Theory of plate tectonics states that Earth's surface the upper mantle an crust, was once made up of enormous rock plates that broke into smaller pieces approximately 300 million years ago and These smaller broken plates form more fluid rock surface in the mantle. as Time passed the plates move and morph into natural land boundaries. This explains natural land phenomena, such as earthquakes and tsunamis.
'He had left the house' can function on its own as a sentence. That's the independent clause. 'before mother arrived' cannot function on its own. it is a sentence fragment, or a dependent clause
