The Shape We’re In: Current Theory about Structure of the Earth

For the past few weeks, we’ve been learning about Earth’s atmosphere and its effects on life on Earth’s surface. Today, we’re going to dig a little deeper, and find that the beauty of this planet isn’t just skin deep.

In the last two decades, many discoveries have proven the Earth itself to be as vibrant and lively as its "living" flora and fauna. Although its changes are measured in the thousands of years of eons, they just as surely take place as the generations of organisms.

When we study plants and animals, we learn first of their basic structure, the cell, before we look into how the organism behaves. Following this example, let’s start out by looking at the structure of our planet, from the innermost part to its surface.

A Cross Section of Our Planet

The Earth has four basic layers:

1. Inner core --- This layer is solid, with a radius of appx. 1300 km. Consists mainly of iron and nickel.

2. Outer core --- Liquid. Moves via convective currents. Mainly molten iron, but probably includes some oxygen, sulfur. Radius of 2200 km.

3. Mantle --- Radius of 2850 km. This section is broken into three additional sections:

a. Lower mantle--- Solid. Thickest part of mantle. Consists mainly of silicon, magnesium and oxygen, with some iron, calcium and aluminum.
b. Asthenosphere --- Thinnest layer of mantle. Like soft plastic, is somewhat fluid. Contains calcium, aluminum and various silicates.
c. Upper mantle --- Topmost layer of the mantle. Solid, rigid. Contains iron, magnesium, silicates and oxygen. This layer and the crust form the rigid lithosphere, which gives rise to the tectonic plates.

4. Crust --- Uppermost layer of the Earth. Thin, only 5 - 50 km. thick.

a. Continental crust --- Lighter in weight but thicker ( 30-40 km) of the two types of crust. Rich in aluminum, oxygen and silica.
b. Oceanic crust --- Thin but heavy crust (5-7 km). Composition of basalt, a rock heavy in iron and magnesium.

Think of the Earth like an egg (but... not to proportion!):

The yolk(when hard boiled) = the inner core
The albumin/"white" = the outer core (liquid)
The membrane = the mantle
The shell = the crust

Continental Drift

Up until about 200 million years ago, all the land on Earth was in one mass, called Pangea. At this time, this mass started breaking up into smaller land masses and drifting away from its original location. This theory, called the continental drift theory, was first introduced by Alfred Wegener (Germany) in 1912. (Died in Greenland in 1930 while exploring the Greenland Ice Cap.) He based his theory on the following observations:

· On the Earth’s crust, lighter rocks rest on top of heavier ones.
· Identified similar fossils of the fern, Glossopteris from 2000 million years ago in current South America, Africa, Australia. India and Antarctica.
· Found that the layers of rock sequences for this time period were the same in these areas.
· Study of glacier positions showed that one common glacier covered all of these areas at that time.

Wegener’s skeptics cited this problem with the theory: lack of explanation for movement of the areas.

However, in 1928, Arthur Holmes (Scotland) suggested that continents moved due to convection currents due to heat from within the Earth.

In the late 1940s and early ‘50s, there was a revival of the continental drift theory, based on new knowledge of the sea floor and magnetic properties of rocks.

Mid-Atlantic Ridge:

This underwater ridge had been known about since the 19th century:

· About 2000 meters across
· About 6000 meters below sea level
· Discovered with earthquake studies --- There is a global network of mid-ocean ridges, along which there is an active earthquake zone. At the top of each ridge is a trough or rift (an opening or split in the ridge) appx. 30 m. across.
· This ridge extends throughout all of the oceans in a network of about 60,000 km.

Ridges and Sea Floor Spreading:

Harry Hess in 1962 proposed that the ocean floor is actually formed at the rift of new mid-ocean ridges. So enthusiastically did he describe his theory that his idea was called geo-poetry The ocean floor and the rock underneath are formed by magma that rises from the molten Earth. The theory states that the ocean floor moves laterally away from the ridge, then plunges into oceanic trenches along continental margins.

· Ex: Ocean crust formed at the East Pacific rise plunges into a trench next to the Andes in South America. This occurs because convection currents push the ocean floor from the mid-ocean ridge to the trench. (Convection currents might also help move continents, like a conveyor belt.)

· Ex: Making toffee at Christmas. Sugar and butter melt. Heat makes this thick goo flow and move. Stirring with a spoon furthers convection movement, and it moves to the edge of the pan. When poured onto a surface to cool, it oozes outwards, towards edges.

Robert Dietz refined the theory, by suggesting that sea floor spreading occurred at the base of the lithosphere, not at the base of the crust.

The hypothesis was confirmed by studies of the magnetic fields found in rock from the ocean floors. Geologists can detect magnetic anomalies where magnetic fields weren’t always where they are today. In 1963, Vine and Drummond proposed that lava erupted at different times along the rift at the crest of the mid-ocean ridges and preserved different magnetic fields.

· Ex: Lava erupting in the past when the north magnetic pole (NMP) was in the northern hemisphere preserved a positive magnetic anomaly, whereas lava erupting in the past when the NMP was in the southern hemisphere preserved a negative magnetic anomaly. (Lava erupting today has a positive anomaly, because the NMP is in the northern hemisphere.)