It depends on what the abilities and technology of the aliens are.
Explanation:
Aliens could have any sort of tech or abilities. Such as, some aliens could have chameleon DNA. This would allow them to blend in, and thus make it to where they would be here, but we just wouldn't know. The same thing goes for if they have cloaking devices or such. Also, it depends on how advanced they were. If they were just now in existence, they probably wouldn't be advanced enough to hide. Also, we must consider what our planet is like. In "War of the Worlds", disease and microscopic organisms kill off all of the Martians. The same would happen to aliens landing, unless they somehow had some immunity thanks to their genetics. Also, we must consider diet. What if our world doesn't hold their kind of prey? In that case, I doubt the aliens would be on this planet. There are many other factors that could be listed, however, I will just leave it to you to decide. I just believe that it depends.
The World Trade Organization (WTO) is the only global international organization dealing with the rules of trade between nations. At its heart are the WTO agreements, negotiated and signed by the bulk of the world's trading nations and ratified in their parliaments.
Answer: Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. At a spreading center, basaltic magma rises up the fractures and cools on the ocean floor to form new seabed.
