Ed Golson Outdoor Education

changes to the earths surface

Earth's surface has changed greatly since the beginning of time and is still changing today. With the use of modern technology, scientists can observe how the Earth's surface changes over time.  Sometimes scientists cut down into the Earth's surface, looking at it in a cross section, or they take a picture from above called an aerial view. 

Changes can originate from below the surface and are caused by earthquakes and volcanoes.  The surface of the Earth can also be changed by an impact from outer space, such as from a meteor strike.  Most commonly, however, the changes to Earth's surface are due to weathering and erosion.

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The Earth is made up of large plates below its crust.  At the boundaries of these plates movement can occur.  When these plates move, its causes changes on the surface.  There are four types of plate boundaries:

  • Divergent boundaries -- where new crust is generated as the plates pull away from each other.  Picture two giant conveyor belts, facing each other but slowly moving in opposite directions as they transport newly formed oceanic crust away from the ridge crest.   

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Convergent boundaries -- where crust is destroyed as one plate dives under another. Such destruction (recycling) of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks (is subducted) under another. The location where sinking of a plate occurs is called a subduction zone. The type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved. Convergence can occur between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates

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  • Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other. Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes.

*Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are unclear.

A fault occurs where rocks are displaced.  The way these rocks are displaced depends on the type of force on the rock.  There are three types of faults:  normal, reverse, and strike-slip. All three types move differently.  Normal faults move vertically usually at divergent boundaries.  Reverse faults move both vertically and horizontally.  When two plates move toward each other there is compression stress on both sides resulting in one rock being pushed up over the other rock.  Finally there are strike-slip faults that move horizontally past one another on a nearly vertical plane. 

San Andreas Fault

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Surface faults usually occur near the top of Earth's surface, however; fast movement or great stress on the plates can cause the same effect as a surface fault.  The behavior of faults generally ends up in an earthquake, but interior plate movement can also cause earthquakes and also can create another fault. 

Volcanoes are another way that Earth's surface can change.  There are three types of volcanoes: shield, composite, or cinder cone.  Volcano classification is determined by the type of magma it produces and how the volcano was originally made.

Shield volcanoes are not the common cone-shaped structures that produce basaltic magma.  These volcanoes are usually wide and dome shaped found at divergent plate boundaries.  Composite or stratovolcanoes are the cone-shaped mountains that produce andesitic magma.  These volcanoes are found at convergent boundaries.  Finally cinder cone volcanoes are smaller structures that produce either basaltic or andesitic magma.  These volcanoes are found at convergent boundaries.

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Some volcanoes erupt violently while others just flow.  Some simply sputter out.  How can you tell which ones do which and why?  Shield volcanoes erupt smoothly because of the way they are designed which allows gases to escape freely.  Composite/stratovolcanoes erupt violently because the top of the volcano has a layer of rock and the pressure and gases build up making the volcano literally blow its top.  Cinder cone volcanoes make craters by making steep sided mounds where the magma once was. 

There are also ways on Earth's surface that change the geology.  Weathering is one of them.  There are physical and chemical ways of weathering.  Physical weathering is breaking down materials into smaller fragments such as breaking a large rock into thousands of pieces.  Chemical weathering involves changing one material into another such as changing coal into diamond.  Erosion is also related to this.  Moving water, wind and ice can cause erosion by moving particles from one place to another or by dislodging particles from a bigger piece.  Did you know the Grand Canyon started out as a river?   

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