Snowball Earth Had Seasons and Climate Cycles, Scottish Rocks Show

Scientists have found proof in ancient rocks from Scotland that Earth kept changing weather patterns even when it was locked in ice from pole to pole. These rocks, pulled from islands off Scotland's west coast, show layers that match one year each, pointing to seasons, sun-driven cycles, and ocean shifts like today's El Niño during the worst ice age 700 million years ago.

Background

Earth went through a time called Snowball Earth around 720 to 635 million years ago. In this period, ice covered the planet all the way to the tropics. From space, it looked like a giant ball of snow. This happened during the Cryogenian period, with the Sturtian glaciation as the longest stretch, lasting 57 million years from about 717 to 658 million years ago.

For years, people thought this ice shut down all weather changes. No seasons, no ocean waves pushing air around, just a dead quiet freeze. The ice blocked air from mixing with sea, so short weather shifts stopped for millions of years. But now, rocks from the Port Askaig Formation on the Garvellach Islands change that story.

These islands sit in the Inner Hebrides, a group of rocky spots far from big towns. The rocks there hold over 1 kilometer of layers from the ice time. They start with warm sea rocks from before the freeze and shift right into ice deposits without a big gap. That makes them special. No other spot keeps such a full record of the slide into full ice.

Teams from the University of Southampton spent years out there, measuring and cutting samples. They looked at 2,600 thin layers, called varves. Each one builds up over a winter or summer, like tree rings but in stone. The site gives a year-by-year look at what went on under the ice.

Key Details

The rocks show clear signs of yearly changes. Dark and light bands stack up from freeze and thaw each season. In calm deep water under ice, bits of mud and sand settled out differently summer to winter. That built the layers.

When experts checked the thickness of these layers with math tools, patterns popped out. Cycles repeat every year, then every few years, ten years, and up to hundreds. They match what we see now: seasons from Earth's tilt, ups and downs from the sun's output over 11 years, and wiggles in ocean heat like El Niño every 2 to 7 years.

"These rocks are extraordinary. They act like a natural data logger, recording year-by-year changes in climate during one of the coldest periods in Earth's history."

— Dr. Chloe Griffin, Research Fellow in Earth Science at the University of Southampton

Dr. Griffin led the work on the layers. Her team used microscopes to see how the bits formed. Stats on thickness showed the repeats. Some match modern ocean-air dances that move heat around the globe.

How the Cycles Worked

Models run by climate experts back this up. A full ice cover kills most weather shifts. But if just 15% of the ocean stays open, especially near the middle, old patterns kick back in. Warm spots in the tropics let air and water talk again. That fits the rocks from a time when ice pulled back a bit, maybe making a slushy belt of open sea.

This was not the whole Snowball Earth. It lasted thousands of years at most, a break in the long freeze. The main time stayed cold and still, with ice thick everywhere. But these blips show the weather system wants to move, even in tough spots.

"These rocks preserve the full suite of climate rhythms we know from today—annual seasons, solar cycles, and interannual oscillations—all operating during a Snowball Earth. That's jaw dropping. It tells us the climate system has an innate tendency to oscillate, even under extreme conditions, if given the slightest opportunity."

— Professor Thomas Gernon, Earth and Planetary Science at the University of Southampton

Professor Gernon helped guide the study. His group also dated tiny crystals in the rocks using uranium and lead. That locked the age to the Sturtian time. Dr. Elias Rugen spent five years on the islands. He called the deposits the best Snowball Earth record anywhere. They let you read the frozen past one year at a time.

Dr. Minmin Fu ran the models. She found small open water patches could spark the cycles seen in the stone. The Garvellach site stands out because it holds the shift from hot seas to ice without missing time. That helps track how the planet tipped into the deep freeze.

What This Means

This find shifts how we see Snowball Earth. It was not always locked still. Life under the ice faced changing cold, not just steady dark. Ocean food chains and tiny life forms dealt with yearly shifts and longer beats. That may have pushed early complex life to adapt or grow.

Today, it shows Earth's weather does not quit easy. Even near total ice, small cracks let old patterns run. As our world warms and ice melts in spots, this says ocean-air links can restart fast. It gives a window on how systems bounce back from big chills.

The rocks also prove Scotland holds top clues to deep time. More work there could map other ice breaks or life signs. Teams plan to check nearby sites and run finer models. They want to see if these cycles spread global or stayed local. Dating more layers will pin down how long the open spots lasted.

For now, the Garvellach Islands stand as a key to a frozen past that pulsed with life-like rhythms. The story from 700 million years ago reminds us how tough and active our planet's heart stays, ice or no ice.