Rocks...tackle the tricky bits

Here we focus on those scientific concepts that are the hardest to explain to children. We break it down into what pupils need to know and outline the background science. Even though much of the background science does not need to be taught to primary aged children, it is useful for you as a teacher when addressing misconceptions and children’s challenging questions.

Please refer to your national curriculum documents when planning your sequence of work and ensure that you teach the correct knowledge for your year group.

What do children need to know about rocks? 

Pupils may have picked up a variety of ideas about rocks through their everyday experiences. Valuing their opinions and knowledge is important before developing it further by undertaking hands-on investigations and enquiries. Learning about rocks builds on prior understanding about the differences between things that are living, dead, and things that have never been alive. Before the topic, children may also have compared rocks with a variety of everyday materials, including wood, metal, plastic, glass, brick, paper and cardboard and begun to think about their uses. We have included some ideas for how to teach rocks in part two of this topic guide.

Key scientific concepts: Pupils need to...

1. Compare and group together different kinds of rocks based on their appearance and simple physical properties.

2. Describe in simple terms how fossils are formed when things that have lived are trapped within rock.

3. Recognise that soils are made from rocks and organic matter.

The rocks topic connects to your pupils’ learning in related topics and prepares them for the tricky topics evolution and inheritance. Children will then recognise that living things have changed over time and that fossils provide information about living things that inhabited the Earth millions of years ago.

Background science 

Key ideas about rocks

Many of the misconceptions children have about rocks arise from the use of ‘everyday language’ when describing them. Although children in primary schools do not need to know all the definitions of geological terms, it is useful that teachers use them correctly. This factsheet from the Geological Society is a handy summary of the basic geology.

The terms rock, stone, and pebble are often used in an intuitive, non-scientific way and this can lead to confusion. To a geologist, the word rock is used for a category or type of rock which can be identified by what it was made of and how it was made. Rocks are naturally occurring solids and are not manufactured.

Another misconception that children often have is that rocks and minerals are the same thing. Rocks are made of one or more types of minerals. Minerals are naturally existing chemical compounds; they are the ‘ingredients’ of rocks and different types of rock have different combinations of them. Minerals can form crystals or grains/granules in the rock. The most common minerals in rocks are feldspar and quartz

Examples of pure feldspar and an example of pure quartz. 

Some rocks are made from interlocking mineral crystals that fit tightly together. Rocks with lots of crystals are usually harder and glint in the light. Other rocks seen to be made from layers; these are often gritty and break more easily. Many rocks are made up from broken fragments, or grains/granules, of older rocks and minerals which have been compacted together. Grains can vary in size and so rocks can be described as:

• Coarse grained means the grains are larger than rice e.g., granite and conglomerate.

• Medium grained means that the grains can be seen without magnification e.g., sandstone and many limestones.

• Fine grained means that the grains cannot be seen without magnification e.g., slate and basalt.

Rocks are made in three main ways which are summarised in this BBC film:

1. Molten rock can cool to become igneous rock. This can happen beneath the Earth’s surface slowly which encourages crystal growth e.g., granite. Alternatively, the molten rock can cool quickly, when volcanoes push molten rock above the Earth’s surface e.g., basalt and obsidian.

2. As the sun, wind and rain erode and ‘weather’ a rock, small particles break away. When these collect and are compacted, they form sedimentary rock such as conglomerate, limestone and sandstone.

3. Some rocks that already exist can undergo changes as a result of intense pressure and hot temperatures to become metamorphic rock. Slate is a metamorphic rock of sedimentary origin (shale) and so is marble which was originally limestone. Meanwhile, the igneous rock granite can change into a metamorphic rock called gneiss when it is subjected to intense heat and pressure.

Key ideas about fossils

A fossil is the preserved remains or traces of a dead organism from a past geological age. Fossils are typically older than 10,000 years. There are several types such as footprints (trace) and insects preserved in amber (true form), but the most common way fossils are made is that they are mineralised; they are turned into rock! This process can be broken down into steps and is depicted on this BBC film and this Natural History Museum film:

1. After the living thing died, the soft parts decomposed leaving the hard parts behind. We tend to think these ‘hard parts’ will be skeletons because of the incredible dinosaur fossils in museums. However, the hard shells or exoskeletons of invertebrates can also be fossilised. In fact, according to the Natural History Museum, nearly 99% of the fossils we find are marine animals such as shellfish.

2. The conditions to form a fossil are rare: the shells or skeletons needed to be buried quickly by small particles of rock called sediment. This was more likely to happen in the oceans or swamps. As more layers of sediment built up on top, the sediment around the ‘hard parts’ compacted and turned to rock.

3. Over time, the bones then gradually dissolved as water seeped through the rock. Minerals in the water replaced the bones, leaving a replica (fossil) in the rock of the original bones.

Key ideas about soil

A misconception that children have about soil is that it is dirt – just brown stuff on the ground! Soil is actually a complex mixture of organic matter (the remains of animals and plants that were once living in it) and the sediments of rocks (these have formed when the sun, wind and rain erode rocks). It contains channels of air and water and is a home for many invertebrates and microbes. Many of these creatures feed on the constant supply of leaf litter and create humus (dark-brown nutrient-rich compost).

Children need to be taught that the type of soil depends upon the rock sediment in the area:

• Sandy soil has large particles which means there are lots of small air gaps. Water drains through them easily so it often feels dry. A tablespoon of sandy soil will not ‘roll’ into a ball in your hands.

• Clay soil has small particles which means there are very few air gaps and water does not drain through it easily. It often feels sticky and damp. A tablespoon of clay soil will roll into a ball and even form a sausage!

• Loam soil contains the good mixture of sand and clay particles and is thought to be the best type of soil for gardening with plenty of air and water. If you try the rolling test, a loam soil will make a ball but fall apart when you try to make a sausage!

Another BBC video explains these key ideas succinctly.

The key vocabulary children will need to develop when learning about rocks, fossils and soils can be found in the downloads section of Explorify.

Ideas to try with your class 

Now you have the tricky scientific concepts under your belt, try our ideas to help you explore rocks with your class in a way they will understand in part two of this topic guide!

Image credits: Mary Anning's Ichthyosaur via Museum of Natural History Oxford 

James St. John via Wikimedia 

Stux via Pixabay