Earth Movements: How do natural events affect our earth?


The Planet Earth

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We can compare the planet Earth to an egg. The yolk represents the core, the white represents the mantle, and the shell represents the crust. Like with the egg, the crust is the thinnest part (only 5-50km deep). It is made of solid rock, mainly basalt in the oceanic crust, and granite in the continental crust. The continental crust is thicker than the oceanic crust. It is especially thick where there are mountains.

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Because the granite of the continental crust is less dense, it floats above the denser basalt below in the ocean.

Basalt and Granite

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Scientists started noticing similar rock formations in different continents. Additionally, scientists found glacial striations (scratches on rocks caused by glaciers) and glacial deposits of the same age and type of rock in different continents.

Fossils of Ammonite

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Furthermore, scientists also started noticing fossils of freshwater animals in two different continents. Fossils are remains or imprints of prehistoric organisms (living things) preserved in rocks. Because those animals could not have swum across the salty ocean, scientists began theorizing that the continents were once all together in one big continent called Pangaea, two hundred million years ago. Over millions of years, the continents began drifting apart. This is called the theory of continental drift. If you look at the shape of the continents, they look like they used to fit together into one big puzzle.

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Continents keep drifting 5cm a year, which is the rate at which a fingernail grows. What enables them to drift are plates, or pieces of the Earth's crust. These plates float on the magma of the mantle. Currents in the magma move these plates, which makes the continents on top to move too. Scientists believe there are eight larger plates and several smaller ones.

Some plates, called continental plates, have continents as part of them. Continents are the parts of the crust that rise above sea level. Oceanic plates only consist of the ocean floor. The theory of plate tectonics states that all of the Earth's crustal plates are slowing moving in different directions at different speeds on top of the semi-liquid mantle. Some move towards each other, some move away, and some slide past each other. What drives all of these movements, is convection currents.

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A convection current is when heated molecules of a fluid (gas or liquid) expand, become less dense, and move up. As the hot fluid cools, it becomes denser and moves back down. This process keeps happening, like a conveyor belt.

Convection Current

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Convection Currents in the Earth's mantle

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Video of Convection Currents

At the boundary between two plates, are ridges. A ridge is an edge where there is a peak and a slope on each side. Under the ocean are mid-ocean ridges, which form at the boundary between oceanic plates that are moving away from each other.

Mid-ocean ridge

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This is part of a process called ocean-floor spreading. The convection currents in the magma, push up through the cracks between two oceanic plates, and solidify to form new crust. The new crust is created along the center-line of the ridge and added to the plate edges as the plates slowly separate, creating new areas of ocean floor. Because the ocean floor spreads at different rates in different spots, the mid-ocean ridges are often zigzagged.

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As ocean floor spreading creates new crust, old crust is being destroyed by a process called subduction. Subduction happens when two plates collide, pushing the edge of one plate below the edge of the other. This process can happen between two oceanic plates, two continental plates, or an oceanic and a continental plate.

Oceanic plates are mostly made of dense basalt, and continental plates are mainly less dense granite. When an oceanic plate and a continental plate collide, the denser basalt is subducted (overridden) by the less dense granite. The oceanic basalt sinks, and eventually remelts back into the mantle's magma. In the spot where the plates collided, a deep trench (ditch) forms. The deepest trench on earth is the Mariana Trench in the Pacific Ocean. It is deeper than the height of Mt. Everest!

Video of Earth's Convection Currents Moving Plates

Mariana Trench in the Pacific

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At these trenches, there tends to be a lot of earthquake and volcanic activity. When volcanoes erupt, they can form new islands over time. Because the Earth is spherical, the chain of islands form in arcs rather than in straight lines.

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Most of the world's mountains are part of mountain ranges. In general, mountains form when crustal plates collide and fuse (stick together) to form a larger plate. When the two plates collide, the continental crust at the plate boundary is pushed up into many mountains (a mountain range). Rocks are folded and faulted.

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Volcano Map

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Volcano Map

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Earthquake Map

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Video of the History of the Planet Earth

Resources

1.

2. Earth Movements Glossary

3. PBS Video: How Something Becomes a Fossil

4. PBS article: Becoming a Fossil

5. Scholastic Article: Major Fossil Find

6. History Channel: Deepest Place on Earth

7. History Channel: Oceans

8. Video: How Volcanoes Form

9. Video: Convection Currents

10. BBC on volcanoes

11. Videos and text on convection currents

12.

13. Video: Continental Drift

14. Earth Science Videos:

http://www.neok12.com/Earth.htm

15. Schooltube Video on Convection Currents

16. Vimeo Video: Convection Moves Plates

17. Earthquake-Proof School Tables, Video and Article

18. Vimeo Video: Convection Currents