Chocolate Rocks
CHOCOLATE ROCKS TEACHER NOTES AND HANDY HINTS The Chocolate Rocks experiment is an adaptation of the Hawai i Space Grant Consortium s Edible Rocks experiment (see: http://imina.soest.hawaii.edu/spacegrant/class_acts/ediblerockste.html). The experiment will take approximately one hour to complete, but can be expanded or contracted, depending on the amount of time and resources available and the nature of the students. The lesson could be entirely based on pictures of rocks and edible samples if actual edible samples are impractical. The number of samples described and the length of any discussions may be extended or reduced by the teacher as appropriate. The discussion and/or the scope of the activity may be extended to include other rock types and their equivalent edible form. The choice is yours. Activity sheet aims and benefits The aim of this activity is to introduce students to rocks and meteorites through the observation and description of the physical characteristics of edible samples chosen as models of real rocks or meteorites. It has been designed as a comfortable introduction to describing meteorites. It helps students to become better observers by making a connection between the familiar (chocolates) and the unfamiliar (meteorites). Edible rocks are used in scientific context, showing students the importance of observation, teamwork, and communication skills. Hints 1. This one is a real winner with the students, so some are likely to get over excited make sure all your samples don t get eaten at the start of the lesson! Save some of the edible samples to be handed out simply for the pleasure of eating them at the end of the lesson. 2. The edible samples are likely to melt in hot weather, especially when being handled. Store them in a cool, dry place until they are needed. 3. Make sure none of the students has an allergy to any of the ingredients of the edible samples before starting the experiment, particularly if any samples are substituted for others containing nuts! Extensions 1. Extend the discussion to include the concepts of igneous, sedimentary and metamorphic rocks. Get the students to suggest other everyday things that may be used as analogies for rocks. What other edible samples might they be able to look at, and what kind of rocks would they be similar to? 2. Discuss the formation history of the samples. Why do some have phenocrysts or vesicles when others do not? 3. Discuss ways in which the samples may have been modified (e.g. by weathering). Are the surfaces or the interiors of the samples more likely to be modified? What would the effect of running water over the samples? Would these weathering process have happened on Mars or on the Earth? How would they be different on the different planets? Chocolate Rocks (Teacher Notes) 1
4. Tie this module in with the Impact Cratering module if you are running it. Would these samples have formed impact craters when they landed (yes, but only very small ones)? What kinds of rocks form large impact craters? Why do some rocks survive travelling through the Earth s atmosphere to land on the surface when others don t? Field note answers 1. an iron meteorite = Dairy Milk 2. a chondritic meteorite = Rocky Road 3. a meteorite with a fusion crust = Milky Way 4. an igneous rock from an explosive eruption = Chocolate Aero 5 an igneous rock that cooled slowly as a magma = Chocolate Brownie 6. an igneous rock that cooled quickly as a magma = Cadbury s Mint Crisp CHOCOLATE ROCKS RECIPES Chocolate Brownies: 50g (2oz) cocoa powder 200g (7oz) self-raising flour 250g (8oz) soft margarine 300g (10oz) caster sugar 4 eggs 200g (7oz) glace cherries 100g (4oz) chocolate drops Pre-heat oven to Gas Mark 4/180C/350F. Sift cocoa powder and flour. Cream sugar and margarine together. Beat in eggs, adding a little of the flour mixture each time. Fold in remaining flour mixture, cherries and chocolate drops. Spread mixture into a tin and bake for 30-60 mins, checking constantly. Cool before slicing. Remove cubes as needed for samples, but keep rest intact. Rocky Road: 400g (14oz) chocolate pieces (melted) 100g (4oz) marshmallows. Line a small, but deep tin with aluminium foil and pour in about half the melted chocolate. Add marshmallows, and mix until coated. Pour remaining chocolate over the marshmallows and spread flat. Refrigerate until cold. Remove some cubes for samples, but keep rest intact. For other Rocky Road samples, add fewer marshmallows, cut in half. Chocolate Rocks (Teacher Notes) 2
CHOCOLATE ROCKS FIELD NOTES 1. Sample is a solid, dark brown, dense mass, with no obvious fusion crust. 2. Sample consists mostly of coloured, soft, rounded to angular blebs, approximately 10mm in diameter, completely surrounded by a dark brown, uniform matrix. 3.Sample has homogeneous light brown interior with a few small vesicles. The exterior is a fairly regular, dark brown, fusion crust with some patterning. 4.Sample has a dark brown fusion crust with some patterning. Interior consists of a brown vesicular rock. There are no obvious inclusions. 5.Sample largely consists of a dark brown, vesicular matrix, with two phases present as phenocrysts. The first is red, rounded, approximately 15mm in diameter, and makes up ~10% of the sample. The second is orange, approximately 5mm in diameter, and makes up about ~5% of the sample. 6.Sample consists mostly of a solid, dark brown, dense matrix, with approximately 15% green, hard phenocrysts, and no obvious fusion crust. Chocolate Rocks (Field Notes)
CHOCOLATE ROCKS STUDENT NOTES If you landed on Mars, what would you see? If you landed on Mars you would see lots of rocks! You would be able to collect rock samples and study them to find out what kind of rock they are and what happened to them in the past, in the same way that geologists study rocks to find out more about the Earth. We can study Mars from the Earth too by using pieces of rocks from Mars that have fallen to Earth as meteorites. What are meteorites? Meteorites are mostly pieces of rock, but some can be made of different materials such as iron. All meteorites fall to Earth from space, and most come from broken asteroids which are themselves large lumps of rock and metal. How can we identify meteorites? Meteorites come in a variety of types and a wide range of sizes and shapes, but most meteorites have two things in common: 1) they have a dark brown or black glassy crusts on the outside. 2) they contain enough iron metal to attract a magnet. The outside crust of the meteorite is produced as the rock is heated by friction when it comes through the Earth s atmosphere. This outer part of the meteorite melts and forms a fusion crust, which often has marks and indentations like thumb prints. The inside stays cool and is usually light grey to black in colour, but some may be light brown, or if they have been weathered and rusted, dark brown. How many meteorites come from Mars? Scientists have 13 meteorites that have come from Mars. They are thought to be from Mars because they are much younger than other meteorites, and because gases found inside them have the same composition as the Martian atmosphere. They are similar to rocks found on Earth called basalts, which form when molten rock is erupted from a volcano. This molten rock is called magma (before erupting onto the surface) and lava (when flowing on the surface after eruption). The magma can change when it is below the surface. As it cools, crystals can form (called phenocrysts) - the faster the magma cools, the smaller the crystals are. For larger crystals (up to 1cm) to form, the magma must cool very slowly. Also, as the magma rises to the surface, the pressure decreases, releasing bubbles of gas (very similar to the way a bottle or can of Coke bubbles when you first open it). If these gases are released very quickly, explosive eruptions can occur (equivalent to opening that Coke can after shaking it up). These bubbles can leave their imprint on a volcanic rock, leaving it like a frozen sponge with lots of holes in it. The number of these holes or vesicles, varies greatly between rocks. Question: Look at the surfaces of the samples. Are they similar to one another? If these were lava flows on Mars and you were looking down on the surface from above, would you be able to tell what the rock was made of or looked like inside? On your mission to Mars you will be landing on the surface. What will you be able to learn about the rocks when you get there? How will you study them? Chocolate Rocks (Student Notes and Activity Sheet) 1
CHOCOLATE ROCKS ACTIVITY SHEET Aims and objectives The aim of this experiment is to describe in scientific terms, samples of edible rock supplied to you by the teacher. These descriptions will be used by others to identify the samples you observed, and attempt to describe the history of the sample. Procedure 1. Your group will be given three different rock samples by the teacher. Do not handle the samples or taste them! 2. Make a detailed sketch of each sample, showing the appearance of the surface and interior. You may use separate sketches for the surface and interior if you wish. Label the various features in your sketch, but do not use food terms (i.e. chocolate or cake). 3. Write two or three sentences describing the physical characteristics of the interior and exterior of the sample. Make your description as clear and complete as you possibly can, as others will be using your description to identify a sample later. 4. Nominate one person from your group. This person will tell the rest of the class one of your descriptions. The other groups will then have to identify that sample from the examples at the front of the class. Repeat this step for each group in the class. Try not to describe the same sample as the last group if possible. How descriptive were you? Was it easy to match each of the descriptions to the right sample? 5. Listen to the teacher read out the descriptions from the "Field notes" sheet provided. This is how a scientist may describe the same samples you have worked with. Were you able to identify the sample from these descriptions? How did they differ from your descriptions of the same sample? GLOSSARY Angular having angles or sharp corners. Bleb a small, rounded inclusion. Chondrite a type of meteorite, characterised by chondrules. Chondrules a small round body found as an inclusion in some meteorites. Homogeneous all of the same kind. Inclusion a material enclosed within another. Matrix material surrounding inclusions or phenocrysts. Meteorite a rock that has fallen to Earth from space. Phase a homogeneous form of matter, separated from other forms by its surface. Phenocryst a large crystal found in some types of rock. Vesicle a small hole in a volcanic rock produced by gas bubbles. Chocolate Rocks (Student Notes and Activity Sheet) 2
CHOCOLATE ROCKS OBSERVATIONS SHEET Name... Sketch the interior and exterior of your three samples in the boxes below (large box = interior, small box = exterior). Label each of your sketches carefully. Sample One Sample Two Sample Three Give a short, 2-3 sentence description of each sample, without using food terms: Sample One :......... Sample Two:... Sample Three:.... Chocolate Rocks (Student Notes and Activity Sheet) 3