Felsic Volcanoes are found in long mountain chains that wrap their way around the edges of continents and oceans.

They also form long island arcs.
(i.e., Aleutian Island arc)

These volcanoes are very violent and steep and form Stratovolcanoes.

 

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A Pyroclastic Flow similar to this buried the city of Pompeii!

People were sealed in tombs with volcanic ash.

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Composition of the Crust

Composition of Continental Crust – made mostly of the plutonic, felsic, igneous rock granite. Density = 2.7 g/cm³

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Composition of Oceanic Crust – made mostly of the volcanic, mafic, igneous rock basalt. Density = 3.0 g/cm³

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Meanwhile, strange things appeared in the HAWAIIAN Islands' volcanic rocks.

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Paleomagnetism: Ancient magnetics of the earth

It so happens that when lava solidifies, it captures the earth’s magnetic field when the rocks cool.  People studying these lava flows started noticing an interesting pattern.  At first, they tossed the results out as just instrument failure until the pattern kept repeating itself over and over again on all the different Hawaiian Islands.  Basalt from some lava flows would point north in the direction that compasses point in today, while lava-cooled basalt layers that were deeper underground pointed north in the opposite direction (South Today).  This was a great mystery at the time.

Using the key, color the layers of basalt as you dig into the earth and go back deeper in time.

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It took a war to help figure out these mysteries. During World War II, German U-BOAT submarines caused havoc to the American and British Navy. In order to find these elusive submarines, a clear map and an understanding of the ocean floor were necessary. The U.S. Navy crisscrossed the Atlantic Ocean, dragging behind them magnetometers (something like a compass) that mapped the magnetic field of the ocean floor as they looked for these German submarines. They discovered that interesting patterns had emerged on the ocean floor.

The mid-ocean ridge system has been compared to a wound in the earth that never heals.
They look like stitches in a baseball.

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These Mafic Mountains show divergent plate boundaries.

The Earth is being ripped apart at these ridge systems, and deep mafic magma is rising to create a new ocean floor.

Rising an average of about 4,500 meters (m) above the seafloor, the mid-ocean ridge is higher than all the mountains in the United States except for Mount McKinley in Alaska (6,194 m). Though hidden beneath the ocean surface, the global mid-ocean ridge system is the biggest topographic feature on the surface of our planet.

Life on the Ridges – Strange life has recently been discovered along with these mid-ocean ridge systems. In places where sunlight does not shine, large communities of animals thrive. What allows these animals to survive in conditions that would seem so hostile?

Bacteria feed off this energy coming from the earth.
They chemically synthesize food using heat energy from the earth.

This process is called
Chemosynthesis!

Other animals feed on the bacteria.

One of the very few food chains that does not get its energy from the sun.
Hot hydrothermal vents give energy to this strange deep ocean life.

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The magnetism of the Mid-ocean Ridge

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Color the magnetic stripes on one side of the ridge using your key.

Mid-ocean Ridge

Now figure out what the other side will look like and color.

Take out your reference tables and draw some magnetic stripes along the ridges using the same color scheme.

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Scientists also checked the age of the ocean crustal rocks and found that the youngest rocks were at these mid-ocean ridges, and the rocks get older as you travel away from the ridge.

What does this suggest?

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Three things have to happen to achieve this pattern.

1.  Lava must be coming up at the ridge, capturing the Earth's magnetic field.
2.  The tectonic plates must move apart at the ridges.
3.  The magnetic field of the Earth must reverse its polarity.

This proves that the seafloor is spreading in the Mid-ocean Ridges!

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Sea Floor Spreading in a nutshell

When igneous rocks cool they trap the earth's magnetic field.
The earth's magnetic poles have reversed many times in the past.
This reversal is captured and shown as magnetic striping along mid-ocean ridges.

Conclusive Evidence: The seafloor is being created and is spreading apart at the mid-ocean ridges.