Overcrowding and slums developed in many impoverished countries in the 20th century, mostly due to large migrations to cities from people looking for work.
Answer:
Radiometric dating of basalt on the ocean floor provide us with an absolute age for the planet earth because the oceanic crust is recycled too frequently at subduction zones,
Explanation:
Radiometric dating of basalt on the ocean floor can not provide us the absolute age of the planet earth due to the subduction. In the process of subduction oceanic plate went below the continental plate as it is less denser. It then melts there and its recycling takes place frequently. So the radiometric dating of basalt on oceanic plate can not provide us the true information regarding the age of the earth.
The U.S. has more than 87,000 dams greater than six feet high (and two million overall). While many dams continue to provide benefits such as flood control, irrigation, and water supply, for other dams the cost of maintenance or the negative effects on communities, fish, and tribes justifies their removal.
Dam owners and regulators decide whether to remove a dam by weighing many factors including: the cost of removal and the ability to replace any lost power generation against avoided long-term maintenance; safety concerns; benefits to endangered fish populations; increased recreational and commercial fishing; and restoration of cultural values of nearby tribes.
By 2020, roughly 70% of dams will be more than 50 years old, inviting us to reconsider the value to the public of long-term investments in this infrastructure.
