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ELEMENTS OF GEOLOGY Understanding important stages of geological time

1Formation of the Earth

In human terms, the age of the Earth is immense. The Earth and solar system were formed 4,560 million years ago. It all began when an interstellar cloud (nebula) of gas and dust began to collapse. This collapse may have been triggered by a shockwave from a nearby supernova or star explosion.

Collisions of dust particles in the cloud built up small asteroids and eventually protoplanets. One of those protoplanets was Earth. The proto-Earth was molten and was subject to frequent bombardment by comets, asteroids, and other planetary debris. At one stage a large asteroid struck the proto-Earth. The debris, ejected into space after the impact, gradually coalesced to form the Moon.

The beginning of life on EarthThe atmosphere of the early Earth was composed of gases such as sulphur dioxide, nitrous oxide, methane and ammonia. This is a similar gas mix to that produced by volcanoes. If the Earth was still surrounded by this gas mix today, it would be toxic to most organisms.

Nonetheless, even with this toxic mix of gases the first life –bacteria – evolved. These bacteria adapted to these harsh conditions and became more diverse. One type of bacterium – the Cyanobacteria – began to produce oxygen as a metabolic by-product.

The oxygen built up in the atmosphere over time and enabled the rise of animal life. The first animals to evolve were worm-like and only their burrows are preserved in rocks. Due to increasing pressure from predators, however, animals began to secrete hard outer skeletons for protection, e.g. shelly animals such as snails.

EvolutionOxygen eventually built up to its present levels, generating the ozone shield that screens out UV radiation and allows complex forms of life to live on land. Fish, amphibians, insects and reptiles evolved. The rocks of the Burren and Cliffs of Moher formed roughly when amphibians and insects were evolving.

Dinosaurs, which are the largest land-based animals to ever have lived on Earth, evolved next. Mammals evolved shortly afterwards but were small and insignificant at this time. A large meteorite impact is commonly thought to have killed the dinosaurs, but this is only one of several contributing factors. Other factors were (1) the release of large amounts of toxic gases and dust into the Earth’s atmosphere by prolonged and catastrophic volcanic eruptions in what is now India and (2) changes in the configuration of the Earth’s continents.

The appearance of grass allowed the evolution and diversification of the grazing animals which dominate temperate and subtropical zones of the Earth today. The expansion of grasslands between 2.5 and 1 million years ago (caused by a change to a drier climate), and therefore of grassland grazing animals, may have triggered the evolution of humans.

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ELEMENTS OF GEOLOGY Understanding important stages of geological time

2Geological Time

The Earth is immensely old. We know from modern dating techniques that it is 4,560 million years old. To put this into perspective, humans evolved only 2 million years ago. By comparison, the rocks of North Clare are between 341 million years and 318 million years old approximately. This is still relatively young in terms of the total age of the Earth!

The diagram below depicts the age of the Earth as a 12 hour clock. Note how the rocks of the Burren form only at 11.09 pm and humans appear at only 18 seconds to midnight.

Geological time as a 12 hour clock

Measuring geological timeGeologists date events in Earth’s history by measuring tiny amounts of radioactive elements (e.g. uranium) in certain types of rocks. Because radioactive elements decay over time at a constant rate, if you measure the amount of what’s left (often only in thousandths or millionths of a gram) you can work out how long ago a particular rock was formed. Radioactive dates are usually accurate to within 0.2 – 2 million years.

The oldest rocks in Ireland occur on Inishtrahull, north of Malin Head. They are approximately 1750 - 1780 million years ago. The oldest rocks in the world are found in northwest Canada and they are 4280 million years old.

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Rocks of the Burren form 11:09

Humans evolve 11:59:42

No life on earth

BacteriaBacteria and algae

AnimalsThis mineral, zircon, contains uranium and is commonly used for dating rocks

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ELEMENTS OF GEOLOGY Understanding important stages of geological time

Earth’s geological time periods

3 Event Date (millions of years ago)

Distance from “today” on washing line of time (cm)

Period

End of the last Ice Age 0.01 ~ 0 Quaternary

First humans 2 0.02 Neogene

Extinction of the dinosaurs 65 6.5 Cretaceous

First grass 90 9 Cretaceous

Atlantic Ocean opens 150 15 Jurassic

First dinosaurs 225 22.5 Triassic

Rocks of the Cliffs of Moher form 318 31.8 Carboniferous

Rocks of the Burren form 330 33 Carboniferous

First life on land 475 47.5 Ordovician

First animals with hard skeletons 542 54.2 Cambrian

First ozone layer 580 58 Proterozoic

First oxygen in the atmosphere 2700 270 Proterozoic

First life on Earth 3500 350 Archean

Earth’s crust forms 4300 430 Hadean

Formation of the Moon 4520 452 Hadean

Formation of the solar system 4560 456 Hadean

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Earth’s Geological Timescale

Eon Era Period Events Start Date (millions of years

Pha

nero

zoic

Cen

ozo

ic QuaternaryModern humans

Recent Ice Ages 2.58

Neogene Mammals diversify & dominate 23

Paleogene 65

Mes

ozo

ic

Cretaceous

Dinosaurs extinct

First primates

Flowering plants 145

JurassicFirst birds

Dinosaurs diversify 199

TriassicFirst mammals

First dinosaurs 251

Pal

aeo

zoic

PermianMass extinctions

Reptiles diversify 299

Carboniferous

Formation of large coal deposits

Rocks of the Burren formed

First reptiles 359

Devonian

First amphibians

First Ferns

Insects 416

SilurianFirst land creatures

First jawed fish 443

OrdovicianFirst jawless fish

First land plants 488

Cambrian

First Fish

First hard-skeleton animals

Mass extinctions 542

ProterozoicFirst single cell & multi-cellular life 2500

ArchaenFirst prokaryotes (have no cell nucleus) 3800

HadeanFirst life on Earth

Formation of the Earth 4600

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ELEMENTS OF GEOLOGY Understanding important stages of geological time

4Ireland during the Carboniferous Period

The rocks of North Clare were deposited during the Carboniferous period in Earth history, between 359 - 299 million years ago. During this time, the shapes and locations of the continents were very different from the way we know them today.

The part of the Earth’s surface that would eventually form Ireland was located approximately 10º south of the equator and it was covered by a tropical sea.

The Earth’s climate was also very different during the first part of the Carboniferous period. It was warmer and more humid, and there were no distinct seasons. The average global temperature was 20ºC then, but is only 12ºC today.

The composition of the atmosphere was also different during the Carboniferous – oxygen levels were much higher - 35%, compared with 20% today.

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Reconstruction of the geography of the world during the Carboniferous Period, approx. 350 million years ago

The images show the plants and animals that were most common. Reptiles and mammals had not yet evolved!