Inside the Earth - A Lot On My Plate
Our current idea on how the continents moved from where we think they were to where they are now is much more complete than Wegener's Continental Drift hypothesis was. Our current model, Plate Tectonics, has become as important to the study of Earth Systems as the discovery of the atom was to Chemistry and Physics.
The idea behind Plate Tectonics is this: Earth's crust is broken up into about 14 major pieces, or "plates," and several smaller plates. These plates move on convection currents of magma from the mantle independently from each other. Two or more plates meet at a plate boundary. The direction of plate movement relative to each other at the boundaries can cause earthquakes, volcanoes, and tsunamis, along with other distinct landforms.
The theory of Plate Tectonics is complex, but the understanding of it makes MUCH of the study of Earth Systems more understandable. The theory builds on the good observations of Continental Drift, and adds more evidence and explanation.
How did the shores of North and South America move apart from Europe and Africa? Data collected from across the Atlantic Ocean floor shows that the crust at the Mid-Ocean Ridge is almost new (0-5 million years old) compared to the crust near the coastlines (about 180 million years old). The sea floor is spreading apart and making a few new inches of crust every year, which does not seem like much, but over a long period of time is substantial.
This is also confirmed with data that shows patterns in the magnetic fields "frozen" in the rocks of the ocean floor. As new crust is created where the floor is spreading, iron and other metals in the rock line up with Earth's magnetic field as the rock cools. Over long periods of time, the magnetic field changes directions, and that shows in the rock record as stripes or bands. The bands on each side of the Mid-Ocean Ridge not only match in age, but in the direction of the magnetic pointing.
If new crust is being created in the middle of the ocean, Earth must be getting bigger, right? Not exactly. While new crust is being created in one place, it is being recycled or destroyed in another.
The last bit of evidence supporting Plate Tectonics deals with the fact that most of Earthquakes and volcanoes occur in specific places around the globe - they are usually not in random locations. Have you heard of the Ring of Fire? It's basically the coasts of the countries surrounding the Pacific Ocean. Volcanoes and earthquakes are very common in this area. These disasters occur here because there are many plate boundaries, and as we will learn, where there is a boundary, there is ACTION!