Metamorphic rocks are formed from other rock that has been subjected to very high temperature and pressure.
When one of the earth's plates comes in contact with another either by both traveling opposite directions, straight into each other, or from one slipping under the other (subduction zone), the pressure/friction of the plates against each other melts the rocks and after they cool they are metamorphic. Example is in Washington state when the Juan de fuca plate is going under the North American plate the pressure/friction of this melts the rocks which then form metmorphic rock.
There are all sorts of ways to reconstruct the history of life on Earth. Pinning down when specific events occurred is often tricky, though. For this, biologists depend mainly on dating the rocks in which fossils are found, and by looking at the “molecular clocks” in the DNA of living organisms.
There are problems with each of these methods. The fossil record is like a movie with most of the frames cut out. Because it is so incomplete, it can be difficult to establish exactly when particular evolutionary changes happened.
Modern genetics allows scientists to measure how different species are from each other at a molecular level, and thus to estimate how much time has passed since a single lineage split into different species. Confounding factors rack up for species that are very distantly related, making the earlier dates more uncertain.
These difficulties mean that the dates in the timeline should be taken as approximate. As a general rule, they become more uncertain the further back along the geological timescale we look. Dates that are very uncertain are marked with a question mark.
