The Rock Cycle and Soil (Cambridge Primary Science 6 Part 2).
here is the part 1: https://youtu.be/KxdtTfBtx_g?si=SQP9AEj6jkbuYo6N
part 3: https://youtu.be/FPBiNkuLqCA
Sedimentary rocks always form in layers like the layers in the canyon in the photograph. This is an important characteristic of sedimentary rocks. Types of sedimentary rock. Three of the most common types of sedimentary rock are sandstone, shale, and limestone. Sandstone is a sedimentary rock which consists of sand grains stuck together. Most sand is made of quartz because this is a very hard mineral. Sand is the most resistant to weathering processes at the Earth’s surface. Sandstone can be red, orange, yellow, or gray in color. Shale is formed of very fine grain sediments. The sediments are softer than sand grains. Shale is usually gray in color. If you scratch shale, it leaves a mark. Shale is often soft enough to break with your hands. Look at the photograph of shale opposite. Notice the layers. Limestone is made from layers of shells. The shells were covering sea animals that died and sank to the bottom of the sea. Limestone is usually white or gray in color. Chalk is a pure type of limestone. Fossils. Another characteristic of sedimentary rocks is that they sometimes contain fossils. Fossils are the preserved remains of animals and plants which we find in sedimentary rocks. How do fossils form? A fossil can only form if the animal or plant is buried quickly in a place where there is very little air. This is why we only find fossils in sedimentary rocks which form in water on the seabed or a lake bed. When animals that live in or near the sea die, their bodies are washed into the sea. The soft parts of the animals rot away. Over a long period of time, minerals in the water gradually replace the bones of the animal. Eventually the minerals harden into rock. So we have a copy of the original animal as a fossil like the dinosaur in the photograph. This fossil is in limestone and it was formed about 150 million years ago. Sometimes only the imprint of the animal in the sediments is left behind. This kind of fossil is called a mold. Look at the mold of a dinosaur’s footprint in sandstone in Namibia. This footprint is also about 150 million years old. This is a fossil mold of a leaf in shale that is 350 million years old. Sometimes a mold fills with minerals that harden to a solid. This makes a cast of the animal. The photograph below shows two fossils of ammonites in limestone. The molds filled with minerals which hardened to form the shape of the original animal. Ammonites do not exist on Earth anymore. They lived 240 million years ago. So that is how old this fossil is. Metamorphic rocks and the rock cycle. What are metamorphic rocks? The word metamorphosis means change. Metamorphic rocks are existing rocks which have been changed. The rocks may be changed by heat. When this happens, all the minerals in the rock melt and then form new crystals. Sometimes rocks are changed by huge pressure. The pressure squeezes the rock so much that the new rock has lots of thin layers. Sometimes rocks are changed by heat and pressure together. Then the new rock has different minerals and lots of layers. What causes the heat and pressure? Look at the diagram. It shows layers of sedimentary rocks buried beneath the Earth’s surface. Magma rises up from the mantle and forms an intrusive ignous rock mass. Just imagine this happening. A huge mass of extremely hot liquid rock pushes solid rocks aside. This causes all the surrounding rocks to be affected by the heat and the pressure. These rocks are changed into metamorphic rocks. Types of metamorphic rock. Let’s see which metamorphic rocks form when sandstone, limestone, shale, and granite are heated until they melt and are squashed by pressure. Quartzite. When sandstone is heated, all the sand grains melt and make new quartz crystals. This forms the metamorphic rock called quartzite. It looks similar to sandstone, but the crystals shine and the rock is much harder than sandstone. This rock is so hard that it is sometimes used to make sculptures. The photograph shows a quartzite statue of Anku which was made in 1850 B.CE almost 4,000 years ago. Marble. When limestone is heated, it changes into a metamorphic rock called marble. All the minerals melt and form new crystals. Marble is extremely hard and very shiny. It can be white, red, blue, or green depending on which substances were in the original limestone. Marble is a very beautiful rock and it lasts for a very long time. People use marble to carve statues and gravestones and decorate public buildings and places of worship. The picture shows how different colored marble has been used to decorate a building in India. The white marble in the photograph has been cut into blocks to use for carving statues and making furniture. Slate. When shale is put under intense pressure, it forms thin layers. This is the metamorphic rock called slate. Remember that layers of sediments build up on the seabed. The bottom layers are buried by the top layers. If there are many layers, the bottom layer is put under huge pressure from the layers above. Burial changes the rock into a metamorphic rock with many layers like slate. People use slate for making roof tiles and floor tiles because it is hard and longasting. The photograph shows slate roof tiles. Nice. When granite is heated, all the minerals melt and make new crystals. Pressure causes the new rock to have stripes or bands of crystals. The metamorphic rock is called nice. Look at the photograph of a piece of nice. Notice that the crystals look similar to granite, but they are in bands. Nice is very hard. People crush this metamorphic rock to make roads. The rock cycle. The rock cycle shows us how the rocks of the earth are used over and over again. One rock can be changed into another type of rock. These processes take a very long time, hundreds of millions of years. Now follow the cycle with your finger. Begin at magma. The bold words refer to the labels on the rock cycle diagram. Magma cools to form ignous rock. There are three arrows coming out of ignous rock. Weathering breaks up the ignous rock. Erosion wears down the rock pieces to form sediments. If it is an intrusive ignous rock, it in the earth’s crust. Heat and pressure from magma can change the ignous rock into metamorphic rock. Ignous rocks are melted back into magma. Look at the arrow between sedimentary rock and metamorphic rock. This arrow says burial, heat, and pressure. Remember that layers of sediments build up on the seabed. The bottom layers are buried below the earth’s surface by the top layers. If there are many layers, the bottom layer is put under huge pressure from the layers above. It is much hotter below the Earth’s surface. Heat and pressure changes the rock into a metamorphic