The layers of the Earth

The crust is the outer hard layer of the Earth. The crust is the part of the lithosphere. The crust is made up of different types of rocks: igneous, metamorphic and sedimentary rocks. Below the crust is the mantle. The upper part of the mantle is made up of peridotite, a rock denser than the rocks common in the crust. The crust and the upper mantle make up the lithosphere. The lithosphere is broken up into tectonic plates that can move. The Earth’s crust is less than 1% of Earth’s volume.

The crust has two different parts. One is the continental crust (under the land) and the other is oceanic crust (under the ocean). The continental crust is thicker and the oceanic crust is thinner. The thickness of the crust can be anywhere from 5 to 70 km. The temperature of the crust increases with depth. Where the crust meets the mantle the temperatures can be between 200^oC and 400^oC. About 75% of the Earth’s crust is composed of two elements – oxygen and silicon. These usually occur in combination with common metals such as aluminium and iron. (% by weight: oxygen, silica, aluminium, iron, and all others).

Lesson 5 Plate tectonics

A map showing the Earth's major tectonic plates.

Plate tectonics is a theory of geology. It has been developed to explain large scale motions of the Earth's lithosphere. This theory builds on older ideas of continental drift and seafloor spreading.

Dissipation of heat from the mantle is the original source of energy driving plate tectonics. Exactly how this works is still a matter of debate. The driving forces of plate motion continue to be active subjects of on-going research.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which float on the fluid-like (viscous-elastic) solid asthenosphere. The relative fluidity of the asthenosphere allows the tectonic plates to undergo motion in different directions. This map shows 15 of the largest plates.

The tectonic plates of the world were mapped in the second half of the 20th century.

Memorize the words:

Continental drift – дрейф континентів seafloor spreading – розсування морського дна dissipation – розсіювання driving force – рушійна сила on-going – існуючий, той, що триває distinct – виразний, чіткий fluid-like – рідиноподібний viscous – в’язкий, тягучий to undergo – зазнавати

Answer the questions:

1. What is the driving force of plate movements?

2. How does the lithosphere exist?

3. What do the lithosphere plates float on?

4. What allows the tectonic plates to undergo motion?

5. When were the tectonic plates of the world mapped?

Lesson 6 Movement of plates

The lithosphere consists of tectonic plates. There are eight major and many minor plates. The lithospheric plates ride on the asthenosphere. These plates move at one of three types of plate boundaries: constructive, destructive and transform.

• At a divergent plate boundary (constructive) two plates move away from each other, and hot magma (liquid rock) is pushed upwards through the cracks. These kinds of boundaries make ocean rifts, undersea volcanoes or ridges.

• At a convergent plate boundary (destructive), two plates move towards each other. This forms islands, volcanoes and high mountain ranges.

• At a transform plate boundary, two plates move parallel to each other. As they move they grind against each other. This kind of plate boundary causes earthquakes.

In other words, when tectonic plates move:

1. together = mountains; volcanoes. For example: the Andes mountain range in South America, the Himalayas, also the Pacific Ring of Fire.

2. away = earthquakes, trenches. The Mid-ocean Ridges are examples.

3. side to side = earthquakes. The San Andreas Fault in California is an example of a transform boundary.

The lateral movement of the plates varies from:

• 1–4 centimeters (0.39–1.6 in) per year (Mid-Atlantic Ridge). This is as fast as fingernails grow.

• 10 centimeters (3.9 in) per year (Nazca Plate). This is as fast as hair grows.

Memorize the words:

to ride – їхати верхи divergent – що розходиться convergent – що сходиться transform – видозміна, перетворення rift – розлом, розколина to grind – молоти, розмелювати lateral – горизонтальний, латеральний to vary – мінятися, змінюватися

Answer the questions:

1. What are the three types of boundaries at which tectonic plates move?

2. What is a divergent boundary?

3. What is a convergent boundary?

4. How do plates move at a transform boundary?

5. How quickly do tectonic plates move?