ELEMENTS OF GEOLOGY Understanding important stages of geological time

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1 1 Formation 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 Earth The 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. Evolution Oxygen 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.

2 Geological Time 2 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 pm and humans appear at only 18 seconds to midnight. Measuring geological time Geologists 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 million years. Humans evolve 11:59:42 Rocks of the Burren form 11:09 Animals No life on earth This mineral, zircon, contains uranium and is commonly used for dating rocks Bacteria and algae Bacteria The oldest rocks in Ireland occur on Inishtrahull, north of Malin Head. They are approximately million years ago. The oldest rocks in the world are found in northwest Canada and they are 4280 million years old. Geological time as a 12 hour clock

3 3 Event Earth s geological time periods 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 Neogene Extinction of the dinosaurs Cretaceous First grass 90 9 Cretaceous Atlantic Ocean opens Jurassic First dinosaurs Triassic Rocks of the Cliffs of Moher form Carboniferous Rocks of the Burren form Carboniferous First life on land Ordovician First animals with hard skeletons Cambrian First ozone layer Proterozoic First oxygen in the atmosphere Proterozoic First life on Earth Archean Earth s crust forms Hadean Formation of the Moon Hadean Formation of the solar system Hadean

4 Earth s Geological Timescale Eon Era Period Events Start Date (millions of years Cenozoic Quaternary Neogene Modern humans Recent Ice Ages 2.58 Mammals diversify & dominate 23 Paleogene 65 Dinosaurs extinct Cretaceous First primates Phanerozoic Mesozoic Jurassic Triassic Permian Carboniferous Flowering plants 145 First birds Dinosaurs diversify 199 First mammals First dinosaurs 251 Mass extinctions Reptiles diversify 299 Formation of large coal deposits Rocks of the Burren formed First reptiles 359 Palaeozoic Devonian Silurian Ordovician First amphibians First Ferns Insects 416 First land creatures First jawed fish 443 First jawless fish First land plants 488 First Fish Proterozoic Archaen Hadean Cambrian First hard-skeleton animals Mass extinctions 542 First single cell & multi-cellular life 2500 First prokaryotes (have no cell nucleus) 3800 First life on Earth Formation of the Earth 4600

5 4 Ireland during the Carboniferous Period The rocks of North Clare were deposited during the Carboniferous period in Earth history, between 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. Reconstruction of the geography of the world during the Carboniferous Period, approx. 350 million years ago 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. The images show the plants and animals that were most common. Reptiles and mammals had not yet evolved!