As I discuss in this question, one of the early movements that was important to the development of Vaishnavism was the ancient Pancharatra movement, whose sacred texts consisted of detailed procedures to worship the sage Narayana, an ancient incarnation of Vishnu who was the son of Yama god of death and twin brother of the sage Nara. Among the oldest Pancharatra texts are the Satvata Samhita, Paushkara Samhita, and Jayakhya Samhita. But there is another Pancharatra text even older than those, and it is found within the Shanti Parva of the Mahabharata! It's called the Narayaniya, and it's an 18-chapter religious dialogue between Yudishthira and Bhishma on the importance of worshipping Narayana/Vishnu, similar to the Bhagavad Gita, another 18-chapter religious discourse between Krishna and Arjuna found in the Bhishma Parva of the Mahabharata.
In any case, in this chapter of the Narayaniya section of the Mahabharata, Krishna tells Arjuna the different names of Krishna/Vishnu and their meaning. In particular, he says this about the name Krishna:
I till the Earth, assuming the form of a large plough-share of black iron. And because my complexion is black, therefore am I called by the name of Krishna.
The word Krishna certainly means dark, and Vishnu's incarnation Krishna is called that because he has dark skin. But my question is, what does Krishna mean when he says that he takes the form of a "large plough-share of black iron" and tills the Earth?
Is this black plough a metaphor for something? Is it similar to descriptions of the Vadavagni fire evaporating ocean water, which I discuss here? In this book Swami Chinmayananda suggests that "[t]he Lord ploughs the hard stupidities in us and prepares the heart-field."
But are there any other scriptures that describe Vishnu tilling the Earth in the form of a giant plough?