How Rocks and Fuels Form

Today you will learn how sedimentary rocks and fossil fuels form, and be able to describe each step.

Picture a river in Texas after a big rain. The water looks brown because it is carrying tiny bits of rock, sand, and mud. These tiny pieces are called sediment, and moving water, wind, and ice carry sediment from place to place. When the water slows down near a lake or the Gulf of Mexico, the sediment sinks and settles to the bottom.

Year after year, more layers pile on top. The bottom layers get squeezed by the weight above them. Water gets pressed out, and minerals act like natural glue that sticks the grains together. After a very long time, those pressed and glued layers become sedimentary rock, a rock made of cemented layers of sediment. Sandstone and limestone are two kinds you can find in Texas. If you have seen a cliff or road cut with stripes, you were looking at those layers.

Sometimes plants and animals get buried in the layers too. Their shapes can be preserved as fossils in the rock. That is why people find seashell fossils in the middle of Texas, far from the ocean. Long ago, a shallow sea covered that land.

Fossil fuels form in a similar way, but they need heat and pressure. When tiny sea creatures and plants died, they were buried under thick layers of mud. Over millions of years, heat and pressure slowly changed them into oil and natural gas, which are fossil fuels made from the buried remains of living things. Swamp plants turned into coal the same slow way. Texas has many oil wells because ancient seas once covered the state.

Try it: If you found a layer of sandstone with seashell fossils inside it on a Texas hillside, what does that tell you about what covered that land long ago?