”all but a few hundred of them extinct” means most extinct？
Most of us, when we talk about volcanoes, think of the classic cone shapes of a Fuji or Kilianjaro, which are created when erupting magma piles up. These can form remarkably quickly. Altogether there are some ten thousand of these volcanoes on Earth, all but a few hundred of them extinct.
1. all but a few hundred of them extinct.
Does it mean most volcanoes are extinct volcanoes, only a few volcanoes are active volcanoes?
These are volcanoes so explosive that they burst open in a single big crack, leaving behind a vast hole, the caldera.
2. Does “ crack“ here mean sound？
Poet William Stafford once said that we are defined more by the detours in life than by the narrow road toward goals.
3. Does it mean our life is full of detours rather than narrow road toward goals?
most universities already allow highly qualified students to graduate early by testing out of certain classes and obtaining a number of college credits
4. Is “test out of ”a phrase, I can not find it.