The age of the Earth is ______ billion years.
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Which element is the most abundant one in the crust (by weigh)?
A. Si
B. O
C. Al
D. Fe
Silicate minerals are the most common type of minerals,
Metamorphism can also be divided into prograde and retrograde metamorphism
Metamorphic grade: is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.