Learn Icelandic: Mörg jarðlaganna eru úr dökku basalti. - Many of the geological layers are made of dark basalt.

In formal or written Icelandic, to express "many of the [noun]", you use the quantifier followed by the noun in the genitive definite form. Here, mörg (many) is followed by jarðlaganna, which is the genitive plural definite form of the neuter noun jarðlag (geological layer).

If "jarðlaganna" is in the genitive case, why is "mörg" in the nominative neuter plural?

The quantifier must agree in gender with the noun it refers to, which is neuter. It takes the nominative case because "many of the layers" is the subject of the sentence. The partitive structure forces the noun into the genitive, but the quantifier keeps the grammatical case required by its role as the subject.

Why do we use the preposition "úr" here instead of "af"?

While both can translate to "of" in English, úr is specifically used to indicate the material something is made out of. Here, the geological layers are made of basalt. Úr always takes the dative case, which is why we see dökku basalti.

Why does the adjective "dökku" end in "-u"?

It is modifying basalti, a neuter noun in the dative singular. Because there is no definite article attached to the noun (it is just "dark basalt", not "the dark basalt"), the adjective uses the strong declension. For strong neuter dative singular adjectives, the standard ending is -u.