Earth's History

Continental drift

The hypothesis that all the continents are moving very slowly across Earths surface continental drift. Continental drift is the gradual movement of continents over Earth's surface due to tectonic plate motion.

Geologic time scale

A timeline of Earth's history based on major changes. Geologic time scale. The geologic time scale is a system that scientists use to organize Earth's history into different periods based on significant geological and biological events.

Half-life

The time it takes for half the atoms of a radioactive isotope to decay is called half-life. Half-life is the time needed for half of a radioactive substance to change into something else.

Index fossil

A fossil from an organism that existed for a short time, it is used to determine the age of other fossils. Index fossil. An index fossil helps scientists date rock layers and other fossils found in those layers.

Law of superposition

The oldest rock layers are at the bottom and youngest are at the top. Law of superposition. This law states that in undisturbed sedimentary rock layers, the layers on the bottom are older than those on top.

Plate tectonics

The theory that Earth's crust is made of moving plates. Plate tectonics. Plate tectonics is the scientific concept that explains how the large plates of Earth's outer shell move and interact, causing earthquakes, volcanoes, and the formation of mountains.

Radioactive decay

The breakdown of an unstable atomic nucleus, this releases particles and energy. Radioactive decay. Radioactive decay is when an unstable atom releases energy and particles to become more stable.

Relative age

The age of one object or event compared to another. Relative age. In science, relative age is determining the order of past events without knowing their exact ages.

Relative dating

A method of comparing fossils or rocks to determine which is older. Relative dating. It involves comparing the order of rock and fossil layers to determine their relative ages.

Sedimentary rock

A rock that forms when sediment is pressed and cemented together. Sedimentary rock. Sedimentary rock is created from small particles or minerals that settle and harden over time into solid layers.

Trace fossil

A fossil formed in soft sedimentary rock, like a footprint, trace fossil. A trace fossil is evidence of an animal's activity, such as footprints or burrows, preserved in rocks.

Uniformitarianism

The theory that geologic processes are the same today as they were in the past. Uniformitarianism. It is the idea that the natural processes shaping the Earth have been consistent over time.