Notes Unit 3:
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Causes
Earthquakes are caused by the shifting of plates or rock slabs. A fault occurs along a plane of weakness. When stress is acting on a rock it will built until the rock reaches its elastic limit, and breaks.
Waves
When faulting occurs, seismic waves are released
and spread in all directions. Waves travel in at different speeds
through objects of different density, moving faster through dense
objects, and slower through less dense objects. Still some types
of waves are not able to travel through liquids, so scientists
have been able to
ascertain that the outer core of the earth is liquid, the inner
core is solid, and the mantle is a semi-solid.
Measuring Earthquakes
Two different scales are used to measure earthquakes.
The Richter scale is used to measure the vibrations produced by earthquakes. These vibrations are measured using a seismograph. The Richter scale ranges from 1 - 10, with each number 10 times as powerful as the one before it.
The Mercalli scale is used to measure the damage done by these earthquakes. It ranges from 1 - 12, twelve being total devastation. Unlike the Richter scale, the Mercalli number cannot be tabulated immediately, for the damage must be surveyed.
For an up the minute list of recent earthquakes and their locations, click here.
For recent earthquakes in New York click here.
Effects
Ground shaking- Bedrock shakes very little, but loose sediments such as in a landfill shakes a lot. Buildings are the number one cause of deaths by an earthquake and buildings built on fill fall most often.
Ground failure- The vibrations cause rockslides. Landsides can wipe out entire villages.
Surface faulting - The surface actually cracks and rips apart due to the earth's movement. In the famous Alaskan earthquake neighbors ended up 1/4 mile apart!
Tsunamis - Large waves can be caused by earthquakes. These waves travel as fast as 747 planes (over 500 mph), and rise to as much as a 100 foot wall of water. Oddly if you were out to sea, you would have no sensation of the tsunami passing you. But the same wave will devastate coastal communities.
Focus v Epicenter
The focus is the point under the ground where the waves are actually produced, and the epicenter is the point in the crust directly above the focus. The epicenter is the first place to feel the tremors of an earthquake, and often the areas suffering the most damage from the quake.
We tend to think of earthquake waves as a series of identical waves that spread from the epicenter. In fact, there are two main different types of waves that spread from the epicenter, S and P-waves.
S-Waves -
S-Waves or transverse waves are waves that resemble ocean waves. As this type of wave travels through a medium, the medium is displaced, or moved, perpendicular to the direction of the wave. These waves case the most damage to buildings because they move along the surface of the Earth and shake it like a blanket. S-waves also can not go through liquids (like the outer core) and are slow. They are the second wave on scene, so they are called Secondary waves.
P-Waves -
P-Waves or longitudinal waves travel in a series of compressions and expansions. It is easiest to imagine an slinky. If you stretched it out on the floor, and pushed in on one side, then the very end of the slinky would compress, then the area next to that would compress, with the end expanding, and so on. The compression and expansion would occur in the same direction as the movement of the wave. P-waves travel very fast. They are the first wave to arrive, so they are called Primary waves.
If S-waves don't go through liquids, how do we know that the outer core is liquid? Click here to find out why.
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Magma is formed on the very top of the mantle, a melted region which is called the aestenosphere. The continents "float" atop this layer. Sometimes this magma is forced to the surface of the earth, and forms magma, which may eventually form lava. Click here for a volcano eruption animation.
Basaltic lava forms shield volcanoes, and granitic lava forms cinder cones and composite volcanoes. A shield volcano is relatively flat, and a cinder cone is steep and nasty. But the composite volcano (or stratovolcano) is the most violent volcano of all- Mt. St. Helen's erupted with a force equivalent to 500 atomic bombs!
Type 1: Shield Volcano
Type 2: Cinder Volcano
Type 3: Composite Volcano
To look up current volcanic activity, click here.
To check out volcanoes in different regions go here.
Check out an MPEG video on Mt. St. Helen. This volcano sent out a cloud of burning ash 100 mph. The ash fell as far away as New Jersey.
Formation and Structures of Volcanoes
Sills form parallel to the rock beds (horizontally), when magma forces between rock layers. The Palisades along the Hudson river is a sill. Dikes cut across layers when magma melts through layers. Dikes can cut perpendicularly or at an angle..
Laccoliths are similar to the sill, but they have an arc on top. Batholiths are the remnants of a magma chamber.
Continental drift
Continental Drift is the original theory of plate tectonics proposed by a meteorologist named Alfred Wegner in 1915. He came up with idea that Pangaea (a supercontinent) broke up 200 million years ago when continents "drifted" or floated across the oceans. At the time people thought it was a crazy idea and that Wegner was a kook. But Wegner had some good evidence.
Fossil Records - Often the same fossils are found on the edges of two continents that are thousands of miles away from each other. The only explanation is that these continents must have been connected at some point in time.
Fit of the Continents- Wegner noticed that the elbow of South America fits perfectly into the curve of the West Coast of Africa.
Identical Rocks- The rock types on either side of the Atlantic match up perfectly
Climatic clues like glacial scratches and location of coal deposits also were noted.
Plate Tectonics
Now we know it's the plates that move not the continents. Sections of the hard and brittle outer layer of the Earth called plates shift above the partially melted astenosphere below (in the mantle).
Proof of Plate Tectonics came as recently as 1968 when evidence of magnetic reversals where discovered.
Magnetic Reversal- The Earth's magnetic field reverses polarity (north becomes south and vice versa) periodically. When a rock solidifies it records the direction of the Earth's magnetic field by the orientation of its magnetite crystals. The pattern of north-south magnetic polarities in the ocean crust are identical on either side of the mid-ocean ridges.
Age of Ocean Plates- The age of the ocean crust is a mirror image pattern on either side of the mid-ocean ridges as well. The oldest crust is near the continents and the new stuff is right on the rift boundary.
Earthquakes and volcanoes- Earthquakes and volcanoes mostly occur on plate boundaries.
Tilted rock layers- Layers of rock are found throughout the earth are often found to be tilted, even though the beds were originally deposited in flat horizontal beds. The only way that this can be explained is that the beds must have been moved.
Plate Movements
The movement of rising magma causes convection currents- the force behind plate tectonics.
The plates move only two to six inches a year.They have moved for billions of years, and will continue to do so for billions more
Click here for an animation on plate movement.
Plate Tectonics Animated Cartoon
THREE types of movement
Convergence-
Convergence is when two plates collide.
When a continental plate collides with a oceanic plate, subduction occurs. The oceanic plate is more dense than the continental plate, and instead of colliding at the same level, the oceanic plate usually falls below the continental plate, or subduction. This subducted oceanic plate "sinks" into the mantle, where it will be melted and turned into igneous rock. The continental plate usually has volcanoes formed above the subduction zone.
When two continental plates collide, non-volcanic mountains are the result. The plates can't subduct because their density is too low. Very high mountain ranges result like the Himalaya Mountain Range in India.
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Divergence-
Divergence is when plates are being separated.
Most of the areas of divergence are on the oceanic crust. The most active area of divergence is in the Atlantic Ocean. The plates separate, magma fills the newly opened are, and this cycle repeats itself over and over. Iceland is a result of the magma that leaks out of one of these ridges.
Another area of divergence is the Great Rift Valley in Africa, where the ground literally split apart, only to be filled with lava. this process is still occurring, but at such an incredibly slow pace that it is very hard to monitor.
Transform-
Transform boundaries occur when plates move by slipping side-to-side. No volcanoes will form, but lots of earthquakes can occur. The San Andreas fault is a transform fault.
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San Andreas fault moves about 2 centimeters a year, but some of the land is locked. When it does open it could be disasterous for San Fransciso and Los Angeles. |
