This educational song covers geology, from the shifting tectonic plates below us to the mountains above. We describe different types of rocks and explain where they’re located and how they’re classified. You’ll hear about volcanoes, earthquakes, metamorphic rocks and more.
Deep in the Earth's crust, almost a mile beneath the ocean floor, a pool of superheated magma pushes upward against the surface layer of rock. At several thousand degrees Fahrenheit, the heat and pressure are awesome. Suddenly the rock layer buckles slightly. A ridge on the ocean floor rises several meters, trembles, and then slams downward and outward. Molten rock, or magma, and superheated steam explode as it falls and two huge rock plates, each thousands of miles wide, shudder and are pushed a few centimeters apart.
A few centimeters may not sound like much, but an entire continent has suddenly been moved. The resulting earthquake sends an undersea shock wave racing silently across the ocean. Minutes later, thousands of miles away, that shock wave, called a tsunami, crashes into the shore. A wall of water drowns hundreds of people as they sleep.
The explanation of the forces that unleash killer tsunamis, massive earthquakes, and even exploding volcanoes is called plate tectonics. It's the study of the huge rock plates upon which each of our continents sit, and it explains how mountains are born, valleys created, and how the Earth's rocky crust or mantle came to be shaped as it is today.
Picture the Earth like a giant peach floating in space. On its surface is a thin skin called the crust. Instead of fleshy fruit, just below that is the rocky mantle. Deep in the center, like the peach's pit, sits the Earth's core. We can study the inside of the Earth by using vibrations called seismic waves measured on a machine called a seismograph. Because seismic waves behave differently in different materials, we know that the Earth has five layers: lithosphere, asthenosphere, mesosphere, outer core, and inner core.
The inner core is solid, extremely hot, and under immense pressure from all the rock above it. The outer core is molten, so it's liquid, and also very hot. This core extends 1,800 miles around from the very center of the Earth.
After the core, the next layer is the mesosphere. This layer is mostly solid and extends outward another 1,500 miles. The asthenosphere is a thin layer, about 150 miles thick, at the border between the mantle and the crust. It is still very hot, and nearly (but not quite) a liquid. Because of this, pieces of the Earth's crust can move very slowly above it.
The outer layer of crust, the lithosphere, is broken into huge blocks of rock called tectonic plates. The borders between these plates are called faults. When molten rock or magma squeezes to the surface at a fault, it releases pressure, exploding to the surface as a volcano.
As we've learned, most of the lithosphere is covered by ocean. This water and the atmosphere around it help shape the Earth's crust. Volcanoes bursting from inside the crust can also change the atmosphere and oceans by shooting hot gases or ash into the air, or by boiling water that has seeped down into the crust. When a tectonic plate moves, the Earth shakes, creating an earthquake. If it shakes underwater, the movement can cause tsunamis, which wash away soil, sand, rocks, and buildings, effectively changing the shape of the land.
Tectonic plates move because of the flow of heat inside the Earth. Deep in the asthenosphere, heat sets up a convection current - remember, that's when heated things (liquids, in this case) rise and cooled things fall. Since the asthenosphere is not quite liquid rock, convection currents move rock very slowly. Where the crust is thin, the heated rock may rise. This usually happens in the middle of an ocean, creating an ocean basin. Where the crust is thick, like under a continent, rock sinks deeper into the mantle.
When tectonic plates move, they rub against each other, causing great stress on the rock. When rock is pushed together, the pressure's called compression; when rock is stretched apart, it's called tension. At points of tension, like under the ocean, magma may come to the surface. At points of compression, layers of rock may bend upward, downward, or break entirely.
A bending layer of rock is called a fold. Over millions of years an upward fold, also called an anticline, can form mountains. A downward fold, or syncline, can create a deep valley. When the rock plate breaks entirely, a crack, or fault, is formed. A fault block, or huge cracked block of rock, can be pushed upward, also creating a mountain. Many landscapes of our country were formed this way. Take the eastern U.S., for example: Folded layers of rock created the Appalachian mountain range, and hills in New York or New England reveal folds as well. Just looking at them tells us how they were formed millions of years ago!
Most volcanoes and earthquakes occur along the boundaries of tectonic plates. The edge of the Pacific Ocean is a good example, where a ring of mountains, faults, and volcanoes have given the area a nickname: the "ring of fire." The area includes California and Alaska, both of which have seen many large earthquakes over the years, as well as Mount St. Helens in Washington, which erupted in a huge explosion in 1980, sending millions of tons of ash into the atmosphere.
The Earth's crust is made of rock, which is composed of minerals. Minerals are solid, naturally occurring non-living materials that have a structure of crystals. There are hundreds of different kinds of minerals, but about 90% of the ones on Earth are silicates. Silicate minerals include oxygen and silicon along with some other combination of elements like iron, aluminum, or potassium. Since each mineral is made of different elements, each has a unique
set of properties: For example, diamonds are very hard, iron is strong, and halite (salt) tastes good on food.
Based on these properties, several tests are used to identify minerals. A streak test shows a mineral's color. The mineral is scraped against a rough porcelain plate so it leaves a streak of dust, the color of which is used to identify the mineral. A second test concerns the mineral's reaction to acid. Some minerals like calcite will bubble or dissolve in a weak acid.
Luster is how shiny, metallic, or dull a mineral appears. Some minerals like fluorite glow under ultraviolet light. Others are magnetic (magnetite) or radioactive (uranium).
Each mineral fractures in a different way as well. Mica, for instance, cracks into thin flat sheets, while feldspar breaks into smooth, flat surfaces called cleavage.
Another test is hardness. Mohs hardness scale rates minerals on a scale from one to 10. Talc, a very soft mineral, has a hardness of one. Quartz, a very common mineral, has a hardness of seven. Diamond, the hardest mineral of all, is rated a 10. So if you think you've found a diamond, try scratching it with a piece of glass or stone. No other mineral can scratch a diamond.
A rock is a solid mixture of two or more minerals. There are three classes of rock: igneous, metamorphic and sedimentary. Each is formed differently, so they have specific characteristics.
Cooling magma forms igneous rock. Some igneous rock cools slowly deep in the Earth, causing visible mineral crystals to form inside. Granite is a strong, very common igneous rock with coarse, easily seen crystals of quartz, feldspar, and mica. Other igneous rocks are spewed quickly from volcanoes, forming lighter rocks like pumice, which is very weak and has a great deal of air inside it.
Sedimentary rock forms from sand and other sediments that, over millions of years, compact and cement together into layers. Sedimentary rock may contain bones, shells, or the imprint of living things - remains called fossils, which can often tell us about the climate, plants, or animals that lived when the sediment was laid down. Sedimentary rock may be very fine like limestone or very coarse like conglomerate. Some kinds, like sandstone, may lie in multicolored layers hundreds of feet thick. Crystals from minerals dissolved in water may form within cracks or pockets in sedimentary rock, or may replace the remains of living material that has decomposed.
Metamorphic rock is formed when sedimentary rock is squeezed by heat and pressure. This might happen when a pocket of magma comes close to the surface, where rock layers are folded by pressure, or in the crust along fault lines. Some metamorphic rock contains visible mineral crystals formed by the pressure and heat. Marble is an example of a metamorphic rock that's very durable and good for use in buildings and walkways.
Over millions of years, rocks form, break down, and eventually re-form into new rocks. This is called the rock cycle. When igneous rocks become weathered and eroded, they turn into grains of sand, which is deposited on the ocean floor. Over time it compacts into sedimentary rock, which may then later be compressed into metamorphic rock. This metamorphic rock may again erode into sediment, pushed deep into the Earth where it melts into magma. This magma may erupt in a volcano and start the whole rock cycle again.
Rock breaks down to form sediments by two different processes: weathering and erosion. Weathering is the process by which rocks are broken down into smaller rocks. Chemical weathering is when water, weak acid, or air breaks down a rock by a chemical reaction.
Some minerals dissolve in water, which may cause the rest of the rock to break apart. Other minerals react with oxygen in a process called oxidation; for example, when iron oxidizes, it turns into rust.
Mechanical weathering is when rock is broken down by a mechanical process. Expansion in heat and contraction in cold can cause a rock to crack. If water gets in a crack, it can freeze, expand, and widen it. Sand blowing against a rock or small rocks in a fast-moving stream can break down or scrape a larger rock. Glaciers tumble rocks along their base, which scrapes the ground below. Even tree roots growing into cracks or burrowing animals can break rock.
Erosion is the transporting or movement of sediments from one place to another, via water, wind, ice, or gravity. Over time mountains and hills erode and wash down into the oceans. Rain, waves and currents, glaciers, volcanoes, and wind can all cause erosion. Gravity contributes to erosion as well, both directly (think of landslides, rockfalls, and mudslides) and indirectly (via streams, rivers, and glaciers).
Deposition is the process in which eroded sediments are dropped or deposited. At the mouth of a river we frequently find a delta, which is a piece of land created by sediment deposited where a river flows into the sea. The delta of the Mississippi River is hundreds of miles long. Soil is composed of sediments mixed with organic materials, the remains of formerly living organisms like decomposing leaves, rotting roots, and dead insects or animals. Soil also contains air and water, can include volcanic ash, and is constantly changed by bacteria, earthworms, and other small creatures.
The most destructive volcanic eruption in history struck in April 1816 on Mt. Tambora in Indonesia, killing 92,000 people. The top 4,000 feet of the mountain were blown into the sky, ejecting more than 93 cubic miles of debris into the atmosphere. The ash caused lowered temperatures and freakish weather worldwide - that summer, it snowed in New England.
On December 26, 2004, the second strongest earthquake ever recorded struck off the coast of Indonesia. The resulting tsunamis, some more than 100 feet high, killed 225,000 people.
Along the bottom of the Pacific Ocean, about a mile and a half below the surface, is an ocean ridge of volcanoes. Gases and superheated, mineral-rich waters bubble up from vents in the ocean floor. Bacteria feed on the minerals, as do strange creatures like tube worms up to six feet long and clams more than a foot across. Iceland was formed by a series of undersea volcanoes in the northern Atlantic Ocean. Underground magma heats water that still provides enough geothermal power to heat three quarters of the island's homes and generate a fourth of its electricity.
Look at a map. See how South America has a big bump where Brazil is and how, across the Atlantic Ocean, Africa has a dent? Don't they kind of look like two puzzle pieces that should fit together? Well, Earth's continents are a puzzle, and millions of years ago they did fit together.
About 250 million years ago most of today's land areas were connected as one supercontinent we call Pangaea. Gradually, different tectonic plates pulled Pangaea apart; as some sections of these giant plates rose, others fell back into the Earth's crust or were pushed apart. This slow movement is called continental drift. So, yes, Africa and South America did fit together once upon a time; similar fossil remains and rock layers on each of the continents show us where the continental plates used to be attached.