From the linker's (GNU ld) man page (remember, link options are just being passed through to the linker):

-larchive
--library=archive

Add archive file archive to the list of files to link.  This option
may be used any number of times. ld will search its path-list for
occurrences of "libarchive.a" for every archive specified.

On systems which support shared libraries, ld may also search for
libraries with extensions other than ".a".  Specifically, on ELF and
SunOS systems, ld will search a directory for a library with an 
extension of ".so" before searching for one with an extension of ".a". 
By convention, a ".so" extension indicates a shared library.