Hi, I am learing about earth science and I need to know about the metric system. What are the main meaurments o .. Mass, Wieght, Density, and Volume.. For example the amin measurment of lenght is a meter . Thank you

Here is a good start:
http://en.wikipedia.org/wiki/Metric_system

Well a meter = 39.4 inches for a start
1 inch = 2.5 centimeters

Check out the website above.

You can even google metric conversions

On this matter, is there any reason why US has never converted to the metric system?
Has it ever been looked at?
Just interested that's all!

There have been numerous acts of legislation to make metric official in the USA. I think the obstacle here is psychological. The old units of length are based on body parts, which we can relate to. Other units are based on ordinary objects. Another reason, given in the early 19th century, was that the names for metric units are cumbersome and long. I do agree that a unit with a ten-letter name can be overwhelming when we are accustomed to units with four and five-letter names. I am referring here to millimeter, centimeter, inch, mile, and pound. Other reasons for the USA not converting was due, mostly in the 19th and part of the 20th centuries, is that the USA manufactures most of its own goods, and rarely imported. With our factories now overseas, nearly all of them using metric, it is time we make the switch to metric so we can do business with them more easily.