This supercontinent was composed of most of Earth's landmasses, which were joined together in a single, large landmass.

The supercontinent you're referring to is known as Pangaea. Here's a step-by-step explanation:

1. The concept of a supercontinent refers to a large landmass formed by the convergence of multiple smaller landmasses. This is a process that occurs over millions of years due to the movement of Earth's tectonic plates.

2. Pangaea is the most recent supercontinent, existing during the late Paleozoic and early Mesozoic eras, which were about 335 million to 175 million years ago.

3. The name "Pangaea" comes from Ancient Greek and means "all lands." This is fitting because, at the time, Pangaea included almost all the Earth's landmasses.

4. Over millions of years, Pangaea eventually broke apart due to tectonic forces. This process formed the continents and ocean basins that we see today.

5. The idea of Pangaea was first proposed in 1912 by Alfred Wegener, a German meteorologist and geophysicist. His theory, known as continental drift, was controversial at the time but is now widely accepted.