We now know that the continents move. They are part of plates that slide around the liquid middle of the Earth like graham crackers on a lake of milk, or like rocks on a lake of lava . . . we'll come up with a better example in a bit. This probably makes you think of all kinds of questions. How fast do they move? What happens when they move too much or too fast? And here's maybe the best one: how do we even find these answers?
Deep beneath the waves, huge mountain ranges run across the ocean floor. You can find them halfway between the continents. This is where lava from the middle of the Earth pours out of cracks in the crust and then cools into new crust. Think of very hot soup sitting outside in the snow. The top of the soup will freeze into a thick, hard skin. This is the crust. Sometimes the hot soup beneath it might bubble up, making a bump in the skin. These bumps are like the mountain ranges we see under the ocean. In other parts of the ocean there are long cracks where we believe the crust melts and sinks back into the inside of the Earth. This sinking and rising is what moves the plates. Plate tectonics
is the theory that explains that there are plates on the surface of Earth that move.
How do we measure our lives? You break things up into small stages, like when you first crawled, when you first walked, when you started to talk, when you began school, went to high school, college, and so on. We can measure the Earth's changes the same way, but each stage is just a lot longer. To make it easy, let's start by looking at how much the continents move each year -- about the length of a grapefruit, or as far as your fingernails grow in one year. That' s not very far at all, considering that they've moved thousands of miles over time! Maybe you have heard that the life of a person is like the blink of an eye compared to how long the Earth has been around. This is true. The Earth would have about 4,500,000,000 candles on its birthday cake. So when we look at the things that have happened on Earth over all those years, we need a way to measure it other than by years or grapefruits. Geologic Time
is how we talk about Earth's history in stages, instead of years. This includes everything from the Earth's start to the first living things to the time of the dinosaurs to the world today. Think of it as a calendar for the Earth, just with major events for each day instead of days or years.
Boil an egg. You do not really have to do this; you can just imagine you're doing it. Now, lightly tap it on the counter, sending cracks through the shell. The Earth's crust is like that cracked eggshell. Each piece of eggshell is like one plate. Without the cracks, the plates would not be able to move. What cracked the Earth's crust? Meteors from space? Cooling of the Earth's lava in the middle? We have no idea! Lithospheric plates
are what we call the broken pieces of crust that make up the surface of the Earth. The plates either slide alongside, under, or over each other.
Let's go back to when the land was all one piece. How did the crust first break? The answer is that many things were pushing and pulling on it. There were the waves underneath it, the moving lava in the center of the Earth, even the pull of the sun and moon. With all of this movement above and below, the land was just too heavy to hold itself together. This was the death of Pangea, the first supercontinent
-- which is the word for most or all of the continents together as one. Was it bad that it all broke apart? No! It may have brought living things to places with different weather and different food, helping to create different kinds of living things -- like humans! Just think, if it had stayed together, we might never have been here. And that would be boring.
Pangea, one big happy family.
I have thrown a lot of things at you. Soup! Grapefruits! Fingernails! Eggshells! Yes, the Earth's crust acts like all of these things! It's no wonder it's taken us well over a hundred years to figure out how it works and how to measure it. If you are confused, that's fine! So were scientists when they started! You are in good company.