Samacheer Class 7 Social Science - Interior of the Earth

Weichart – Gutenberg discontinuity
2. The earthquake waves are recorded by an instrument known as ……………….
Seismograph3. Magma rises to the surface and spreads over a vast area is known as ……………….
Lava
4. An example for active volcano ……………….
Mauna Loa in Hawaii /Mt. Strampoli / St. Helens / Pinatubo
5. Seismology is the study of ……………….
Earthquakes

The chief cause of earthquakes is the sudden slipping of portions of the earth's crust along fractures or faults. When stress builds up along these fault lines due to the movement of tectonic plates, the sudden release of this stress causes the earth's crust to shift abruptly, producing earthquakes. The sudden shifting and upheaval of landmass creates vibrations that we experience as earthquakes. Another significant cause of earthquakes is volcanic activity. When magma beneath the earth's surface moves or when volcanic eruptions occur, they can trigger earthquakes. Additionally, the collision and movement of lithospheric plates at plate boundaries generate enormous pressure and stress, which when suddenly released, result in earthquakes. Human activities such as mining, reservoir construction, and underground explosions can also trigger minor earthquakes in some cases.

The innermost layer of the earth is called the core, also known as the barysphere. The core is separated from the mantle above it by a boundary called the Weichart-Gutenberg discontinuity. The core is divided into two distinct parts based on their physical state and composition. The outer core is rich in iron and exists in a liquid state due to the extremely high temperatures at this depth. The inner core, composed of nickel and ferrous materials (Nife), is solid in state despite having even higher temperatures than the outer core because of the immense pressure exerted by all the layers above it. The core is the hottest layer of the earth and plays a crucial role in generating the earth's magnetic field through the movement of liquid iron in the outer core.

No one has been able to take samples from the interior of the earth because the innermost layers, particularly the core, are extremely difficult to access. The central core of the earth has very high temperatures, reaching thousands of degrees Celsius, and experiences immense pressure due to the weight of all the layers above it. These extreme conditions of temperature and pressure make it impossible for humans to reach the core directly or to extract samples from it using current technology. The deepest boreholes drilled by humans have only penetrated a few kilometres into the earth's crust, which is a tiny fraction of the earth's radius of approximately 6,371 kilometres. The intense heat and pressure increase dramatically with depth, making further drilling extremely challenging and impractical. Therefore, our knowledge of the earth's interior is based on indirect methods such as the study of earthquake waves, analysis of meteorites, and laboratory experiments that simulate the conditions of the earth's interior.

Despite the greater thickness, the continental crust is less dense than the oceanic crust because it is composed of both light and dense rock types. The continental crust is primarily made up of lighter granitic rocks, which have a lower density compared to the denser basaltic rocks that form the oceanic crust. Although the continental crust is thicker, reaching depths of about 35 to 40 kilometers, its overall density is lower due to the predominance of lighter silica and alumina-rich materials. In contrast, the oceanic crust, though thinner at about 6 to 7 kilometers, is composed of heavier basaltic material rich in silica and magnesium, making it significantly denser. This difference in density is a crucial factor in understanding plate tectonics and the behavior of the Earth's lithosphere, as denser oceanic crust tends to sink beneath lighter continental crust at convergent plate boundaries.

SIAL and SIMA are two distinct layers that make up the Earth's crust, each with different compositions and characteristics. SIAL is the upper part of the crust and consists primarily of granite rocks. It forms the continents and continental platforms. The main mineral constituents of SIAL are silica and alumina, which give this layer its name. SIAL is relatively light in density, which is why continental masses float higher on the mantle. SIMA, on the other hand, is the lower part of the crust and consists of denser basaltic rocks. It forms the ocean floors and underlies the SIAL layer. The main mineral constituents of SIMA are silica and magnesium, from which its name is derived. SIMA is significantly denser than SIAL, which explains why oceanic crust is heavier and tends to sink beneath continental crust. Together, these two layers form the complete crust of the Earth, with SIAL dominating continental regions and SIMA dominating oceanic regions.

Active volcanoes and dormant volcanoes are two different categories of volcanoes based on their level of activity. Active volcanoes are those that erupt frequently and show regular or periodic volcanic activity. There are approximately 600 active volcanoes in the world today. Examples of active volcanoes include Mt. Stromboli in Italy, Mt. St. Helens in the United States, Mt. Pinatubo in the Philippines, and Mauna Loa in Hawaii. These volcanoes pose a continuous threat to nearby populations and are closely monitored by scientists. Dormant volcanoes, also called sleeping volcanoes, are those that have shown no signs of activity for many years but may become active at any time in the future. They are not extinct, meaning they still have the potential to erupt. Examples of dormant volcanoes include Vesuvius Mountain in Italy, Mt. Fujiyama in Japan, and Mt. Krakatoa in Indonesia. The distinction between active and dormant volcanoes is important for understanding volcanic hazards and for planning safety measures in volcanic regions.

Earthquakes have numerous devastating effects on the Earth's surface and human settlements. Earthquakes cause vibrations that often trigger landslides in mountainous regions, which can bury villages and block valleys. The collapse of buildings is another major danger of earthquakes, particularly in areas where structures are not earthquake-resistant. Most houses that collapse during earthquakes are made of mud and bricks, which prove to be death traps when the ground shakes violently. Fire is another great danger associated with earthquakes, as ruptures in gas lines and electrical systems can ignite fires that spread rapidly through affected areas. The underground water system is severely disturbed by earthquakes, causing great disturbance in water supply and quality. Earthquakes can also trigger tsunamis and floods, especially when they occur beneath oceans or near water bodies. These floods and waves cause great loss of life and property damage in coastal regions. Additionally, earthquakes can cause changes in the Earth's surface, including the formation of new fault lines, uplift or subsidence of land, and changes in the course of rivers. The overall impact of earthquakes on human life and infrastructure is profound and far-reaching.

There are three types of Volcanoes based on eruptions.
Active Volcanoes
Dormant Volcanoes
Extinct Volcanoes
Active Volcanoes:
It erupts frequently. Most of them lie in the Pacific Ring of Fire belt.
There are about 600 active Volcanoes in the world. Mauna Loa in Hawaii is the world’s biggest active Volcano.
Ex: Mt. Stromnoli, St. Helens, Pinatubo.
Dormant Volcanoes:
It has shown no sign of activity for many years but they may become active at any time.
It is also called a sleeping Volcano.
Ex: Vesuvius mountain, Mt. Fujiyama, Mt. Krakatoa.
Extinct Volcanoes
The top of extinct Volcanic mountains has been eroded.
Ex: Mt. Popa, Mt. Kilimanjaro and Mt. Kenya.

There are three major zones of volcanic activity in the world, each with distinct characteristics and locations. The first major zone is the Circum-Pacific belt, also known as the Pacific Ring of Fire. This is the volcanic zone of the convergent oceanic plate boundary and includes the volcanoes of the eastern and western coastal areas of the Pacific Ocean. This zone is popularly termed the Pacific Ring of Fire and has been estimated to include approximately two-thirds of the world's active volcanoes. The second major zone is the Mid-Atlantic belt, which runs along the Mid-Atlantic Ridge and includes volcanic activity associated with the divergent plate boundary between the North American and Eurasian plates, and between the South American and African plates. The third major zone is the Midcontinental belt, which includes volcanic activity in continental regions, often associated with hotspots or areas of crustal weakness. These three volcanic belts are also the principal earthquake belts of the world, as volcanic and seismic activities are closely related and occur in regions of intense crustal deformation and plate movement.

The causes of volcanic activity are rooted in the internal heat and structure of the Earth. The temperature increases at the rate of approximately 10°C for every 35 meters of depth, with increasing pressure. Due to this temperature gradient, the interior of the Earth is in a semi-molten state called magma. This magma is extremely hot and exists under great pressure within the Earth's crust and upper mantle. As the magma is subjected to intense pressure, it dissolves a great volume of combustible gases, including water vapor, carbon dioxide, and sulfur dioxide. The accumulation of these gases increases the pressure within the magma chamber. When the pressure becomes too great, the magma and its dissolved gases seek a way to escape. This causes the volcanic material to burst forth through weak spots or fractures in the Earth's crust, resulting in volcanic eruptions. The weak spots are typically found along plate boundaries, at hotspots, or in areas where the crust is thin or fractured. The combination of high temperature, pressure, and dissolved gases makes volcanic eruptions a powerful natural phenomenon.

The distribution of earthquakes and volcanoes in the world is not uniform but concentrated in specific zones related to plate tectonics. Distribution of Earthquakes: In the Pacific Ring of Fire, approximately 68% of all earthquakes occur, making it the most seismically active region in the world. Another 31% of earthquakes take place in the Mediterranean-Himalayan belt and Asia Minor region, which is associated with the collision of the African, Eurasian, and Indo-Australian plates. The remaining percentage of earthquakes occur in Northern Africa and the Rift Valley areas, as well as in the Ganga-Brahmaputra Valley region. Distribution of Volcanoes: Volcanoes are located in regions that are intensely folded or faulted, typically along plate boundaries. There are approximately 600 active volcanoes in the world, with thousands of dormant and extinct ones. Volcanoes occur along coastal mountain ranges, offshore islands, and in the midst of oceans. The major volcanic belts include the Circum-Pacific belt, the Mid-Atlantic belt, and the Midcontinental belt. These volcanic belts are also the principal earthquake belts of the world, demonstrating the close relationship between volcanic and seismic activities. Both earthquakes and volcanoes are concentrated in zones of active plate movement and crustal deformation.

The Earth's interior is very hot due to the composition and structure of its innermost layers. The innermost layer of the Earth is called the core, which has two distinct parts: the outer core and the inner core. The outer core is rich in iron and exists in a liquid state due to the extreme temperatures. The inner core is made of nickel and iron, which remain in a solid state despite the extremely high temperatures because of the immense pressure exerted by all the layers above it. The central core has a very high temperature, estimated to be around 5,200 to 6,000 degrees Celsius, which is comparable to the surface temperature of the Sun. This extreme heat in the core is generated by the radioactive decay of elements such as uranium, thorium, and potassium, which release energy as they decay. Additionally, residual heat from the formation of the Earth billions of years ago is still being released. This internal heat is the primary source of energy for many geological processes, including volcanic activity, earthquakes, and the movement of tectonic plates. The heat gradually conducts outward through the mantle and crust, driving convection currents in the mantle that move the plates.

Volcanoes are both constructive and destructive in nature, having significant impacts on the environment and human societies. Constructive effects of volcanoes include the enrichment of surrounding soil with nutrients, as volcanic ash contains minerals that are highly beneficial to plants and agriculture. Some plateaus and plains are formed due to volcanic eruptions, creating new landforms and expanding habitable areas over geological time. Volcanoes also help cool the Earth by removing heat from the interior and releasing it into the atmosphere, contributing to the regulation of the Earth's thermal balance. Destructive effects of volcanoes are numerous and severe. Volcanic eruptions can cause earthquakes, fast-moving floods, mudslides, and rockfalls that destroy everything in their path. The dust and ash that come out of volcanoes are very harmful to living things, as they can damage respiratory systems, contaminate water supplies, and block sunlight, affecting agriculture and climate. Lava flows can destroy homes, forests, and agricultural land. Volcanic gases such as sulfur dioxide can cause acid rain and air pollution. The pyroclastic flows, which are superheated mixtures of gas and rock, can travel at high speeds and incinerate everything in their way. Despite the destructive nature of volcanic eruptions, the constructive aspects demonstrate that volcanoes play an important role in shaping the Earth's landscape and supporting life through soil enrichment.

Volcanoes can form islands through a process of repeated underwater eruptions and accumulation of volcanic material. Some volcanoes are found under oceans, submerged beneath the water surface. When an underwater volcano keeps erupting over a long period of time, it continuously deposits lava and volcanic material on the ocean floor. As successive eruptions occur, the volcanic material accumulates and builds up in layers. Eventually, the volcano grows tall enough that its peak rises above the ocean's surface, and thus an island is formed. This process is called submarine volcanism and is responsible for the formation of many islands around the world. Examples of islands formed this way include the Hawaiian Islands, which were created by volcanic activity over a hotspot in the Pacific Ocean. As the Pacific Plate moves over the hotspot, new volcanoes form and old ones become dormant or extinct, creating a chain of islands. The Galapagos Islands were also formed through similar volcanic processes. Once an island is formed above sea level, it may continue to grow if volcanic activity persists, or it may become dormant and be subject to erosion. This process demonstrates how volcanic activity shapes the Earth's surface and creates new landmasses.