|Athro, Limited Earth Science Geologic Sections Intrusions|
|Interpreting Geologic Sections|
This section includes two intrusions. An intrusion is produced when liquid magma is forced through existing rocks. An intrusion may force its way to the surface, and erupt as a volcano with lava flows, or it may remain entirely underground. When an intrusion cuts across pre-existing strata (a dike), it is pretty easy to see that they came first and the intrusion came later. However, when a body of igneous rock is found interbedded with and parallel to beds of sedimentary rocks, it isn't immediately obvious if it is an intrusion within the beds (a sill), or a flow on the surface that was later covered by more rocks. One clue as to the order of events is the presence or absence of a contact metamorphic zone where the igneous rock contacts the sediments. If the overlying sediments are metamorphosed where they come in contact with the igneous body, then the igneous body must be an intrusion.
It is sometimes not possible to determine the exact order of all of the events in a section - this is especially true with intrusions. It is important to identify this uncertainty, and to search for additional evidence, beyond that visible in a single section, that can resolve the order of events.
Sometimes the order of events is ambiguous. With the information available in this section it is possible only to determine
the order of some of the events. It is not possible to determine which of the two intrusions is older. It is also not possible to determine whether the lighter colored intrusion (H) is younger or older than the erosion and deposition of the sandstone (S) at the top of this section. What sorts of things might you look for to resolve this ambiguity?