Planet Earth
Section: Earth and Space | Syllabus: Cambridge Primary Science (0846)
Igneous Rock
- How it forms: hot magma or lava cools and solidifies
- Identifying features: contains crystals, no layers, no fossils, often hard
- Examples: granite, basalt, obsidian
- Memory tip: Igneous = "Ignite" (fire) - formed from hot molten rock
Igneous rock forms when magma or lava cools and solidifies
Sedimentary Rock
- How it forms: layers of sediment (sand, mud, dead organisms) pressed together over time
- Identifying features: visible layers, may contain fossils, often crumbly or grainy
- Examples: limestone, sandstone, chalk
- Memory tip: Sedimentary = "Sediment" (bits that settle) - formed from layers
Sedimentary rock forms from layers of sediment pressed together over time
Fossil Formation
- Step 1: the animal dies
- Step 2: it is quickly buried by sand and clay
- Step 3: the soft tissues decompose, leaving the hard bones
- Step 4: more sand and clay build up on top and harden into rock
- Step 5: the bones are gradually replaced by minerals
- Step 6: erosion of the rock years later reveals the fossil
Fossils form in sedimentary rock over millions of years
Worked Example: Identifying Rock Type
- Question: a rock has visible layers and contains a fossil of a shell. What type of rock is it? Explain your answer. [2 marks]
- Answer: sedimentary rock [1]. Sedimentary rocks form from layers of sediment and can contain fossils because they form at low temperatures [1]
Metamorphic Rock
- How it forms: existing rocks are changed by heat and pressure deep underground
- Identifying features: bands or stripes of different minerals, very hard, no fossils (destroyed by heat)
- Examples: marble, slate, quartzite
- Memory tip: Metamorphic = "Metamorphosis" (change) - changed by heat and pressure
Metamorphic rock forms when heat and pressure change an existing rock
Skill: Building an Identification Key for Rocks
- A rock identification key uses a series of yes/no questions about observable features to work out which type of rock you have
- each question must be phrased so it only has a yes or no answer - a question like "how well can you see the crystals?" does not count
- Useful yes/no questions to ask about a rock include:
- does the rock have layers?
- does the rock have crystals?
- does the rock have visible grains?
- does the rock contain fossils?
- does the rock scratch easily (is it hard or soft)?
- is the rock rough or smooth? Is it shiny or dull?
Worked Example: Building a Key
- Question: suggest two questions you could use in a key to identify a rock. [2 marks]
- Answer: any two yes/no questions about an observable feature, e.g. "Does the rock have layers?" [1] and "Does the rock have crystals?" [1]
Soil Components
- Soils can be classified based on the size of their particles and what they contain
- Sand: large particles - water drains through quickly
- Silt: medium-sized particles - holds some water
- Clay: very tiny particles - holds lots of water but can become waterlogged
- Organic matter (humus): dead plant and animal material - contains nutrients (and minerals) for plants
- the higher a soil's organic content, the more minerals it usually contains, and the better it holds air and water
- Soil also contains minerals, small rock fragments, air (in the gaps between particles), and living organisms (such as earthworms and insects)
Classifying Soil by Colour
| Soil Colour | Composition | Property |
|---|---|---|
| Black | Lots of organic content | Smells bad |
| Brown | Organic content and sand | Traps air and water |
| Red | Iron and sand | Poisons some plants |
| Yellow | Mostly sand | Water leaves the soil |
| Grey | Mostly clay | Water remains in soil |
Sandy Soil
- Main content: mostly sand
- Properties: drains quickly, doesn't hold nutrients well, warms up fast
Large sand particles leave big gaps, so water drains through quickly
Clay Soil
- Main content: mostly clay
- Properties: holds water well, can become waterlogged, slow to warm
Tiny, tightly packed clay particles hold water but drain slowly
Loam Soil
- Main content: a mix of sand, silt, clay, and humus
- Properties: best balance - holds water AND drains well, rich in nutrients
Loam mixes all the particle sizes together, giving the best balance for plant growth
How Soil Composition Affects Plant Growth
- The composition of soil can change over time, which affects how well plants grow
How Soil Supports Plant Growth
- Plants need water - soil must hold enough water for roots to absorb
- Plants need nutrients - these come from organic matter (humus) in the soil
- Plants need air - roots need oxygen, so soil shouldn't be too compacted
How Soil Composition Can Change
| Change | Effect on Plants |
|---|---|
| Adding compost (organic matter) | Supports growth: adds nutrients, improves water retention |
| Soil becomes compacted | Hinders growth: less air for roots, water cannot drain |
| Heavy rain washes away topsoil | Hinders growth: nutrients are lost |
| Drought dries out soil | Hinders growth: not enough water for roots |
Worked Example: Adding Compost
- Question: a farmer adds compost to their field each year. Explain how this helps plants grow. [2 marks]
- Answer: compost adds organic matter / humus to the soil [1]. This provides nutrients for the plants / improves the soil's ability to hold water [1]
Erosion
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