Presentation on theme: "Our Ever-Changing Earth! Part 1: Continental Drift and Seafloor Spreading."— Presentation transcript:
Our Ever-Changing Earth! Part 1: Continental Drift and Seafloor Spreading
For centuries, people have wondered about the strange things happening on the surface of our Earth. How did the continents and oceans form? Why were fossils of seashells and other sea creatures found on the tops of mountains? How do mountains form? What causes earthquakes and volcanoes? Why do they occur in some places but not others? What is it like at the bottom of the ocean? What is it like under the surface of the land?
Moving Continents? Since as early as 1598, as better maps were drawn, people wondered why some continents appeared to “fit” together so nicely, especially South America and Africa.
Finally, in 1915, a German meteorologist named Alfred Lothar Wegener was the first to try to explain the possibility of “moving” continents.
Alfred Lothar Wegener ( ) Studied meteorology, astronomy, climatology and geophysics. Very intelligent and interesting man. Explored Greenland several times. Actually died (froze to death on a rescue mission) while on Greenland!
In 1915, Wegener published his book describing his Theory of Continental Drift And he filled the pages with evidence to support his theory. He used information from fossils, rocks, ancient climates, and more, to back up his ideas.
Alfred Wegener’s Theory of Continental Drift. The Evidence! One of the most interesting fossils he studied was Glossopteris, an extinct tree that had large seeds – too big to be carried by wind and too weak to survive a long ocean voyage. How was that a possible clue?
And he found that fossils of many other plants and animals matched up too.
He also noticed similarities in rock formations from South America and Africa. Coincidence?
And other people helped him by noticing that ancient mountain chains lined up. But they could not explain the “gaps” in the ocean. Alfred thought he could!
He also noticed similarities in glacial deposits from hundreds of millions of years ago.
He found that many other deposits, including coal deposits, matched up perfectly as well Why might coal be so fascinating to Wegener?
Wegener suggested that all the continents were once connected into one supercontinent he called Pangaea. Pangaea is a word that means “all land”. He believed it existed about 300 million years ago. (We now believe it was more like 225 million YA) He believed it then separated into two continents – one called Gondwanaland and one called Laurasia, separated by the Tethys Sea. Gondwanaland and Laurasia later separated into the continents we know today.
Unfortunately for Alfred Wegener, his theory of continental drift was rejected by most people. Many considered his ideas crazy, and Wegener a madman! His theory had evidence, but it did not explain HOW such massive things (continents) could move. For years, long after his death, his theory was mostly ignored. Click on image for video (2:14)
But a very interesting thing happened in the 1940s and 1950s that got people thinking again about Alfred Wegeners theory.
It is called sonar. During WW II (1940s) sonar was used to look for submarines. It also gave people their first look at the bottom of the ocean. What they found surprised everybody…
Mid-ocean ridges. Miles of them! They are the longest mountain chains in the world – and they are underwater. They are like the seams of a baseball, running all around the Earth. They have several parallel ridges with a steep valley down the middle (twice as deep as the Grand Canyon).
They also found deep sea trenches. These are the deepest valleys in the world – and they are also under the sea. They have extremely steep sides. Some of them are 3 to 5 miles deep – the deepest spots in the ocean.
In 1960, American geologist Harry Hess made a startling hypothesis. Hess suggested that the ocean floor was separating at these ridges. He proposed that molten magma rises there, causes the spreading, cools, hardens, and becomes new sea floor (oceanic crust).
Throughout the 1960’s, more and more evidence supported the idea of seafloor spreading. Observation: The deep-sea submersible ALVIN (from Woods Hole Oceanographic Institute) observed evidence of “pillow lava” at the ridges. Drilling Samples: The age of the crust varied as you moved farther from a ridge, but the crust was never older than continental crust.
More evidence... Depth of Sediments: Sediments were thinner nearer the ridge and thicker farther away. Why? Magnetic Striping: The newly forming ocean crust was actually “recording” the magnetic reversals of the Earth every 600,000 years.
But one question remained: If new crust is forming at the mid- ocean ridges, why isn’t the Earth getting larger and larger?
The Answer: Subduction Zones. Subduction is where ocean crust is being forced under other crust. It is believed to be melted back into magma. Recycling? Subduction zones also explain those deep sea trenches! Click on image for video (4:02)
Finally, when it was all added together… The Theory of Plate Tectonics!