The metaphor is a type of comparison. It does not use the words 'like', or 'as'. If the comparison uses 'like,' or, 'as', then it's a simile. As in the following (presume Marty is a human):Marty eats like a horse. ß This is a simile. Similes tend to be a bit more specific. This one focuses on how Marty eats. It means he eats a lot and maybe sticks to a vegetarian diet. I don't know if horses ever eat meat. That would be bad, getting chased and eaten by a horse.
Marty is a horse. ß This is a metaphor. This tells us the entirety of Marty will remind someone of a horse in some way. The imagination is more free to roam. He is large-framed and powerful. Perhaps Marty is a good runner, or maybe he likes to stand around in fields. So on, and so forth.Ok - a metaphor is comprised of two parts. There is a vehicle and there is a tenor. The vehicle is the concrete image used to convey the tenor. The tenor is the idea associated with the comparison. Getting back to Marty; the horse is the vehicle. We all know what a horse looks like, though your image might not match mine. Right now I'm thinking of a rainbow pony. Anyway, the image (vehicle) used in the comparison with Marty, brings certain ideas about the guy. Those ideas are the tenor.
In order for a metaphor to work, there has to be literal truth connecting the vehicle and the tenor. In other words, if there is nothing at all similar between Marty and the horse, the metaphor falls apart. It's a bad metaphor and should be thrown into the scrap heap for recycling. Stay with me, I'm about to get to the point.I have a memory from second grade. It's the first time I heard the idea of 'Mother Earth'. We had these cool magazines called 'Weekly Readers'. I remember this article about some people somewhere referring to mother earth. It was all about picking up your trash. It was along the lines of Woodsy the Owl (give a hoot, don't pollute) and an icon named Johnny Horizon who looked a whole lot like the Marlboro Man, minus the smokes. And that's all well and good. I try to pick up after myself and throw away the trash where it belongs. This is usually behind the seat in the truck.
But getting back to Mother Earth…the comparison doesn't work. There is no literal strength to the comparison.The earth is not my mother. My mother lives in Missouri. If the earth was my mother I would grow up to become a planet. I am not doing that. I'm growing (mostly outward these days) to be an old human. I am, however, picking up certain habits and mannerisms of both of my true biological parents.
The earth is a planet. It does not reproduce. If the earth could give birth it would birth baby earths (say that six times fast). But now we're to think of the earth as our mother? There's an entity that gives birth to completely unrelated multiple thousands of species? Sorry, doesn't happen. And what a mother it would be! Think about how many millions of her own offspring she would be guilty of butchering on a yearly basis. She has these children and doesn't bother to feed them or protect them or care for them, offering only an incredibly hostile home and allows her children to fend for themselves, brother eating brother? Mother Earth puts Clytemnestra to shame (there's a Google Search for ya).The idea of Mother Earth isn't good poetry. It doesn't stand up to explication. Know what I mean?