Learn Turkish: Bu formasyonlar sadece taştan mı ibaret? - Are these formations composed solely of rock?

The word ibaret (meaning composed of or consisting of) always requires the preceding noun to be in the ablative case (-DAn). Because taş ends in the voiceless consonant ş, consonant harmony changes the suffix to -tan.

The lesson title mentions ibaret olmak. Why isn't there a form of olmak at the end of this sentence?

In present-tense nominal sentences (sentences where the main verb is simply to be), the copula is often implied rather than explicitly stated. You could formally say ibaret midir?, but in everyday speech, it is completely normal to just end on the word ibaret.

Why is the question particle mı placed before ibaret instead of at the end of the sentence?

The question particle mı comes immediately after the word or phrase being emphasized or questioned. By saying taştan mı ibaret?, the speaker is specifically questioning if it's made of rock (as opposed to something else). If they asked taştan ibaret mi?, it would be grammatically correct but would place slightly more focus on the fact of it being composed.

Is sadece strictly necessary here, or does ibaret already imply only?