How is the process of freezing magma similar to that of freezing water? How is it different?
It is similar because in both processes the temperature is decreasing and both substances are solidifying. They are different because magma can be made of many different types of rock and water is just two elements.
What are the sources of heat in the Earth? How did the first igneous rocks on the planet form?
The Earth is heated by planetismal accretion, gravitational compression, differentiation into the crust, mantle, & core, and radioactive decay.
The first igneous rocks formed from freezing magma.
Describe the three processes that lead to the formation of magmas.
Decrease in pressure: a decrease in pressure permits melting because atoms are able to break free of solid mineral crystals. This occurs when hot mantle rock rises to shallower depths in the Earth.
Addition of volatiles: when volatiles mix with mantle roc
Why are there so many different types of magmas?
Because it depends on the source, how it interacts with its surroundings (assimilation), and whether crystals sink as they form (partial melting and magma mixing).
Why do magmas rise from depth to the surface of the Earth?
Because it is less dense than surrounding rock and becuase the weight of the overlying rock creates pressure that squeezes magma upward.
What factors control the viscosity of a melt?
Temperature: hotter magma is less viscous, volatile content: volatile atoms tend to break apart bonds and may accumulate to form bubbles, and silica content: mafic magmas are less viscous than felsic magmas.
What factors control the cooling times of a magma within the crust?
The depth of the intrusion, the shape and size of a magma body: the greater the surface area for a given volume of intrusion, the faster it cools, and the presence of circulating ground water: water passing through magma absorbs and carries away heat.
How does grain size reflect the cooling time of a magma?
Extrusive rocks cool rapidly and have small grains. Intrusive rocks cool much slower and have more time to grow larger grains.
What does the mixture of grain sizes in a porphyritic igneous rock indicate about its cooling history?
It cooled slowly at first and then was ejected from the volcano causing it to cool rapidly.