Q. Are encyclopedias a credible source?

This depends on the encyclopedia*, of course, but generally speaking -- sure, by and large, encyclopedias are credible. But.

Credibility (stemming from the accuracy of the information and the authority of the author/publisher) is only one aspect to consider when doing research.

Encyclopedias provide summaries and overviews of topics. They're great for when you're starting your research and need to learn what search terms to try out, or what directions you could take your exact topic focus towards.

For college-level research papers, though, you're typically expected to go past this surface-level of information. Encyclopedias (including Wikipedia, in this librarian's opinion*) are fine places to start, but you can't finish your research there, and they probably shouldn't appear in your final bibliography. 

For example, let's say you're researching pottery for a humanities class.

Wikipedia on pottery is a somewhat long article, and of course it has links to lots of other related articles. However, it still provides an overview of the topic. Jomon pottery is just one paragraph -- 5 sentences -- in the section on the beginnings of pottery. Pottery of the early Americas is likewise a short paragraph (albeit linked to a dedicated article). 

This information is an introduction, not an exhaustive analysis of the topic in question. 

If we jump over to the library database JSTOR and search for "early pottery," we can find entire scholarly articles on the subject that spend many times the word count discussing it. Not just one paragraph, but 16 pages or 32 pages on early pottery traditions of the American southeast, 12 pages on early pottery kilns in the Middle East, on pottery in Xianrendong Cava, China, on Southern Coastal Tanzania, and on and on and on. All these articles -- we haven't even turned to books -- represent many times more detail than the encyclopedia offers, with a lot more context and expertise.

* Just to be clear regarding Wikipedia -- the crowd-sourced nature of its information is an issue, even with references listed (because who's checking that those references actually support what's being said?). But you also shouldn't typically be citing Encyclopedia Britannica, either. Use encyclopedias to get familiar with the basics of your topic as you prepare to do your deeper research.