Ok heres what you want to know.
Ill try to keep it short. When heavy planets form initially they heat up. and are molten. than as they start to form a atmosphere the surfaced start to cold and form a think layer of mass, which over time condensates to a local point do to rotation of the planet( Pangaea) the subsurface layer( under the liquid, typically water based, forms a solid crust. This a common scenario fyi and and about 85 percent of starts have planets and about 1 in 1000 stars have some from of live. Now what happened here is that a very large meteor pinged earth like a pool ball, near what was than northern africa roughly, according to most maps, and fractured the crust like a egg shell, this is also very common occurance over such a long period of time. hence the creation of the plates, what happens that causes great extinctions in life forms is, as the fractures occur you get massive volcanic activity all at the the same time along the fractured plate boundrys, which causes a number of effectss, the 2 primary being poisonous gases and atmospheric changes in content and Temperature, which tends to crush most forms of life, much of what goes extinct depends on the type and severity of the effects and somewhat on geological locations etc etc etc you can figure the rest ,,it can actually be modeled. if your understanding/science is great enough. please forgive the bad grammer and this is a summary fyi and not complete detail..didnt fell like typing to much or editing to much :) Im tired and its late