[Music] hi and welcome back to three science lessons co uk by the end of this video you should be able to describe how fossil fuels are formed these include coal oil and gas in the last video we looked at how the levels of gases in the atmosphere have changed over time we saw that the early atmosphere contains very large amounts of carbon dioxide but virtually no oxygen whereas the atmosphere today contains plenty of oxygen but only small amounts of carbon dioxide we saw that photosynthesis takes in carbon dioxide and produces oxygen some of that carbon ends up in fossil fuels and we're looking at that in this video the first thing to understand is that fossil fuels are formed over millions of years fossil fuels are non-renewable in other words if we keep using them then they'll run out we're going to explore how coal oil and gas are formed so let's start by looking at coal coal is formed from the remains of ferns and trees if these die in marshy wetlands then they do not decompose and that can be due to a lack of oxygen or acidic conditions these both prevent bacteria from carrying out decomposition over time the plant remains are covered with sediment and compressed high temperature and pressure creates coal now another really important fossil fuel is oil crude oil is formed from plankton which are tiny plants and animals found in the sea when these die they settle in mud on the seabed again if oxygen is not present then they do not decompose over time they're compressed by sediment and heat and pressure then convert them into crude oil the third fossil fuel is natural gas natural gas is mainly the hydrocarbon methane we often find natural gas near deposits of oil and that's because natural gas is formed from plankton in a similar way to oil now the key fact is that old fossil fuels contain trapped carbon and that carbon was part of the carbon dioxide taken in by fog synthesis in a later video we look at the effects that burning fossil fuels is having on the planet remember you'll find plenty of questions on fossil fuels in my ization workbook and you can get that by clicking on the link above [Music]