Section 1: Overview

This section deals with the various processes that occur on our dynamic and ever changing planet. The section explains the various features and properties of ocean and rocks. It also explains the processes that cause earthquakes and volcanoes.

Section 1: Objectives

After completing this section, the student will be able to:

1. explain the features of the ocean floor.
2. classify rocks and define their properties.
3. understand earthquakes and volcanoes.

Topic 1.1  The Ocean Floor and its Properties

Imagine that you are an astronaut aboard the space station that is revolving around the earth. When you look out through a porthole in the direction of earth, you will see a big blue ball floating around in space.

Can you guess why the earth looks blue from space instead of brown or some other color?

It looks blue because most of the earth's surface is covered with water and that's the reason why our earth is also called the 'blue planet'. Did you know that oceans cover about 71% of the earth's total area and are home to a large variety of flora and fauna i.e. plants and animals?

Let's start with a brief introduction of oceans.

Introduction

An ocean is a vast body of salt water that covers a considerable portion of the earth's surface.

Some facts about oceans

• The total area covered by oceans is about 140 million square miles (362 million sq km), or approximately 71% of the earth's surface.

• The average depth of the oceans is 12,200 feet (3,720 m).

• The deepest point present in the ocean is the Mariana Trench (36,198 feet or 11,033 m), in the western Pacific.

• The ocean ridges form a great mountain range called the global mid-ocean ridge, almost 40,000 miles (64,000 km) long and it winds its way around the Earth like the seam on a baseball. It is the single largest topographic feature on Earth.

• The world's highest mountain is the Mauna Kea Mountain in Hawaii. It extends a height of 33,474 feet (10,203 m) from its base on the ocean floor.

There are five oceans on the earth's surface. They are

       1. The Pacific Ocean
       2. The Atlantic Ocean
       3. The Indian Ocean
       4. The Artic Ocean
       5. The Antarctic Ocean.

Amongst these five oceans, the Pacific, the Atlantic and the Indian Oceans are known as the major oceans. These five oceans are not separate bodies of water; they form one continuous oceanic mass. The boundaries between these five oceans are only arbitrary lines, drawn for the sake of convenience.

What do these oceans contain?

They contain:

A wealth of marine life like fish, shrimps, oysters,coral etc. A variety of mineral wealth on the ocean floor. We can find Copper, Cobalt, Nickel, Manganese, Gold etc. in huge quantities on the ocean floor.
Crude oil, which we can extract by drilling the ocean floor.

Oceans - Some Comparisons

• Oceans are so vast and deep that if the highest mountain peak on the earth's surface which is the Mount Everest - at a height of 8848 meters - were to be sliced at its base and dropped into the deepest oceanic trench , the Mariana Trench - of a depth of 11034 meters, the Everest would sink to a depth of more than two kilometers. Now, that is really deep!

• Nearly 330000 cubic kilometers of seawater evaporates from the oceans and reaches the atmosphere every year. It is returned to the earth's surface in the form of rains, snow, fogs etc.

You must have realized the importance of oceans by now. They are essential for the survival of mankind and we must do our best to conserve its animal and plant life. We must utilize this marine wealth in a sustainable way, so that the future generations can also enjoy this wonderful gift of nature.

Now, let's look at the bottom of the ocean or sea.

The Seabed

The seabed is divided into a number of zones, depending on their depth and the distance from the coast.

The Continental Shelf

It lies between the coast and the 200-meter depth contour and is the shallowest zone. The continental shelf occupies 7.5% of the seabed and corresponds to the submerged portion of the continental crust. The continental shelf area is broader where the shore is the edge of a plain, and is narrow if mountains constitute the coast.

The Continental Slope

The downward slope of the continental shelf increases abruptly to form the continental slope, which occupies 8.5 % of the sea floor. The continental slope has a step slope linking the Continental shelf and the deep sea floor lying at an average depth of about 3600 meters.

The Submarine Ridges

In some oceans, narrow, elongated ridges, resembling mountain ranges on the land, rise from the sea floor. These are called Submarine ridges. Peaks of these ridges may rise above sea level to form islands. Some of these mountain ranges are as big as the Grand Canyon of Colorado.

The Continental Rise

The continental slope meets the abyssal plain at a gentler slope, called the continental rise.

The Basins

Basins are a part of seabed and constitute about 65 % of the sea floor. They lie at a depth of 4000 meters and show many mountain ranges and hills, which resemble similar landforms on the continents.

The Ocean Trenches

There are some deep scars on the ocean floor, usually along the volcanic Islands, which are the deepest places on the earth's surface. They are known as the Ocean-Trenches or the Ocean Deeps. Most of them occur in the Pacific and they are also present in the Atlantic and Indian Ocean.

Now that we have an idea about how the seabed looks, let's learn about ocean water.

Different Layers of an Ocean

Ocean water is a complex solution of about:

• 96 % pure water,
  
• 3 % Sodium Chloride (common salt),
  
• 1 % many other chemicals.

The amount of dissolved salts in the oceans varies from place to place or from time to time throughout the ocean. Therefore, different regions of water within the ocean can have very different properties. Did you know that the density of ocean water, for example, is strongly affected by water temperature and the amount of dissolved salts present in it? As a result, density varies from place to place.

Because of these differences in densities, ocean water has fairly distinct layers of ocean water as we go down towards the ocean floor.
Scientists have divided the ocean into 5 distinct main layers. These layers also known as "zones", extend from the surface to the crushing depths of the ocean. Given below are the different ocean zones.

Zone I - Epipelagic Zone

The first zone or layer from the surface is known as the Epipelagic Zone. This zone extends from the surface to a depth of 200 meters (656 feet). All the water present in this zone is fairly well mixed. Under ideal conditions, a beam of sunlight can penetrate this zone and reach it's bottom. Temperatures in the Epipelagic zone vary with climate and season, but the worldwide average temperature of the surface zone is 22 degrees Celsius.

Zone II - Mesopelagic Zone

The second zone is the Mesopelagic Zone, which extends from a depth of 200 meters (656 feet) to 1000 meters (3281 feet). This zone is also referred to as the twilight zone and is also known as the mid-water zone. The sunlight in this zone is extremely faint. It is in this zone that we begin to see the twinkling lights of bioluminescent creatures. A great variety of strange and bizarre fish can be found here.

Zone III - Bathypelagic Zone

The third zone is called the Bathypelagic Zone. This zone extends from a depth of 1000 meters (3281 feet) down to 4000 meters (13,124 feet). The only visible light in this region is the light produced by bioluminescent life forms. The water pressure at this depth is crushing, but a surprisingly large number of creatures can be found here. Sperm whales can dive down to this level in search of food. Most of the animals that live at these great depths are black or red in color due to the lack of light.

Zone IV - Abyssopelagic Zone

The fourth zone is called the Abyssopelagic Zone. This zone extends from a depth of 4000 meters (13,124 feet) to 6000 meters (19,686 feet). Its name comes from a Greek word meaning "no bottom". The water temperature of this zone is near freezing, and there is no light at all in this zone. Very few creatures can be found at these depths (because of very high water pressure). Most of the organisms that are found at these depths are invertebrates, such as basket stars and tiny squids.

Zone V - Hadalpelagic Zone

Beyond the Abyssopelagic zone lies the Hadalpelagic Zone. This ocean zone extends from a depth of 6000 meters (19,686 feet) to 10,000 meters (32,810 feet). These zones are mostly found in deep water trenches and canyons. In spite of the unimaginable pressures and extremely cold temperature, life can be found in this zone too! Invertebrates such as starfish and tubeworms thrive at these depths.

Deposits on the Ocean floor

In view of the present trends of population explosion, humanity's future on the earth may largely depend on its knowledge of the ocean's potential resources of food, minerals and energy.

Continental shelves provide us with large quantities and varieties of marine food such a fish, shrimp, oysters and crabs etc. Beside these, there are large quantities of mineral wealth present on the ocean floor.

Deep-sea sediments are related to temperature of the surface waters, depth and distance from land etc.

There are two types of deposits on the Ocean floor.

     1. Terrigenous deposits
     2. Pelagic deposits.

Terrigenous deposits consist of mineral particles that are derived from the weathering of land rocks present on land. They are carried to the sea by rivers and winds and are found near the coasts.

Pelagic deposits are found on the deep sea-floor, which is covered by a slimy layer called "Ooze". The innumerable shells and skeletons of tiny planktonic animals and algae form ooze.

Deep-sea explorers discovered manganese nodules on the ocean floor many decades ago. These nodules look like potatoes in size and shape and consist of a rocky central part surrounded by concentric layers of metal oxide. These nodules contain enough copper, cobalt, nickel as well as manganese, to fulfill our needs for many thousands of years.

Magnetic Patterns on the Ocean Floor

Deep-sea explorers also discovered zebra stripe-like magnetic patterns of rocks on the ocean floor. These strange patterns were unlike any seen in the continental rocks. It was discovered that basalt, the iron-rich, volcanic rock that makes up the ocean floor contains a strong magnetic mineral called magnetite and this mineral is powerful enough to be noticed on a magnetic compass.

Formation of Magnetic Rock

How does magnetic rock form and why are there variations in the polarity of these rocks?

That is because grains of magnetite behave like little magnets that align themselves with the orientation of the Earth's magnetic field. After a volcanic eruption the magma (molten rock containing minerals and gases) cools to form solid volcanic rock and the alignment of the magnetite grains is "locked in," recording the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling.

Normal and Reverse Polarity Rocks

Rocks generally belong to two groups depending upon their magnetic properties. One group has normal polarity, characterized by the magnetic minerals in the rock having the same polarity as that of the Earth's present magnetic field. The result of this is, the north end of the rock's 'compass needle' points toward the Earth's magnetic north. The other rock group has reversed polarity that is, the polarity alignment is opposite to that of the Earth's present magnetic field. In reverse polarity, the north end of the rock's 'compass needle' points towards the Earth's magnetic south.

Magnetic Striping

As more and more of the ocean floor was mapped, the magnetic variations present on the floor revealed recognizable patterns. These magnetic patterns on the ocean floor were mapped over a wide region and they formed a zebra-like pattern on the ocean floor. It was revealed that alternating stripes of magnetically different rock were laid out in rows on either side of the mid-ocean ridge. Amazingly one stripe had normal polarity and it's adjoining stripe had reversed polarity. This overall pattern, defined by these alternating bands of normal and reversed polarized rock came to be known as magnetic striping.

Key points to remember -

1. The vast body of salt water that covers a considerable portion of the earth's surface is called an ocean.
   
   
2. The vast areas of interconnected oceans contain 97.2 % of the total volume of water present in the world.
   
   
3. The volume of dissolved salts in seawater is called salinity.
   
   
4. No part of the ocean is completely still, in ocean depths, the movement of water is very slow.
   
   
5. The seabed consists of different zones known as
   1. The Continental Shelf  2. The Continental Slope
   3. The Abyssal Plain and  4. The Ocean Deeps.
   
   
6. Ocean deposits are broadly classified in two types.
   1. Terrigenous deposits and 2. Pelagic deposits.
   
   
7. Basalt, the iron-rich, volcanic rock that makes up the ocean floor, contains a strong magnetic mineral called magnetite.
   
   
8. The overall pattern of the magnetic variations, defined by the alternating bands of normal and reversed polarized rock came to be known as magnetic striping.

Topic Practice

Online Quiz
Students: You can take this online quiz as many times as you need in order to insure you understand the topic.