Wednesday, May 20 - Earth Unit - The Oceans
Objective: Understand how the Earth’s oceans impact climate and organisms.
Earth is called the blue planet because about 75% of the Earth’s surface is covered with water. 71% of that are the oceans – they contain about 97% of the Earth’s water supply. The other 3% is freshwater. Most freshwater is frozen in polar ice caps or glaciers.
Water moves continuously from Earth’s surface to its atmosphere in the water cycle. The heat causes the water to evaporate and become water vapor. When the water in the oceans evaporate, the salt is left behind.
The water vapor rises in the air and cools as is rises. The cooling causes the water to change back into liquid (the process of condensation). These cooled water droplets form into clouds. Then, some of the water droplets combine to form larger drops which fall to the Earth as rain, snow or hail. Precipitation is always freshwater.
The three major oceans are the Pacific, Atlantic and the Indian Ocean. There are two smaller oceans near the Earth’s poles – the Arctic and the Southern. While continents form natural boundaries for the oceans, the are all interconnected and share certain characteristics.
Salinity – This is the concentration of salt dissolved into the ocean water. Much of the salt comes from rocks on land that have been broken down and dissolved. Runoff from land (such a fertilizers) also contribute to the salinity of oceans. On average the ocean is 3.5% salt and 96.5% water. Salinity can vary – in tropical areas the water is saltier (more evaporation) and in places where a freshwater river runs into the ocean it may be less salty.
Temperature – Temperature can also vary in the oceans. The upper layer of the ocean is about 100 m thick and is the brightest and warmest part of the ocean. The Thermocline layer lies below the surface, and temperature decreases as depth increases. The temperature in the ocean’s bottom layer is pretty uniform – near freezing even in tropical waters.
Density – The density of water varies – based on salinity and temperature (more salt makes water denser, warmer temperatures make water less dense).
Like the wind in the atmosphere – water flows in the ocean.
Surface currents – Water driven by winds. These happen in the first several hundred meters of the oceans. They are also influenced by the Coriolis effect – as a result of the Earth’s rotation on its axis, the currents of the Northern Hemisphere deflect to the right and the current in the Southern Hemisphere deflects to the left.
Density currents – These form when waters of different temperatures and salinities meet. Density currents move cold, salty water along the bottom of the ocean. The dense water travels along the bottom of the ocean toward the equator. Then the water becomes warmer and intermix – that water rises and flows back to the poles.
Waves – Waves are a rhythmic movement that carries energy through space or water, Winds skimming over the surface of the water are the main causes of ocean waves. Waved move the water a short distance in a circular path. The highest point of a wave is the crest and the lowest point is the trough. Wave height depends on three factors: wind speed, wind duration and fetch (the distance wind blows over water).
Tides – The periodic rise and fall of sea level that occurs twice a day in most coastal areas. They form because of the pull of gravity from the moon and the sun. Because the moon is closer, its influence on the Earth’s tides is greater. As the moon moves through its orbit, its gravity pulls on Earth. The water on the side of Earth facing the moon bulges out in the direction of the moon. The water on the opposite side of the Earth is not affected as strongly, but the moon’s gravity pulls on Earth and Earth moves slightly toward the moon – moving away from the water on the opposite side, creating another bulge. These bulges are the high tides. They shift position as the Earth rotates on its axis which is why the tides repeat in a daily cycle.
Sea level is the height of the surface of the oceans. When sea level is low, more land is exposed. When se level is high there is less land and some land habitats might be under water.
Oceans absorb energy more slowly and retain it longer than dry land. If the oceans quickly absorbed and released energy, then Earth’s temperature would fluctuate from 100 degrees Celsius during the day to -100 degrees Celsius at night.
Most oceans life lives in the upper part of the ocean where there is light for photosynthesis. Algae and plants provide the base of the food supply chain of ocean animals. However, many organisms have adapted to the Earth’s deepest waters – learning to live in cold, low-light, low oxygen environment. The ocean also provide a food source for people.
ASSIGNMENT: Read the article about life in the deep ocean. Think about how life at the bottom of the ocean is different from the ocean you see at the beach.