Chaos would happen. Here is a chronological order of events:
The size of the earth does not allow for such a hole to exist for very long. Gravity would compact the crust down and push the magma up, filling the hole. The compacting would cause massive earthquakes along every fault line as the tectonic plates compress together. Mountain ranges would suddenly form in areas such as the Mid-Atlantic Ridge, where the tectonic plates moving apart are suddenly mashed back together. The Mariana Trench would become suddenly much deeper as the Pacific Plate is shoved underneath the Mariana Plate. These massive earthquakes would cause massive tsunamis, obliterating everything along the shoreline that hadn't already been obliterated. A significant portion of living things would die (It is hard to speculate what exact percentage). After that, things would cool down for a while.
What happens if the chunk comes back to Earth? The size of the asteroid that is speculated to have caused the extinction of the dinosaurs was between 7 and 50 miles in diameter. This meteor is around 1,500 miles in diameter. When the chunk crashes back into the earth, the energy released would be at least 30,000 times as much as the dinosaur killing asteroid. The Wikipedia article on the Chicxulub meteor states the following:
The impact would have caused a megatsunami over 100 metres (330 ft) tall that would have reached all the way to what are now Texas and Florida. The height of the tsunami was limited by the relatively shallow sea in the area of the impact; in deep ocean it would have been 4.6 kilometres (2.9 mi) tall. A cloud of super-heated dust, ash and steam would have spread from the crater as the impactor burrowed underground in less than a second. Excavated material along with pieces of the impactor, ejected out of the atmosphere by the blast, would have been heated to incandescence upon re-entry, broiling the Earth's surface and possibly igniting wildfires; meanwhile, colossal shock waves would have triggered global earthquakes and volcanic eruptions.
Imagine that, but several orders of magnitude larger. I think it is reasonable to say that, "Everyone and everything would die."