Lecture 10 - Plate Tectonics, Earth Structure andVolcanoes

Outline

I. Plate Tectonics and Earth Structure

II. How Magma Forms

 

I. Plate Tectonics and Earth Structure

Volcanoes are not randomly distributed across the globe but arefound in bands that also define regions where earthquakes occur. What determines where volcanoes occur and why the association with earthquakes? The answer is that both earthquakes and volcanoes occur at boundaries of large plates that make up the surface of the Earth and that move around with respect to each other producing earthquakes.

The part of the Earth that makes up the plates is called the lithosphere, and it is strong. The lithosphere is underlain by a weaker layer called the asthenosphere. The weakness of the asthenosphere makes it easy for the lithosphere to glide over it. The layers of rock that make up the interior are:

The plates move because the interior of the Earth is hot and the Earth is trying to get rid of its heat. It transfers heat from the interior to the surface by convection, which requires moving hot material to colder regions and colder material to hotter regions. Hot solid rock from the interior is less dense than surrounding rock and very slowly rises towards the surface of the Earth transporting its heat with it. At the surface, the hot rock cools and becomes very strong forming the plates. The cool material is more dense than surrounding rock and sinks down. These cells of rising warm material and sinking cool material are known as convection cells. The plates are the top cool part of this cell. Areas where the cold plates dive back down into the mantle are called subduction zones, they are regions of plate convergence. Areas where hot material rises to the surface make up the mid-ocean ridges where new ocean floor is being created. These are divergent plate boundaries. Magmas with different compositions are erupted at these different plate boundaries, with basaltic magma being produced at mid-ocean ridges and andesitic magma erupting at subduction zone volcanoes.

II. How Magma Forms

How and where do magmas form and why are there 3 major types of magma (basaltic, andesitic and rhyolitic)?

Magma is produced when temperatures at depth in the Earth exceed the melting temperature of rock. Rocks of different composition have different melting temperatures and these temperatures increase as pressure or depth in the Earth increases. In the regions of the Earth where the pressure and temperature are favorable to melt rock, we get magma.

The different compositions of magma are due to what material melts to make the magma and how much it melts. Since rocks are made up of many different minerals, if we only melt a little bit of the rock, the melt will not have the same composition as the solid rock but the composition of the minerals that melt first at lower temperatures. Think about a chocolate chip cookie that sits in the sun and begins to melt, the chocolate chips have a lower melting temperature than the cookie so they begin to melt first. If you separated this meltf rom the cookie, you would have a melt with an entirely different composition from the cookie. The same is true for rocks. So the 3 major types of magma come from melting different rocks, but only partially melting these rocks.

  1. Basalt- comes from partially melting mantle rock (peridotite). The first minerals to melt in peridotite combine to give us basalt.
  2. Andesite- comes from partially melting oceanic crustal rocks (basalts). The first minerals to melt in basalt combine to give us andesite.
  3. Rhyolite- comes from partially melting continental crustal rocks (diorite or andesite composition). The first minerals to melt in diorite combine to give us rhyolite. Because the SiO2 content of rhyolite is very high, it is very viscous and often cools and solidifies before it reaches the surface. Therefore, most rhyolitic magmas form granites.

 

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