It can help to go over the main ideas or important moments of the story and then see what those are portraying, whether there is a lesson or not. you can also look at the ideas of the characters and what they are thinking to determine the theme.
Researchers found a total of 138 volcanoes in the West Antarctic Ice Sheet overlying the West Antarctic Rift System, 91 of which had never before been identified. Basically they have never seen those types of volcanoes before.
Emigrate means to leave one's country to live in another.
Immigrate is to come into another country to live permanently.
Migrate means to move & can refer to either.
If the Beluga Whale makes a sound that hits a fish moving towards it, the frequency of the sound that the Beluga gets back will be increased (higher pitch). If the fish was moving away when the sound hit it, the sound will come back to the whale in a lower frequency (lower pitch). So depending on what sound the whale hears reflected off the fish, it can tell if the fish is moving away or closer.