forces - How an asteroid enters the Earth make a completely different outcome?

In this scenario there are different ways an asteroid could enter the Earth's crust but does the aftermath differ? Think of the surface of the Earth as thin sheet of ice and the magma the water underneath much like an egg. Would an asteroid, the size of a small city 10 miles or more wide necessarily wipe out the Earth? Would most of the force go strait through the crust and the impact be absorbed by the magma? Would the shape make a difference like hitting flat like a belly flop compared a smooth entry like a Olympic diver with no splash? Would an asteroid hitting the Earth at steep angle cause the Earth's rotation to slow in half or more?

There are many questions on this but the main answer I am asking is with the right angle of impact, shape and direction of impact make a difference if the same asteroid would equal extinction or not?

Answer

This is an interesting question and one that probably needs detailed simulation to settle. But one can make the following broad prediction: the shape of the meteorite would have minimal effect on the outcome, for the following reasons:

1. 
   

   At the kinds energies let slip in the moments of impact and the kinds of pressures and temperatures that prevail, all kinds of matter behave in ways pretty near to those of an ideal gas. The forces between molecules that give rise to the everyday "solidness", "hardness" and "sloshiness" of solids and liquids are minuscule compared with those arising from the impact. The gas approximation is made, very successfully it would seem, in the modelling of the extreme environments met in the center of explosive blasts, particularly in the modelling of the detonation of thermonuclear weapons. The main mechanism slowing the impactor down is a rocket-like thrust: as the impactor lets slip enormous energy, vaporizing the Earth's crust, the backthrust from the swiftly expanding gasses allows the momentum to be transferred to the Earth;

   
   


2. 
   
   

   In many "penetration" type scenarios, a kind of negative feedback where increased penetration speeds and energies beget increased resistive forces means that penetration depth is only very weakly dependent on impact speed or impactor shape. Newton was well aware of this kind of mechanism and indeed proposed the law that, for impactors of similar density to that of the impacted body, the penetration depth is independent of the impact speed and equal to the length of the impactor (measured along the direction of relative impact velocity). This surprising law is discussed in tpg2114's answer to the question "Platform Diving: How deep does one go into the water" in some detail. Apparently the surprising rule has pretty solid experimental backup and indeed if one makes detailed fluid-dynamical calculations using ram pressure drag, as I did here in answer to the same question, this behavior does come out of the mathematics. Ram pressure drag is probably a good model for this kind of problem.

   
   

There are many groups around the world who have studied known impactor events in detail through computer simulation. See, for example, this study at Princeton of the Chixulub impactor.