Download Factors that Shape the Earth

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Factors that Shape Earth’s Surface
Unit Notes
 Plate Tectonics
Continental Drift
About 250 million years ago, the earth’s continents were all connected as one big landmass called Pangaea.
The theory of continental drift states that Pangaea split apart forming individual continents that have
moved to their present positions. Evidence to support this theory includes:
- The outlines of some continents appear to fit together like jigsaw puzzle pieces
- Identical fossils, rocks, and mountain ranges found on continental coastlines separated by vast oceans
- Coal deposits and fossils are found in climates where organisms couldn’t possibly have survived
Sea-floor Spreading
Earth’s crust is made of continental crust (thicker but less dense) and oceanic crust (thinner but more
dense). When a continent splits apart, the continental crust separates, allowing the magma beneath it to
rise, forming a mountain ridge of denser rock. Ocean water fills in the space between the splitting land
masses and covers the ridge. As the land continues to split into two continents, new oceanic crust is
formed between them at the ridge, so the farther away from the ridge, the older the material. As new
crust forms, old crust is forced into trenches along the continents where it is returned to Earth’s mantle
and forms magma again. These trenches are called subduction zones. Evidence of sea-floor spreading
includes heat measurements, volcanic activity, age measurements, and orientation of magnetic crystals in
the rocks.
The theory of plate tectonics combines the ideas of continental drift and sea-floor spreading and explains
how the continents move. According to the theory, the Earth’s crust is broken into tectonic plates that
“float” on the liquid magma-type material in the mantle. This movement occurs because some areas of the
mantle are hotter than others (due to radioactive decay) and it tends to rise in these areas if the
pressure overlying it weakens. The movement of mantle material creates a convection current pattern
and the plates move with the current.
{You should have two important handouts with diagrams of the tectonic plates and a cross section of plate
boundaries on the Earth.}
Plate Boundaries
- Divergent plate boundaries occur where plates are separating, as in ocean ridges.
- Convergent plate boundaries occur where plates are colliding, as in the Himalaya mountains.
- Transform plate boundaries occur where plates are sliding past each other, as in the San Andeas Fault
of California.
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Results of plate movement
 Mountains form in the Earth’s crust from folding (where plates are colliding), faulting (where plates are
separating and one plate drops lower), or from volcanic activity (the build up of magma creates a
mountain).
 Earthquakes occur whenever plates move. They are detected by instruments called seismographs and
measured using the Richter scale: the higher the number, the more powerful the earthquake.
 Volcanoes form at convergent boundaries; lava flows occur at divergent boundaries. Sometimes,
volcanoes occur in the middle of tectonic plates as the plate moves over a “hot spot” (Hawaii is an
example).
 Tsunamis are huge ocean waves (with wavelengths several hundred kilometers long) caused by
earthquakes in the ocean or along a coast. They cause tremendous flooding when they reach a coastline on
the other side of the ocean.
The ‘Pacific Ring of Fire’ is a zone of earthquake and volcanic activity that surrounds the Pacific Ocean
due to the interactions between the Pacific tectonic plate and various plates in contact with it.
Tectonic activities tend to build the land up; weathering, erosion, and deposition try to level the Earth’s
surface.
 Weathering
Weathering is the breakdown of material (usually rocks) and can be either physical or chemical.
 Physical / Mechanical weathering changes the size or shape of a rock but the rock is still the same kind
of rock (granite, shale, etc.)
Examples:
- Ice Wedging / Frost Action -- water enters a crack, freezes, expands, thaws, sinks down further into
crack, freezes, expands, etc.
- Plant roots grow in cracks and break the rock as the roots grow
- Temperature changes (alternately freezing and thawing) can strain a rock and cause it to crack
- Abrasion -- rocks being carried along in a river rub against each other in the water, wearing them down
 Chemical Weathering changes the chemical composition of the rock through a chemical reaction,
producing new substances. Pollution can greatly increase this process.
Examples:
- rainwater on rocks containing iron breaks it down into rust
- water on limestone dissolves the rock, forming a cave
- acids from decaying organic matter can break down rock
The rate of weathering depends on:
 Temperature -- greater in climates with hotter temperatures
 Moisture -- greater in moist climates
 Surface area -- a pile of pebbles will weather faster than a large rock of the same mass because the
pebbles have more surface area
 Chemical composition -- some rocks (i.e. granite) are more resistant to weathering than others
(i.e. limestone)
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Soil forms from weathered bedrock and humus: the decayed remains of dead organisms. The longer the
area has been weathered, the deeper the soil.
 Erosion and Deposition
Weathering breaks down rock into small pieces; erosion is the process that picks up those pieces and
carries them away. Deposition drops them off in new locations.
The presence of plants reduces the amount of soil erosion because roots hold water and soil in place.
Agents of erosion:
Rivers -- rain that falls on the land eventually makes it into rivers, carrying dirt on its way. Once in the
river, the water’s motion moves rock pieces along with it. Erosion is greatest wherever the water is
moving the fastest: on steep slopes and the outside of curves. Deposition occurs whenever the water
slows down (gentle slopes and inside curves) because the water no longer has the energy to carry
particles. Deltas are areas where rivers enter the ocean and particles are dropped, spreading out to form
a triangle.
Glaciers -- snow that falls for many years without melting can build up and turn to ice which can flow
downhill due to gravity and the pressure of the snow on top. When it moves, it scrapes the bedrock under
it, eroding the land and pushing material ahead of it like a bulldozer. When the glacier melts, the material
is deposited in a new location. New York State landscapes were formed by great continental glaciers that
covered the area up to a mile thick about 10,000 years ago.
Ocean Waves -- the physical action of waves and the chemical action of salt water break up the rocks
along coastlines and carry them out to sea or drop them off further down the coast.
Wind -- wind can pick up sand-size particles or smaller that are loose and dry. The stronger the wind,
the larger the material that can be moved. When the wind dies down, the particles are deposited.