There is really nothing to know. All you have to use is context clues and common sense. We know some synonyms for direct are manage and handle. Indirect means not direct, right? And we can't handle or manage earth's interior (core), right? Therefore, the study of earth's interior is an example of indirect observation because we cannot directly manage it.
By people sailing or drifting from southeast Asia
The name for such a hole is "caldera" which is a word of Spanish origin meaning "a pot" or a "cooking pot" (thing of cauldron!) - and this word is used because it resembles the shape of this hole.
Often the Calderas are filled with water forming lakes and often there is an island in the middle of the water.
Marine Pollution involves the introduction into the ocean by humans of substances or energy that changes the quality of the water or affects the physical, chemical, or biological environment.
Chemicals and debris, the majority of which originates on land and is dumped or blown into the water, make up marine pollution. The ecology, the health of all living things, and global economic institutions are all harmed by this pollution.
In the modern world, marine pollution is becoming a bigger issue. Chemicals and rubbish are the two main sources of pollution in our ocean.
Chemical contamination, often known as nutrient pollution, is problematic for the environment, human health, and the economy. This kind of pollution happens when human activities, particularly the application of fertiliser on farms, cause chemical runoff into waterways that eventually empty into the ocean.
Learn more about marine pollution here
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Answer: Probably, harder the mineral it scratches. It makes more sense.