G7-TSFE – Plate Tectonics at Work-700-800L-03

At that time, the causes of earthquakes were still unknown. It was not until much later that surveyors made a discovery. They had noticed that the land on either side of the San Andreas Fault had permanently shifted. They realized this by looking at fences. Any fence across the fault-line was torn apart by the movement. They also noticed that the further they were from the fault-line, the less distorted the land became. This pattern showed that known fault-lines had shifted. For landowners, it was hard to tell where their property lines were compared to their neighbors.

Shortly after the great San Francisco quake, a well-known geophysicist started to study the earthquake. His name was H F Reid. Reid, using the measurements collected by surveyors, developed a theory. He hypothesized that the reason for earthquakes was strain built up within Earth’s crust. He felt it was like a rubber band being stretched. At some point, the strain would become too much. At that point, the land would snap back. The problem with this hypothesis was it did not explain what was this strain to happen in the first place. In 1912, another scientist named Alfred Wegener had a different theory. His theory was Continental Drift.

He proposed that moving continents were causing earthquakes. He had noticed by looking at maps. He noticed all of Earth’s major landmasses fit together. It was sort of like a giant jigsaw puzzle. He also noticed numerous similarities between rock types, geologic structures, and fossils on the matching sides of the continents. This was especially true for plant and animal fossils. This provided evidence for the movement of the continents. Once again though, there was no explanation for what was causing the continents to move. It took 50 more years to get that answer. This happened when Harry Hess confirmed the sea floor was spreading along mid ocean ridges. This evidence helped to explain why the continents were moving.

The San Francisco Bay area rests between two of Earth’s plates. The city is located between the North America and the Pacific plates. The boundary where these two plates meet is named the San Andreas Fault. It extends over 800 miles through the state. The plates slowly side past one another at a rate of about 1.5 inches per year. This might not seem like much, but after a few decades, a lot of energy is stored between them. Not all plates slide past each other like they do in the San Andreas Fault region. In some areas, two plates are converging or being shoved together. For example, the Indian Plate constantly moves northward. This motion causes it to slam into the Asian Plate. This convergence is what has caused the Himalayan Mountain Range to rise up. The land has nowhere else to go but up. In other areas, plates are moving apart from each other or diverging. Examples of this are the Rift Valley in Africa and mid-ocean ridges. When plates move apart, the crust opens, allowing magma to flow upward and fill the chasm that is created.