Describe deep sea trenches and abyssal plains and their relative ages.

Oceanic Trench or Deep Sea Channel, it is a narrow, long and having a sharp inclination depression in the lowermost deep-sea in which happen the extreme oceanic depths. They normally form in places where once the structure of the earth's crust plate is forced below the edge of another.  While Abbysal plain is a beneath the surface of the water plain on the deep ocean floor, typically found at depths approximately 3,000 meters and 6,000 meters. It is in a horizontal position largely between in a mid-ocean ridge and a foot of a continental rise. Almost 50% of the Earth’s surface was sheltered by Abbysal Plains. They are the smoothest and flattest and not so discovered areas on Earth.

81 is a composite number.