Every science has its own theory. Geology did not have it until 1970s when plate tectonics theory was first introduced after extensive research in oceans. Plate tectonics explains, either directly or indirectly, just about every topic discussed in geology. It is the glue that binds everything together. 

Before plate tectonics, geologists had no explanation for these (and other) questions:

1. What causes the tectonic plates to move the way they do?
2. Why is the oceanic crust younger than continental crust?
3. Why do the continents look the way they do, like puzzle pieces or are positioned as they are?
4. Why are Japan and California so prone to earthquakes and volcanoes?
5. How did the Himalayas form?
6. Why do we find evidence of aquatic species at the very top of the Himalayas and other mountainous areas?
7. How do mountains form?
8. Why do the oceans look the way they do?
9. What factors contributed to the occurrence of the ice ages?
10. Why the same species can be found on continents on opposite sides of the oceans?

As you move through this section, keep in mind all of these questions and make note of any other impacts the Theory of Plate Tectonics has had on the field of geology.

Below is a map of Pangaea (started breaking up approx. 200 million years ago) with current boundaries of countries.

Source: https://www.visualcapitalist.com/incredible-map-of-pangea-with-modern-borders/

Just for fun, would you like to see your location in Plate Tectonics history?

Then follow this link: https://dinosaurpictures.org/ancient-earth#0

And finally to wet your appetite for the Plate Tectonic, the latest visualization of Dynamic Earth:

 https://www.nytimes.com/2021/02/06/science/tectonic-plates-continental-drift.html?referringSource=articleShare