Rocks are important because geologists use evidence from them to learn about what the Earth was like in the past. They allow scientists to build a historical record of the planet to learn what events occurred before people lived.
Rocks can answer a number of questions about what Earth was like in the past. They can indicate if a particular part of the Earth was under the ocean or on a mountaintop. Scientists also use them to help determine if the atmosphere was thick or thin and whether the climate was warm or cold at a given time.
Learning about how the Earth existed in the past allows scientists to learn how it works in the present and is likely to work in the future. As an example, geologists use rocks to determine how temperature changes affected Earth and life on Earth in the past. This helps scientists understand how global climate change is affecting the planet.
Granite and basalt may be the most important types of rocks in the Earth's crust. They are both igneous rocks, which means they form from cooling magma. Granite is the most common type of rock on continental landmasses, and basalt makes up most of the ocean floor.