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The Changing Earth

The earth has not always looked the way it looks today. In other words, the United States one billion years ago was in a totally different location than it is today!! How does this happen? And why does this happen? Let's take a look. In order for us to get some understanding of how the earth has changed over time, we first need to understand some of the things that took place, and are still taking place, in the earth.

The Earth's Interior

What about the internal structure of the Earth? Our best clues about the interior come from waves that pass through the Earth's material. When earthquakes shake and shatter rock within the Earth, they create seismic waves which travel outward from the location of the quake through the body of the Earth. Seismic waves are disturbances inside the Earth that slightly compress rock or cause it to vibrate up and down. The velocity and characteristics of the waves depend on the type of rock or molten material they traverse.

Studies of seismic waves have revealed two important types of layering in the Earth: chemical and physical. Compositional layering refers to layers of different composition. Physical layering refers to layers of different mechanical properties, such as rigid layers verses "plastic" or fluid layers. 1

Compositional Layering

Compositional layering was the first type of layering recognized. Seismic and other data indicate that the Earth contains a central core of nickel-iron metal. The core is surrounded by a layer of dense rock, called the mantle, that extends most of the way from the core to the surface. Near the surface, the densities of the rocks are typically lower. The crust is a thin outer layer of lowerdensity rock about 3 miles thick under the oceans and about 18.5 miles thick under the continents.

The core-mantle-crust structure gives us important clues about the history of the Earth and other planets. First, it shows the importance of differentiation processes - processes that separate materials of different composition from one another. Most geologists believe that the key differentiation process in the Earth was melting of much of the inner rock material after the Earth formed. The source of the heat was radioactive minerals trapped in the Earth as it formed. Gradually those minerals released heat as radioactive atoms decayed. The interior of the Earth was so well insulated by overlying rock that the heat could not escape. The temperature rose until the rock melted. When the rock melted, heavy portions like metals flowed downward toward the center, while lighter, low-density minerals floated toward the surface, where they eventually solidified into a crust of low-density rock.

Note that one of the lowest-density and most common minerals to form in cooling, molten rock is called feldspar. Feldspar should have formed and floated toward the surface. Indeed, we find that surface rocks of both the Earth and the Moon are extremely rich in feldspar. One of the rock types formed from mixtures of minerals rich in feldspar is called basalt. Basalt is the common lava that erupts from volcanoes tapping the crust and upper mantle, and basalt is also common on the Moon's surface.

A still lower-density type of rock is granite, the light colored quartz-rich rock commonly formed from molten materials in continents. In some ways, continents seem to be low-density granite "scum" floating on the denser rocks of the basaltic lower crust and upper mantle. 1

Physical Layering

The second type of layering involves layers of different rigidity. This is thus a physical, rather than a chemical layering. Layering by rigidity was created during the cooling of the Earth. The interior stays molten for a long time because it is hard for the heat to get out, but the surface cools fast because it is exposed to the atmosphere and surrounding space, and the heat can easily radiate away. Thus if a planet were melted and left to cool, a solid layer would form on the surface, while the center is still molten liquid. This picture is complicated by the fact that the layers at different depth have different compositions (because of differentiation) and are also at different pressures. They may thus have different solidifying temperatures, and they may actually form alternating layers of solid material, molten material, or partly molten slushy material. A partly molten layer can be visualized as a layer in which some lowmelting-point minerals are not melted.

The solid layer at the surface of the Earth, or any other such planet, is called the lithosphere, and it is underlain by a partly melted layer that is much less rigid and less strong. The underlying partly melted layer is called the asthenosphere . Interplay between the lithosphere and asthenosphere determines the surface features of the Earth, such as mountains, sea floors, and continents.

Scientists now believe the Earth and other planets were fairly hot when they formed. Once each planet formed, its insides cooled by the three processes of conduction, convection and radiative heat loss. Specifically, heat was conducted outward from the hot center through rock; convection may have occurred in some of the more fluid layers where currents can flow; and heat radiated from the surface into space. As the Earth cooled, the surface layers solidified, forming a relatively rigid surface layer of rock - the lithosphere. In the Earth, the lithosphere is about 62 miles deep, and the asthenosphere underlies it, at a depth of 62 to 218 miles. Even though the asthenosphere is not totally molten, it seems to be plastic enough for sluggish convection currents to flow. These currents bring up hot mantle material, creating "hot spots" in the Earth's crust where volcanoes are likely. The current also drag on the underside of the lithosphere, setting up stresses in it and tending to crack it into large pieces.

Just like Arctic explorers crossing ice floes that are floating on the ocean, we are living on a rigid layer that "floats" on a more fluid base. As the asthenosphere shifts, it can stretch the brittle lithosphere only so far before it cracks. This is what we perceive as an earthquake. 1

Plate Tectonics

Tectonics is the study of movements in a planetary lithosphere, such as the movements that cause earthquakes, mountain building, and so on. For more than a century, geologists studied the Earth's tectonics without recognizing the underlying nature of the forces that cause these movements. Only since the 1960s have geologists pieced together a real understanding of how these principles affect the Earth and how they apply to other planets. This new understanding is called the theory of plate tectonics.

As the asthinosphere drags on the more brittle lithosphere, it cracks the lithosphere into large, continent-scaled pieces called plates. Further asthenosphere movements tend to drag and jostle the floating plates, sometimes pulling them apart from each other and sometimes pushing them into each other. Cracks along the margins of plates are usually the sites of volcanoes and earthquakes. Volcanoes form because molten magma from below can squeeze up to the surface through the cracks. Plate collisions cause stresses - earthquakes - as plates rub together.

These stresses often cause violent fractures of the lithosphere rock layers. The fracturing events are better known as earthquakes. Fractures caused by earthquakes are called faults. Colliding plate regions are laced with faults like the famous San Andreas fault. Many earthquakes are caused by movements along these faults.

The theory of plate tectonics also explains why rocks older than 1 or 2 billion years are so rare on the Earth. Most older surfaces have been crumpled beyond recognition or driven downward under other plates, to be re-melted, mixed with mantle material, and perhaps re-erupted as new lava.

Now we can see a connection with the landscapes on other planets. Smaller worlds, like Mars and the Moon, do not have well-developed crumpled mountain ranges or plate boundaries because they cooled faster than big worlds. Their lithospheres got thicker in the same amount of time. Thus their surfaces are more stable and more protected against asthenosphere currents far below. Lava does not so frequently gain access to the surface. Convection can't so easily drive plates apart or cause them to drift into each other. Therefore, the surfaces of Mars and the Moon have much more ancient structures than the Earth does. 1

The surface of the earth is broken into seven large, and many smaller, moving plates -- all of which ride on the asthenosphere. Each plate is approximately 50 miles thick and moves anywhere from 1/2 inch to several inches per year. As plates move, they collide or separate or move over one another. These three basic types of plate movement have been identified and named -- pictured here on the right -- as: extensional movement; compressional movement; and transformational movement.

Click to view a movie illustrating the three types of plate movement.

Think about it, if a single plate moves only 1/2 inch per year, that means it moves:

5" in 10 years

50" in 100 years

500" in 1,000 years

5,000" in 10,000 years

50,000" in 100,000 years

500,000" (or about 8 miles) in 1 million years

How far, in miles would it move 100 million years? (800 miles, right?)
How about 1 billion years? (8,000 miles, right?)
That means approximately 24,000 miles of travel for the plate over 4 billion years. Now consider this, seven large plates and many smaller ones, all moving in one of the three manners pictured above, all at the same time for more than 4 billion years.
Here is an exercise for you. Compute the distance in miles a plate moving at a rate of 1 inch per year would travel in 1 billion years.

Plate tectonics, in conjunction with all the others forces spoken of in this section, have caused a change in the location of the continents over the past 4 billion years. Scientists believe that at one time there was one huge land mass on the earth - a land mass we call "Pangea." Over millions and millions of years, that land mass has changed due to plate tectonics and the continents are positioned as we see them today.

Click to see an illustration of how it is theorized that Pangea was formed.

Mechanisms of Surface Change

Volcanism is the eruption of molten materials from a planet's interior onto its surface. On the Earth, the asthenosphere contains pockets of partly melted materials, as indicated by seismic wave analysis. This underground molten rock, called magma, is under pressure, often charged with gas such as steam, less dense than surrounding rock, and highly corrosive. Therefore it tends to work its way to the surface, especially in regions where fractures provide access. When it reaches the surface, it erupts and is then called lava. If enough lava is erupted, it may accumulate into volcanic mountains. During intervals ranging from years to millions of years, volcanism thus creates new land forms ranging from flat lava flows to craters and volcanic peaks.

Space exploration has shown that volcanism is one of the most important processes forming landscapes on other planets. Some planets have huge lava flows and volcanoes. Study of the Earth's volcanoes helps us understand these alien landscapes. Conversely, study of the other planets' volcanic features helps us understand relations we see among terrestrial volcanoes.

Click for a brief explanation of how volcanism is a part of plate tectonics.

Among all known planets, the Earth undergoes the most active processes of land form destruction. Largely, this activity is due to its thick atmosphere and flowing water, which other planets lack. Erosion includes all processes by which rock materials are broken down and transported across a planet's surface; such processes include water flow, chemical weathering, and windblown transport of dust. Deposition included all processes by which the materials are deposited and accumulated; such processes include deposition of sediments in lake and ocean bottoms and dropping of windblown dust in dune deposits. Most of these processes do not occur on most other planetary bodies. 1

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The Changing Earth The earth has not always looked the way it looks today. In other words, the United States one billion years ago was in a totally different location than it is today!! How does this happen? And why does this happen? Let's take a look.

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How Earth Changes in the blink of an eye

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How Earth Changes in the blink of an eye

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Intro

From islands popping out of the ocean during earthquakes to glaciers calving icebergs every hour, the Earth can undergo dramatic changes right before your eyes.

These seven rapid geological transformations have fascinated scientists and struck fear in the hearts of everyone else for ages.

A new coast

Earthquakes not only rattle the Earth, but they radically change the landscape. The Chilean earthquake that struck on Feb. 27 changed the country's landscape by raising the ground by more than 8 feet (2.5 meters) near the coast and sinking land farther inward, a recent study found.

The massive quake caused marine platforms to rise out of the ocean, thereby shifting the coastline in some places 1,640 feet (500 m) closer to the ocean. [See images of Chile's raised coast.]

Ice conveyor belt

Glaciers huge rivers of ice formed when snow and ice accumulate over hundreds and thousands of years act like a big conveyor belt that pushes ice into the sea. These icy rivers move slowly over time, some eventually dumping ice chunks into the sea, a process known as calving a leading source of additional water for the world's oceans.

Some kinds of glaciers, however, calve as often as once an hour. These kinds of glaciers are called "grounded," meaning they rest on the ocean floor; others float on top of the ocean waters as they run into the sea. Scientists recently observed Alaska's Columbia Glacier undergoing a transition from grounded to floating, which dramatically slowed its calving. [ See scientists studying the glacier.]

Volcano collapse

Massive volcanic eruptions unleash ash and pumice into the sky and can be heard thousands of miles away and even seen from space . But volcanoes can change the landscape in the blink of an eye in a way other than blowing off their tops by triggering huge landslides.

Thousands of years ago, a large collapse of the edifice of the Soufriere Hills volcano on the island of Montserrat in the Lesser Antilles sent landslides into the ocean. Some of these landslides involved nearly 1.2 cubic miles (5 cubic kilometers) of material that travelled underwater for miles.

Volcanic dome collapses occur when dome-shaped lava mounds on top of a volcano break apart due to a gas pressure build-up. Soufriere Hills' eruptions have produced some of the largest volcanic dome collapses ever recorded.

Landslides

Landslides can wipe away villages in the blink of an eye even when volcanoes aren't involved. Heavy rains triggered landslides on the slopes of Mount Elgon in Uganda, on March 1, 2010.

Landslides are common in the region, but these recent landslides are much larger than previous ones. The landslides buried three villages, leaving 83 dead and more than 300 missing as of March 8, reported the United Nations Office for the Coordination of Humanitarian Affairs. The Ugandan government has also stated that deforestation may have played a role in the landslides.

Avalanches

When mountaintop glaciers collapse, they can trigger an avalanche of ice and debris down the mountain. Such was the case for Mt. Kazbeck in Southern Russia when the Kolka Glacier collapsed on Sept. 20, 2002.

In the above image, the dark grey streak shows the gorge that was overrun by ice, rock, water and other debris from the avalanche. The avalanche plowed down the Genaldon River Valley at speeds up to 112 mph (180 kph) and buried parts of a village with a layer of ice and rock 427 feet (130 meters) thick.

Avalanches, along with other deadly natural disasters such as heat waves and floods, could become more common in mountainous regions thanks to climate change, according to a recent study. In the Alps, where temperatures have increased twice as much as the global average temperature since the late 19th century and are predicted to rise by an average of 0.54 to 0.9 degrees Fahrenheit (0.3 to 0.5 Celsius) per decade in the next century, these threats are a real concern.

Rapid megafloods

Some of the most spectacular canyons on Earth (and Mars) were probably formed in the geologic blink of an eye, suggests a recent study that found clues to their formation deep in the heart of Texas.

As anyone living in Comal County, Texas can attest, they can form even faster. Lake Canyon Gorge, a 23-feet- deep (7 meters) canyon was carved in just three days by a flood in 2002. The flood scoured a swath of greenery in this Texas town, leaving sand-colored bedrock rubble in its wake.

A single catastrophic flood capable of cutting into bedrock is extremely rare, but the Comal flood gave scientists a front-row ticket to an event similar to those from the planet's distant past, geologists said.

Rapid megafloods may have formed other canyons in the distant past as glacial ice dams released trapped water. Large floods may be responsible for the formation of some Martian canyons as well, said geologists.

Meteor impact

Many meteors headed for Earth burn up in the planet's atmosphere. Those big enough to make it through leave quite an impression on the landscape.

The Earth's wind, water and vegetation will eventually erase most craters. With few exceptions, even the largest craters are eventually destroyed by the processes of plate tectonics.

The Barringer Crater, also known as Meteor Crater, is a 0.8-mile- (1,300-meter-) diameter, 570-foot- (174-m-) deep hole in the flat-lying desert sandstones that lies 18.6 miles (30 kilometers) west of Winslow, Ariz. [ See other meteor craters across the Earth.]

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7 Ways the Earth Changes in the Blink of an Eye

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From islands popping out of the ocean during earthquakes to glaciers calving icebergs every hour, the Earth can undergo dramatic changes right before your eyes. These seven rapid geological transformations have fascinated scientists and struck fear in the hearts of everyone else for ages. Earthquakes not only rattle the Earth, but they radically change the landscape.

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But most changes happen so slowly we don't notice them at all. The continents slowly creep across the surface of the Earth at an average speed of eight centimeters a year. Over hundreds of millions of years, mountains form, and then slowly erode away.

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The Forces that Change the Face of Earth - Earth's Changing Surface - Beyond Penguins and Polar Bears

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In the Arctic and sub-Arctic, glacial erosion has shaped much of the landscape. Glaciers primarily erode through plucking and abrasion. Plucking occurs as a glacier flows over bedrock, softening and lifting blocks of rock that are brought into the ice.

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As our learning objective mentioned earlier, Earth's surface is constantly changing. Many processes change Earth's landscape instantaneously, right in front of our very eyes, while others take place...

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How many causes does weathering have?

3

How many causes does erosion have?

3

What are the names of two slow processes that change Earth's surface?

Weathering & Erosion

How does temperature weather rocks?

Water takes up more space when frozen. When water freezes inside a rock the rock can crack.

How do chemicals weather rocks?

Some can change the minerals in rocks causing the rocks to crumble or break.

How are caves formed?

Water enters cracks in limestone. Weathered rock is washed away. This continues for many years.

How do water & sand weather rocks?

Rushing water on beaches & river bottoms as well as blowing sand on rocks can break them down.

Name 3 things that cause erosion.

Rivers & Streams, Gravity, & Glaciers

How do glaciers cause erosion?

Huge mass of ice moves along large areas moving rocks.

How does gravity cause erosion?

Can pull rocks down mountains & hills. Strong winds blow soil & sand.

How do rivers & streams cause erosion?

Carry weathered rocks over long distances with their currents.

How do people change Earth's surface?

Cut forests down, drain wetlands, & build roads.

How do roads or parking lots change Earth's surface?

When it rains water cannot soak through the ground and instead rushes away.

Name 3 things that cause Weathering.

Rushing water & blowing sand, changing temperature, chemicals.

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3rd Grade Science Lesson 7 - Slow Changes on Land

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How many causes does weathering have? 3 How many causes does erosion have? 3 What are the names of two slow processes that change Earth's surface? Weathering & Erosion How does temperature weather rocks? Water takes up more space when frozen. When water freezes inside a rock the rock can crack.