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.
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!
Top 10 devastating volcanic disasters, click here.
Check out a 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.
Most of the amazing landforms of the Earth such as our long mountain ranges and volcanoes are caused by plate tectonics.
The plates of our Earth consist of two layers- the crust and the solid outer mantle.
The ocean plate is denser and thinner and the continental plates are less dense and considerably thicker.
You can measure the density of these two plates by dividing mass by volume.
D = M / V
(M) is mass, or the number of molecules in an object.
(V) is volume, or the space mass occupies.
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- The same fossils were found on the edges of continents that were thousands of miles away from each other. Wegner realized it was unlikely that animals (and plants) could evolve to be identical to each other without interbreeding.
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- Glacial scratches and location of coal deposits also supported his theory.
Mountain ranges- Long mountain ranges such as the Appalachian mountains appeared to continue into Europe once you fit the continents together.
Now we know it's the plates that move not the continents. The hard and brittle crust and outer mantle called lithospheric plates shift above the partially melted lower layer of mantle called the asthenosphere.
The Earth is divided into 12 major plates and many minor ones. You are required to be able to name the major plates using the reference tables.
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 but mirror opposites on either side of the mid-ocean ridges.
Age of Ocean Plates- The age of the ocean crust is also a mirror image pattern on either side of the mid-ocean ridges as well. The oldest crust is near the continents and the new rock is right on the rift boundary.
The movement of rising magma causes convection currents- the force behind plate tectonics. Hot magma is less dense and rises to the surface. The plates are pulled apart as the magma moves sideways away from where it rose.
The plates move only two to twelve inches a year. They have moved for billions of years and will continue to do so for billions more. Earthquakes are the inevitable result of moving plates and can actually be mapped to show plate boundaries.
Earthquakes and volcanoes lie along the plate boundaries.
THREE types of plate movement
Convergence is when two plates collide.
When a continental plate collides with a oceanic plate, subduction (sinking) occurs. The oceanic plate is more dense than the continental plate, and instead of colliding at the same level, the oceanic plate sinks below the continental plate. This subducted oceanic plate is then melted and volcanoes form on land (such as the Andes Mountains).
Ocean-ocean crust collisions cause volcanic island arcs such as Japan, New Zealand or the Aleutian Islands.
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 Himalayan Mountain Range in India.
Divergence is when plates are separating. They create long chains of volcanoes underwater or on land.
Most of the areas of divergence are on the oceanic crust. One of the most active areas of divergence is in the Atlantic Ocean. The plates separate, magma fills the newly opened area and this cycle repeats itself over and over with the land masses moving away from each other.
Another area of divergence is the Great Rift Valley in Africa, where the ground literally splits 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 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.
San Andreas fault moves about 2 centimeters a year, but some of the land is locked. When the land releases and a large earthquake does occur it could be disasterous for San Fransciso and Los Angeles.
Hot spots are when an individual magma plume creates volcanoes in the plate above it. However, since the plates slowly move over time, the volcanoes look like a chain of volcanic islands in the ocean. But unlike an island arc, all of the volcanic islands get older as you move away from the active hot spot. Hawaii is a great examples of a hot spot.