Peeling Back The Crust
After Mohorovicic made his discovery, geologists began taking a closer look at the composition of Earth's crust. They found that the crust is not one uniform layer. Instead, there are many important Asthenosphere 60-220 miles (100 km-350 km) Figure 5.3 The Earth is made up of layers of varying depth. differences between the crust that lies under the oceans and the crust that makes up the continents. The most obvious difference has to do with the thickness of the two. While the average thickness...
Early ideas about shifting continents
By the mid-1800s, geologists had collected a great deal of information about the rocks found along the coasts of North America, South America, Europe, and Africa. At this time, an American named Antonio Snider-Pellegrini was working in Paris where he had access to many of these geological reports. As he reviewed the data, Snider-Pellegrini noticed something interesting. Not only did the coastlines between the continents appear to match in shape, but so did many of the rock types found on each...
The Theory Of Seafloor Spreading
Long before Wegener introduced his ideas of continental drift, geologists had been faced with two other great mysteries of the ocean. First, they wondered what happened to all of the sediment that was carried into the sea by the rivers that emptied into the ocean. Scientists had observed how ponds and lakes would eventually fill in with silt and sand carried in by the streams that flowed into them. If Hutton and Lyell were right about the age of Earth and the fact that processes like erosion...
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Figure 3.2 Scientists created a geologic time scale by studying rock layers to divide the Earth's history into different time periods. rock layers from different areas of the planet, they slowly built up a geologic time scale for the planet. This chart neatly divided Earth history into different time segments called eons, eras, periods, and epochs. Having a relative time scale to work with was very helpful, but what geologists really needed was a way of telling absolute time. An absolute age...
Relative vs absolute time
One of the biggest problems that geologists have when trying to piece together the history of our planet is figuring out how old different rocks are. Before the twentieth century, the best that they could do was to come up with a relative age date. A relative age date can help estimate which rock is older when it is compared with another, but it does not tell how old the rock actually is. To understand how radioactivity can change one type of atom into another, we must first look at what an...
Under the
If you stop and think about it, the name Water would probably be a better name for our planet than Earth. Dry land, or earth, only makes up about 30 of the planet's surface. Most of the rest is covered by oceans. Up until the mid-twentieth century, little was known about what the seafloors of our planet looked like. There were several reasons for this lack of information. First and foremost, at certain points past the edge of the continents, the ocean gets really deep Before the early 1900s,...
Earthquakes And Volcanoes Draw Some Lines
One of the most important things that geologists discovered when they conducted seismic studies of the crust was that it was not made up of one single piece. Instead, the crust is cracked and broken into many different pieces. Scientists discovered this fact as they began to learn more about the causes of earthquakes. It had long been known that earthquakes do not happen with the same frequency all over the planet. Some places, like California and Mexico, have many earthquakes. Places like...
Catastrophism Makes A Splash
In the early part of the 1700s, most of the people who studied the Earth believed that our planet had changed very little over time. They believed that the rocks, mountains, and oceans that they saw had been created at the same time that Earth first formed. Earth itself was thought to be approximately 6,000 years old. However, this age was not based on the work of scientists it was proposed by religious scholars who based their estimate on the story of Earth's creation as it was described in...
Drifting Continents
The first step in the development of modern plate tectonic theory can be traced back to the early days of the twentieth century and an idea that became known as continental drift. Simply stated, continental drift is the hypothesis that all of the landmasses on Earth were joined together into one giant supercontinent at some time in the distant past. Over millions of years, this single landmass slowly broke apart and the continents gradually drifted into the positions that we see them in today....
Radioactivity Heats Things Up
After Alfred Wegener's death in 1930, much of the controversy over continental drift began to die down. Most scientists felt that there were just too many flaws in the theory to give it much serious consideration. Rather than pursue it further, they went about their business conducting research in other areas of geology. Nevertheless, Wegener's ideas did not completely die out. In South Africa, a geologist named Alexander Du Toit firmly believed that Wegener's reconstruction of Pangaea was...
Vine And Matthews Earn Their Stripes
The 1950s was a busy time for marine geologists. Not only were Bruce Heezen and Marie Tharp creating the first maps of the seafloor, but other scientists were collecting data on the remnant magnetism of the rocks that were found on the bottom of the ocean. It was during this time that Arthur Raff and Ronald Mason made a fascinating discovery. The two men were conducting a magnetic survey of the Pacific Ocean seafloor off the coast of Oregon. As they sailed back and forth across the mid-ocean...
Stanley Runcorn Discovers Wandering Poles
In the 1950s, a number of geologists were trying to make some sense of how Earth's magnetic field works. One of these men was Stanley Runcorn, a geophysicist at the University of Newcastle in England at the time. While measuring the remnant magnetism found frozen into igneous rocks around Europe, he discovered something very strange It appeared that the position of the north magnetic pole of Earth moved around over time. This was very unexpected. Based on the shape of Earth's magnetic field and...
Harry Hess Uncovers A Deepsea Mystery
Even though sonar had been available for more than 20 years, it was not really put to many scientific uses until World War II, when a naval officer named Harry Hess turned an antisubmarine weapon into an important research tool. In the process, he would not only unlock the mystery of what the seafloor looked like, but give the theory of continental drift a whole new look. Harry Hammond Hess was born in New York City in 1906 and graduated from Yale University with a degree in geology in 1927....
Many Questions Still Remain
Even though plate tectonics has answered many of the questions dealing with how the surface of Earth changes over time, the theory itself still has many unanswered questions. Perhaps the biggest question is also the simplest one What makes the tectonic plates move in the first place The most common answer goes back to Arthur Holmes's idea of convection currents. If you recall, Holmes believed that heat generated by radioactive elements would cause hot spots in the mantle. As the rock became...
Alfred Wegener Takes On The World
The scientific community quickly rejected Taylor's theory on mountain building. The idea of continents being able to move through a solid Earth seemed so far fetched that few serious geologists would accept it. That did not stop everyone from thinking about the idea, however. In the early 1900s, a German scientist named Alfred Wegener picked up on the concept and, before he was through, made many geologists think that the impossible might be possible. Wegener was born in Berlin on November 1,...
Arthur Holmes And The Concept Of Convection
Even though his name is not as famous as either James Hutton or Charles Lyell, Arthur Holmes ranks as one of the greatest geologists of all time. He is best known for his textbook Principles of Physical Geology. First published in 1944, this book is still used by many geology students today. When it comes to plate tectonic theory, however, Holmes can be thought of as the man who moved the continents. Holmes's career as a geologist started soon after his graduation from Imperial College of...
