Plate tectonics: Earth's continents and oceans are on the move

Transcription

1 Plate tectonics: Earth's continents and oceans are on the move By Phillip Heron, The Conversation, adapted by Newsela staff on Word Count 1,075 Level 950L A sign marking where the San Andreas fault line crosses in California. Photo from Flickr. It was once believed that Earth's continents were stuck in place. Fifty years ago, that idea was shaken up. In 1966, J. Tuzo Wilson introduced the idea that Earth's continents and oceans are constantly moving over our planet s surface. This is known as plate tectonics. It explains tectonic activity, which includes things like earthquakes and the building of mountains at the edges of continental landmasses. Today, scientists are still hoping to find out where the surface of our planet has been and where it s going. This article is available at 5 reading levels at 1

2 Evidence For The Theory German scientist Alfred Wegener first proposed the idea of plate tectonics back in He noted that the Earth s current landmasses could fit together like a jigsaw puzzle. Wegener analyzed fossil records that showed similar species once lived in faraway places. However, he couldn't explain how the continents could actually drift. Most other geologists dismissed his ideas. In 1966, Tuzo Wilson built on earlier ideas. He found the missing link: The Atlantic Ocean had opened and closed at least once before, he said. By studying rock types, he found that parts of New England and Canada were once part of Europe, and that parts of Norway and Scotland might have once been part of America. Wilson showed that the Atlantic Ocean had opened, closed and re-opened again. In that process, it swept up pieces of land with it. This was proof that our planet s continents were not stuck in place. They move. This article is available at 5 reading levels at 2

3 How Plate Tectonics Works The Earth is made up of many layers. The top part is the crust, and just beneath that is the mantle. Together, they re called the lithosphere and make up the plates in plate tectonics. We now know there are 15 major plates that cover the planet s surface. They move about as fast as our fingernails grow. Based on research of rocks, we know that no ocean is more than 200 million years old. The oceans opening and closing process explains how the Earth s surface evolves. A continent breaks up when hot, molten rock inside Earth changes its flow. That, in turn, affects the lithosphere, changing the direction plates move. This is how, for instance, South America broke away from Africa. The continent drifted away, the sea floor spread out, and the Atlantic Ocean was formed. In fact, the Atlantic is still opening. It is generating new plate material in the middle of the ocean. This makes the distance between New York and London just a few inches longer each year. Oceans close when their tectonic plates sink beneath another. Geologists call this process subduction. Off the Pacific Northwest coast of the United States, the ocean is slipping under the continent and into the mantle below the lithosphere. In slow motion, it is creating Mount St. Helens and the Cascade mountain range. This article is available at 5 reading levels at 3

4 In addition to undergoing spreading (construction) and subduction (destruction), plates can simply rub up against each other, usually generating large earthquakes. All three processes occur at the edges of plate boundaries. Still, there are a few things plate tectonics can't explain. For example, what produces mountain ranges and earthquakes within continents, far from plate boundaries? Gone But Not Forgotten Recently, scientists have used new technology to more clearly look deep below Earth and the tectonic plates. We have found many scars left over from the ancient collisions of continents. These old scars below the Earth s crust may still determine how certain processes happen on the surface of Earth. There may be hidden activity going on. If these plates, more than 20 miles below the ground, were reactivated, they would cause devastating new tectonic activity. This article is available at 5 reading levels at 4

5 It looks like previous plate boundaries may never really disappear. These structures contribute to geological changes even today. They may be why we see geological activity inside our continents. Mysterious Blobs 1,800 Miles Down Modern geophysical imaging also shows two chemical blobs at the boundary of Earth s core and mantle. These may trace back to our planet s formation. These hot, dense piles of material are 1,8000 miles beneath Africa and the Pacific. They re difficult to study, and nobody knows where they came from or what they do. These blobs sometimes interact with cold ocean floor that has subducted from the surface down to the deep mantle. When this happens, they generate hot plumes of mantle and blob material. This causes super-volcanoes at the surface. Does this mean plate tectonic processes control how these piles behave? Or is it that the deep blobs of the unknown are actually controlling what we see at the surface by releasing hot material to break apart continents? Answers to these questions have the potential to shake the very foundations of plate tectonics. These are some of the exciting questions scientists are working hard to answer. Plate Tectonics In Other Times And Places How did plate tectonics even begin? Scientists still don't know. The early Earth was much hotter inside than it is today, and plate tectonics worked differently. Our planet has changed quite a bit since then. So far, amazingly, Earth is the only planet we know of that has plate tectonics. Soon, we may be able to use what we know about plate tectonics to learn about other planets. This article is available at 5 reading levels at 5

6 A planet's ability to host living things is always linked to plate tectonics. Venus, for example, has no moving plates. Because of this, its air is 96 percent carbon dioxide, which makes life nearly impossible. On Earth, subduction helps push carbon dioxide down into the planet s interior and out of the atmosphere. It only took 3 billion years of plate tectonic processes to get the right carbon balance on Earth in order for living things to survive. A Theory Works Now, But What s In The Future? Geophysicists now believe that every tectonic movement may affect Earth forever. Life here would be vastly different if plate tectonics changed. For one thing, if those continentsized chemical blobs moved, they could cause super-volcanoes, covering the sky in ash for years. It s hard to understand what our future holds if we don t understand our beginning. By discovering the secrets of our past, we may be able to predict the motion of our plate tectonic future. Philip Heron is a scientist studying Earth's movements at the University of Toronto in Canada. This article is available at 5 reading levels at 6