to liquid.
✏️ Quick check · 2 of 7
Barometer
A barometer measures:
Pressure.
✏️ Quick check · 3 of 7
Weather vs climate
Climate is:
Long-term.
✏️ Quick check · 4 of 7
Fresh water
Most fresh water on Earth is found:
Frozen.
✍️ Type your answer · 5 of 7
Of 1000 units of Earth’s water, 3% is fresh.
How many units are fresh?
Enter an exact answer, without rounding.
Multiply.
✍️ Type your answer · 6 of 7
A rain gauge collects 5 mm a day for 6 days.
What is the total, in mm?
Enter an exact answer, without rounding.
mm
Multiply.
✍️ Type your answer · 7 of 7
The highest temperature of the day was 24 °C and the lowest was 12 °C.
What is the temperature range, in °C?
Enter an exact answer, without rounding.
°C
Subtract.
📝 Lesson quiz · 5 questions
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Question 1 of 5
The energy that drives the water cycle comes from the:
Question 2 of 5
Which is a form of precipitation?
Question 3 of 5
Wind speed is measured with an:
Question 4 of 5
Humidity is the amount of:
Question 5 of 5
Runoff is water that:
Lesson 6 · Space
🌗 Earth, Sun & Moon
Day and night, the seasons and the phases of the Moon are all caused by how Earth, Sun and Moon move.
🔄 Rotation, revolution and seasons
Earth rotates on its axis once every 24 hours (day and night) and revolves around the Sun once every 365 days (a year). Its axis is tilted at 23.5°. The tilt, not distance, causes the seasons: the hemisphere tilted toward the Sun has summer.
360° in 24 h → 15° per hour
in 6 hours: 6 · 15 = 90°
revolution: 365 days (plus a leap day every 4 years)
tilt: 23.5°
June: northern hemisphere tilted toward the Sun → summer there
🖼️ See it
The Moon is always half lit by the Sun. The phase is how much of that lit half we can see from Earth.
🌙 The Moon
The Moon orbits Earth about every 27 days, and its phases repeat every 29.5 days because Earth moves too. We always see the same side. A solar eclipse happens when the Moon is between the Sun and Earth, and a lunar eclipse when Earth is between the Sun and the Moon. Tides are caused mainly by the Moon’s gravity.
phase cycle ≈ 30 days: new moon → full moon is half = 15 days
phases: new, waxing crescent, first quarter, waxing gibbous,
full, waning gibbous, last quarter, waning crescent → 8 phases
The Moon does not make its own light. It reflects sunlight, and the phase depends on how much of the lit half we can see.
✏️ Quick check · 1 of 7
Seasons
Seasons are caused by:
Tilt.
✏️ Quick check · 2 of 7
Solar eclipse
A solar eclipse happens when:
Moon in the middle.
✏️ Quick check · 3 of 7
Rotation
One rotation of the Earth takes about:
One day.
✏️ Quick check · 4 of 7
Moon light
The Moon shines because it:
Reflects.
✍️ Type your answer · 5 of 7
Earth rotates 360° in 24 hours.
How many degrees does it rotate in 1 hour?
Enter an exact answer, without rounding.
°
Divide.
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Earth rotates 15° every hour.
How many degrees in 6 hours?
Enter an exact answer, without rounding.
°
Multiply.
✍️ Type your answer · 7 of 7
The Moon’s phase cycle is about 30 days. Full moon is half way through from new moon.
After how many days?
Enter an exact answer, without rounding.
days
Half of the cycle.
📝 Lesson quiz · 5 questions
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Question 1 of 5
Earth’s axis is tilted at about:
Question 2 of 5
How many days does Earth take to orbit the Sun?
Question 3 of 5
A lunar eclipse occurs at:
Question 4 of 5
When it is summer in the Northern Hemisphere, it is:
Question 5 of 5
Tides are mainly caused by:
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 · Layers of the Earth
From the surface inward, the correct order is:
Q2 · Layers of the Earth
The mantle is best described as:
Q3 · Rocks & the Rock Cycle
Magma that cools and hardens forms:
Q4 · Rocks & the Rock Cycle
The rock cycle shows that:
Q5 · Minerals
The Mohs scale measures a mineral’s:
Q6 · Minerals
Rocks are made of:
Q7 · Weathering, Erosion & Deposition
Erosion differs from weathering because erosion:
Q8 · Weathering, Erosion & Deposition
Sand dunes are formed by:
Q9 · Water & Weather
Clouds form when:
Q10 · Water & Weather
Runoff is water that:
Q11 · Earth, Sun & Moon
The Moon always shows the same side to Earth because: