Geology, Water, Weather, and Climate

Earth's Dynamic Crust and the Rock Record

Plate tectonics explains that the outer rigid layer of Earth, known as the lithosphere, is divided into moving plates. These massive plates do not stay still; they constantly shift across the surface. At divergent boundaries, tectonic plates pull apart from one another. This pulling motion creates geological features such as mid-ocean ridges and rift valleys. At convergent boundaries, plates move toward each other. When they collide or press together, they can produce towering mountains, subduction zones, or deep-ocean trenches, depending on the specific plates involved. In subduction, a denser oceanic plate slides beneath a less dense continental or oceanic plate. Meanwhile, at transform boundaries, plates slide past each other horizontally, a movement that frequently causes earthquakes.

The solid materials that make up these plates are classified into three major rock types based on how they form. Igneous rocks form from the cooling and solidification of molten rock, which can occur as magma underground or as lava at the surface. Sedimentary rocks form through processes such as the compaction and cementation of loose sediments, the accumulation of organic matter, or the precipitation of dissolved minerals from water. Metamorphic rocks form when existing rocks are altered by intense heat and pressure without completely melting. Over geological time, the rock cycle demonstrates that any of these three rock types can be transformed into any other type through natural planetary processes.

Geologists read the history recorded in these rock formations using the principles of relative age dating. Relative age dating determines whether an event or rock layer is older or younger than another without determining an exact numeric age. The law of superposition states that in undisturbed sedimentary rock layers, the oldest layers are at the bottom and the youngest are at the top. The principle of original horizontality states that sedimentary layers are initially deposited flat under the pull of gravity. Furthermore, the principle of cross-cutting relationships states that a fault or igneous intrusion is younger than the rock layers it cuts across. Scientists also look for index fossils, which are fossils of organisms that were geographically widespread but existed for a relatively short geological time span, making them especially useful for dating rock layers.