How did photosynthesising cyanobacteria change the Earth’s spheres?

They reshaped all three other spheres. In the atmosphere, the oxygen they released built up (the Great Oxidation Event) and formed the ozone layer that shields the surface from UV. In the hydrosphere, that oxygen reacted with dissolved iron and stripped the oceans of it, changing ocean chemistry. In the geosphere, the precipitated iron formed banded iron formations and oxygen enabled many new oxidised minerals — so one biological change cascaded through the whole Earth system.

What is the role of oxygen in producing banded iron formations?

The early oceans held large amounts of dissolved iron in their oxygen-free water. When cyanobacteria released oxygen, it oxidised the dissolved iron into insoluble iron oxide, which settled to the sea floor. Over time these deposits built up in alternating iron-rich and silica-rich layers, forming banded iron formations — which are the world’s main iron ore, including the Pilbara deposits of Western Australia.

How did life create the ozone layer, and why does it matter?

Once photosynthesis had put oxygen (O2) into the atmosphere, some of it was converted high up into ozone (O3), forming the ozone layer. Ozone absorbs harmful ultraviolet radiation, so before it existed the land surface was hostile to life. By building the ozone shield, life made it possible for organisms to leave the water and colonise the land.

How does life build rock and minerals?

Life builds rock directly — marine organisms make calcium carbonate shells and skeletons that accumulate into limestone, buried plants become coal, and some rocks form from the silica skeletons of tiny organisms. Oxygen from life also enables a large share of Earth’s mineral species (oxidised minerals) that could not form on a lifeless planet, and land plants combined with weathering to create soil.

How life reshaped the planet

Life is not a thin film living on top of an unchanging planet — it reached down and remade the Earth itself. The breathable air, the chemistry of the oceans, and much of the rock beneath your feet are all, in part, the work of living things. This powerbook traces life's grip on the air, the water and the rock — and why the planet we depend on is one that life created.

spheres reshaped

change that did it: oxygen

short sections

How life reshaped the planetSection 1 of 5

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A planet remade by its own life

It is natural to think of life as something that simply exists on the Earth — a thin film of plants and animals living on top of a planet that would be much the same without them. That picture is wrong. Life has reached down and remade the planet itself. The air you breathe, the chemistry of the oceans, and even much of the rock beneath your feet are, in part, products of living things.

To see this clearly, it helps to name the Earth’s great interacting parts — its spheres. The biosphere is all of life. The atmosphere is the air, the hydrosphere is all the water, and the geosphere is the solid rock and minerals. They are not sealed off from one another; they constantly interact, and the biosphere has acted powerfully on the other three.

In the last chapter we watched the biosphere develop — the first cells, photosynthesis, oxygen, complex life. This chapter turns the question around: not how life developed, but what its development did to the rest of the planet. The answer, traced through the air, the water and the rock, is one of the most remarkable facts in Earth science — that life is not a passenger here, but an engineer.

Life is not a passenger on the Earth — it acts back on the air, the water and the rock.

New words

biosphere

All living things on Earth — an active force that has reshaped the planet.

atmosphere

The Earth’s air; its oxygen and ozone are products of life.

hydrosphere

All the Earth’s water; its chemistry was transformed by life.

geosphere

The solid Earth — rock and minerals — much of it shaped by life.

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What is the central claim of this chapter about the relationship between life and the planet?

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Quick reference & sources

The interactive book above is the lesson. What follows is a plain recap you can revise from, plus the sources and how this maps to the syllabus.

Cyanobacteria reshaped every sphere

One biological change — oxygen-producing photosynthesis — cascaded through the whole Earth system. Atmosphere: oxygen built up (Great Oxidation Event) and formed the ozone layer, shielding the surface from UV and enabling land life. Hydrosphere: oxygen stripped dissolved iron from the oceans. Geosphere: that iron precipitated as banded iron formations, and oxygen enabled many new minerals.

Banded iron formations

Early oceans held dissolved iron in oxygen-free water. Life’s oxygen oxidised it into insoluble iron oxide, which settled out in alternating iron-rich and silica-rich layers — banded iron formations. These are the world’s main iron ore (the Pilbara deposits from Module 1), so today’s steel is the rusted record of life oxygenating the sea.

Life builds rock, and a coupled system

Life builds the geosphere directly: limestone from marine shells and skeletons, coal from buried plants, and a large share of Earth’s minerals enabled by oxygen; land plants helped make soil. The lesson is the Earth system — biosphere, atmosphere, hydrosphere and geosphere are coupled, so a change in one ripples through all.

For HSC students

This powerbook covers the Changes in the Geosphere, Atmosphere and Hydrosphere thread of Module 5 (Earth’s Processes), framed by the inquiry question how did the changes to the biosphere affect the Earth’s geosphere, atmosphere and hydrosphere? Working through it, you should be able to:

analyse the changes to the geosphere, atmosphere and hydrosphere caused by the development of the biosphere;
describe the effect of photosynthesising cyanobacteria on each of the spheres;
explain the role of oxygen in the production of banded iron formations;
relate these changes to the idea of a coupled Earth system.

The next chapter learns to read the record of all this change in the rocks — fossils and deep time — and includes the plate tectonic supercycle. Explanations here are original and are a study aid, not a copy of the syllabus.

Sources

Geoscience Australia and published geoscience — the Great Oxidation Event, banded iron formations and the Pilbara iron ore.
Research on the co-evolution of life and the atmosphere, the ozone layer, and ocean chemistry.
Mineral-evolution research (e.g. R. Hazen and colleagues) — how oxygen and life expanded Earth’s mineral diversity.
NSW NESA Earth and Environmental Science Stage 6 Syllabus (2017) — used only to scope the Module 5 content; explanations above are original.

Keep going

The air, the oceans and much of the rock are not the stage life performs on — they are things life built. We live on a planet its own inhabitants remade.