Notes

Earth's Layered Structure



Crust
The crust, the thin, rocky outer layer of Earth, is divided into oceanic and continental crust.

The oceanic crust is roughly 7 kilometers thick and composed of the igneous rocks basalt and gabbro. The continental crust is 8–75 kilometers thick, but averages a thickness of 40 kilometers. It consists of many rock types. The average composition of the continental crust is granitic rock called granodiorite.

Continental rocks have an average density of about 2.7 g/cm3 and some are more than 4 billion years old. The rocks of the oceanic crust are younger (180 million years or less) and have an average density of about 3.0 g/cm3.


More than 82 percent of Earth’s volume is contained in the mantle—a solid, rocky shell that extends to a depth of 2,890 kilometers.

Core
The core is a sphere composed mostly of an iron-nickel alloy. At the extreme pressures found in the center of the core, the iron-rich material has an average density of 13 g/cm3(13 times denserthan water)


Lithosphere
Earth’s outermost layer, which consists of the crust and uppermost mantle, forms a relatively cool, rigid shell called the lithosphere. This layer averages about 100 kilometers in thickness


Asthenosphere
Beneath the lithosphere lies a soft, comparatively weak layer known as the asthenosphere. Within the asthenosphere, the rocks are close enough to their melting temperatures that they are easily deformed.

Thus, the asthenosphere is weak because it is near its melting point, just as hot wax is weaker than cold wax. The lower lithosphere and asthenosphere are both part of the upper mantle



Lower Mantle
From a depth of about 660 kilometers down to near the base of the mantle lies a more rigid layer called the lower mantle. Despite their strength, the rocks of the lower mantle are still very hot and capable of gradual flow.



Inner and Outer Core
The core, which is composed mostly of an iron-nickel alloy, is divided into two regions with different physical properties.

The outer core is a liquid layer 2,260 kilometers thick. The flow of metallic iron within this zone generates Earth’s magnetic field. Just as there is a magnetic field around a bar magnet, an immense magnetic field surrounds Earth. The poles of the magnetized needle on a compass align themselves with Earth’s magnetic field.

The inner core is a sphere having a radius of 1,220 kilometers. Despite its higher temperatures, the material in the inner core is compressed into a solid state by the immense pressure.



Seismic waves bend as they travel through Earth. This information has helped scientists to infer the planet’s layered structure.

During the twentieth century, studies of the paths of P and S waves through Earth helped scientists identify the boundaries of Earth’s layers and determine that the outer core is liquid.

In 1909, a Croatian seismologist, Andrija Mohorovicic, presented the first evidence of layering within Earth’s mantle. By studying seismic records, he found that the velocity of seismic waves increased abruptly about 50 kilometers below eastern Europe. This boundary separates the crust from the underlying mantle and is now known as the discontinuity. The name of the boundary is usually shortened to Moho.

Continental Drift



A German scientist, Alfred Wegener, noticed the similarity between the coastlines on opposite sides of the South Atlantic Ocean. As you can see in the figure, the shapes of South America and Africa are an approximate fit.

In 1915, Wegener proposed his radical hypothesis of continental drift.


According to Wegener’s hypothesis of continental drift, the continents had once been joined to form a single supercontinent. He called this supercontinent Pangaea, meaning all land.

Wegener also hypothesized that about 200 million years ago, Pangaea began breaking into smaller continents. The continents then drifted slowly to their present positions.

Move through the activity to understand how the arrangement of the continents has changed over geologic time.


Rock Types
Anyone who has worked a jigsaw puzzle knows that the pieces must fit together to form a complete picture.

The picture in the continental drift puzzle is one of matching rock types and mountain belts. If the continents were once part of Pangaea, the rocks found in a particular region on one continent should closely match in age and type those in adjacent positions on the adjoining continent.


Ancient Climates
Wegener found evidence for dramatic global climate changes that supported his hypothesis. He found glacial deposits showing that between 220 million and 300 million years ago, ice sheets covered large areas of the Southern Hemisphere. Deposits of glacial till occurred at latitudes that today have temperate or even tropical climates: southern Africa, South America, India, and Australia.



Sea-Floor Spreading



During the early 1900s, a new technology made it easier to map the ocean floor. Sonar, which stands for sound navigation and ranging, is a system that uses sound waves to calculate the distance to an object. The sonar equipment on a ship sends out pulses of sound that bounce off the ocean floor. The equipment then measures how quickly the sound waves return to the ship. The deeper the water, the longer it takes the sound waves to return to the ship.



Deep-Ocean Trenches
As scientists mapped the ocean floor, they found long, curved valleys along the edges of some ocean basins called deep-ocean trenches.

Trenches form the deepest parts of Earth’s oceans. For example, the Marianas Trench in the Pacific Ocean is more than 11 kilometers deep.

Most trenches occur around the edges of the Pacific Ocean, although others occur in the Indian and Atlantic oceans.



Mid-Ocean Ridges
By the late 1950s, scientists had constructed a more complete map of Earth’s ocean floor. The map showed that the mountain range in the middle of the Atlantic Ocean was not an isolated feature. Instead, it formed a mid-ocean ridge, a long chain of mountains extending the length of the ocean.


Often, a deep, central valley runs down the center of a ridge. Called a rift valley, the central valley of a mid-ocean ridge resembles a long canyon. Some parts of the ridge system lack a rift valley.

Composition of the Ocean Floor
Earth’s ocean floors are made of igneous rocks of basaltic composition. Basalt forms when lava reaches the surface and hardens to form solid rock.

Most of the ocean floor is covered with a thick layer of sediment. Scientists found that the sediment layer became thinner closer to mid-ocean ridges and that along the ridge there was no sediment.





The new map of the ocean floor aroused the curiosity of many scientists. One geologist, Harry Hess, thought that the mid-ocean ridges and deep-ocean trenches might help to explain how the ocean floor was formed.

In 1963, Hess published his hypothesis of sea-floor spreading. In the process of sea-floor spreading, new ocean floor forms along Earth’s mid-ocean ridges, slowly moves outward across ocean basins, and finally sinks back into the mantle beneath deep-ocean trenches. During sea-floor spreading, new oceanic lithosphere is formed, and the ocean floor gets wider. Today, the Atlantic Ocean is thousands of kilometers wide. Millions of years ago, the Atlantic would have been a narrow sea, like the Red Sea.


Eruptions Along Mid-Ocean Ridges
How did the mid-ocean ridges form? Scientists have found evidence that the mid-ocean ridges formed as the result of volcanic activity. As shown in the figure, fractures along the central valley of a mid-ocean ridge fill with magma that wells up from the hot mantle below. (Magma is molten rock that forms in the upper mantle and rises through the crust).

Gradually, the magma cools to produce new slivers of ocean floor. Spreading and upwelling of magma continuously adds new ocean floor.

The process can also begin on land when a rift valley forms and splits a continental landmass. Over millions of years, the rift valley widens to form a new ocean basin—like the Red Sea.

Movement of the Ocean Floor
As new ocean floor is added along mid-ocean ridges, the older ocean floor moves outward and away from the ridge on both sides. Rates of sea-floor spreading average about 5 centimeters per year. These rates are slow on a human time scale. But they are fast enough that all of Earth’s ocean basins could have been formed within the last 200 million years