55 L6-Earth Then and Now

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Oxford Read and Discover level 6 Earth Then and Now by Robert Quinn

Introduction Did you know that Earth formed billions of years ago? Our planet has changed a lot since then. The oceans and continents have moved. The plants and animals that we see today were not always here— some are old and some are new. People are new, too. Scientists say that we have only been here for about 200,000 years! How did Earth form? When did plants and animals first appear? Where did the first people live on Earth? How do oceans and continents move? What keeps Earth warm? Discover! Now read and discover more about Earth in the past and Earth today!

Chapter 1 A Ball of Fire Scientists think that Earth is about 4.6 billion years old. Our planet started as an enormous ball of fire! How Earth Formed Scientists think that Earth formed from a cloud of gas, dust, and rock that was around our sun. These materials came together and formed a ball of fire and liquid rock. At that time, the temperature on Earth's surface was very hot, and nothing lived here. Discover! The Great Pyramid in Egypt is about 4,600 years old. Earth is one million times older than that! Earth's Layers After millions of years, Earth cooled down. The surface became a layer of solid rock, called the crust. This is the part of Earth that we live on. The crust is usually about 30 kilometers thick on land, but it's thinner at the bottom of the ocean. Under Earth's crust, there's a layer called the mantle. It's about 2,900 kilometers thick. The mantle is very hot its temperature is about 3,000 degrees centigrade. It's mostly made of liquid rock, called magma. Earth's core is under the mantle, at the center of the planet. The core is about 3,500 kilometers across and it's mostly made of two metals iron and nickel. The outer core is liquid, but the inner core is solid. That's because the other layers push down on the inner core with incredible pressure. Temperatures in the inner core can be more than 6,000 degrees centigrade.

Mountains of Fire There's a lot of heat in Earth's core and mantle. We can see some of this heat when volcanoes erupt and produce lava. In some parts of Earth's crust, magma forms underground pools, called magma chambers. When there's a lot of magma in a chamber, the magma moves up a tunnel to the surface. When the magma gets to Earth's surface, it's called lava. The lava comes out of holes called vents. Some volcanoes also produce lots of gas and ash. The lava and the ash can sometimes form a tall cone. Discover In 2010, a volcano in Iceland erupted and produced big clouds of ash. Planes couldn't fly through the ash, so thousands of people couldn't travel! Underwater Volcanoes Sometimes volcanic vents form under the ocean. When this happens, the lava cools very quickly and makes round shapes, called pillow lava. Underwater lava can also build up and form volcanic islands, like Iceland or Hawaii. One of the newest volcanic islands on our planet is Hunga Ha'apai, near Tonga in the Pacific Ocean. This island appeared after a big underwater eruption in 2009.

Chapter 2 Water and Air Today, water covers about 70% of our planet. Billions of years ago, Earth's surface was dry, and nothing lived here. Earth's atmosphere was also different. It had lots of carbon dioxide and other gases, but no oxygen. How the Oceans Formed At first, there wasn't any liquid water on Earth's surface, but there was lots of water vapor in the atmosphere. This water vapor came from inside the planet when volcanoes erupted. When Earth cooled down, the water vapor condensed and formed clouds in the sky. Then it started to rain. After millions of years, liquid water covered most of our planet's surface! Discover Some of our planet's water came from millions of icy meteorites. When the meteorites entered Earth's atmosphere, the ice heated up and changed to water vapor. Oxygen About 3 billion years ago, something amazing happened living things appeared on Earth! Some of the first living things were tiny blue-green bacteria. These bacteria grew in shallow pools of warm water and we can find their fossils today. The fossils look like rocks with unusual shapes and they're called stromatolites. Blue-green bacteria used sunlight, water, and carbon dioxide to make their own food, like plants do today. The blue-green bacteria also produced oxygen, and after millions of years, there was lots of oxygen in Earth's oceans and atmosphere. Today, the air that we breathe is about 21% oxygen. We couldn't live without it!