It only took temperatures falling to double digits below zero for a few nights, and then winds to calm down a bit, for an already cold Lake Superior to start freezing up good.

The big lake went from about 20 percent ice cover earlier in the week to nearly 50 percent of the lake with at least some ice as of Friday, according to the Great Lakes Environmental Research Laboratory.

A satellite photo taken Thursday shows solid, so-called fast ice, on all of the lake’s protected bays with more ice forming — but still laced with fissures — off Duluth and Superior. The North Shore, which had little or no ice as recently as Wednesday, also is building ice. Ice also has apparently locked-in Isle Royale.

If ice continues to build on the lake, it would have several ramifications. Anglers will get access to lake trout and other fish off the Twin Ports and hikers may get access to the famous ice caves at the Apostle Islands National Lakeshore. It also means quicker access to an ice road for people traveling to Madeline Island who now are traversing the ice by a windsled.

The more lake that’s covered with ice also means less lake-effect snowfall, with less open water to produce water vapor that turns into snow over land.

All of the Great Lakes had an estimated 47 percent ice cover as of Friday, with shallower, smaller Lake Erie nearly entirely ice-covered.