Why Talk Is So Important In Classrooms Fisher, Rey and Rothenberg (2008) 

“Vygotsky (1962) suggested that thinking develops into words in a number of phases, moving from imaging to inner speech to inner speaking to speech. Tracing this idea backward, speech—talk—is the representation of thinking. As such, it seems reasonable to suggest that classrooms should be filled with talk, given that we want them filled with thinking!”

http://www.ascd.org/publications/books/108035/chapters/Why-Talk-Is-Important-in-Classrooms.aspx

Why Is Talk So Important For Learning? Ambitious Science Teaching (University of Washington, 2015)

“Talk is a form of thinking. Research in linguistics and social psychology show that people do not engage in talk merely to communicate something they already know. Rather, to prepare to talk means that one has to formulate what might be relevant to say, but these mental formulations are never very explicit until one begins speaking. In this way, thought is often constructed simultaneously with speech. Speech is a vehicle for all forms of reasoning: comparing ideas, elaborating on them, critiquing them, relating them to everyday experiences, the list goes on and on. Students who get practice at this become better learners, both individually and as a class. It is sobering to think that in many classrooms, students sit, nearly silent, as their teachers do all the talking—and that this experience may literally go on for years.”

http://ambitiousscienceteaching.org/wp-content/uploads/2014/09/Discourse-Primer.pdf

Talking to Learn: Dialogue in the Classroom Australian Council for Educational Research (ACER) Digest (2009)

“Research into talk in classrooms has demonstrated that, even though students’ talk serves vital developmental and learning functions, frequently teachers do most of the talking and children do not often have the opportunity to officially engage in talk that extends for more than a few seconds. For example, research conducted by Smith, Hardman, Wall and Mroz (2004) found that in the typical classroom:  Open questions made up 10% of the questioning exchanges and 15% of the sample did not ask any such questions. Probing by the teacher, where the teacher stayed with the same child to ask further questions to encourage sustained and extended dialogue, occurred in just over 11% of the questioning exchanges. Uptake questions occurred in only 4% of the teaching exchanges and 43% of the teachers did not use any such moves. Only rarely were teachers’ questions used to assist pupils to more complete or elaborated ideas. Most of the pupils’ exchanges were very short, with answers lasting on average 5 seconds, and were limited to three words or fewer for 70% of the time.” 

https://www.trb.tas.gov.au/Shared%20Documents/Talking%20to%20learn-%20%20Dialogue%20in%20the%20classroom.pdf