To study seismic waves, topographic maps, plate tectonics, we should know the structure of the earth. Earth is the only planet known to have an atmosphere containing free oxygen, oceans of water on its surface, and life.
This course covers the following:
- the structure of the earth
- seismic waves
- topographic maps
Structure of the Earth
The Earth is divided into four distinct layers (Figure 1). The layers are defined by the various properties of the materials that compose them.
Crust is a complex structure and is made from many kinds of rocks. It is the thin, outermost layer made of cool, solid rock that floats on the denser rock beneath. The crust can be broken into pieces called tectonic plates. These plates float on a layer of partially molten rock in the mantle. The movement of these plates is called plate tectonics.
There are two main types of crust: oceanic and continental.
- Oceanic crust can be found under oceans. It is made of mostly silicon, oxygen, and magnesium. This difference in composition causes oceanic crust to be denser than continental crust. Since it is denser, oceanic crust sinks deeper into the mantle than continental crust, causing depressions on the Earth’s surface called ocean basins.
- Continental crust can be found under continents. It is made of mostly silicon, oxygen, and aluminum.
Below the crust lies the dense mantle, extending to a depth of 2890 km. It consists of dense silicate rocks and it provides the thermal and mechanical driving forces for plate tectonics Both P and S waves from earthquakes travel through the mantle, demonstrating that it is solid.
Temperatures range varies from 500°C to over 4000°C in mantle. Convection currents happen inside the mantle and are caused by the continuous circular motion of rocks in the lithosphere
The outer core is composed of iron and some nickel. Temperatures range from 4400°C to 6100°C. The Outer Core is about 2200 km thick. It is the second largest layer and made entirely out of liquid magma.
The inner core is solid and is composed mostly of iron and nickel. Even though it is very hot, the weight of all the rock above it causes the inner core to remain in a solid state. Experiments reported in 1997 suggest that the inner core spins at a slightly faster speed than the Earth itself.
The inner layers of the earth remain hot due to the process of nuclear fission. Elements such as, uranium decay and this split releases heat energy. This energy is great enough to melt the surrounding material. The earth’s outer core flows around the inner core, and this motion produces the magnetic field (Figure 2).
Most of what we know about the interior of the Earth comes from the study of seismic waves (Figure 3) from earthquakes. The energy released during the movement of tectonic plates creates waves, and these waves are known as seismic waves. These waves contain vital information about the internal structure of the Earth.
Seismic waves are of four types (Figure 4 & 5):
- P- Waves (Primary waves)
- S- Waves (Secondary waves)
- L- Waves (Surface waves)
- Rayleigh waves
Primary (or “P”) waves are incredibly quick, reaching seismic stations first after an earthquake has erupted. They can travel through both liquid and solid parts of earth’s interior. During the earthquake, these waves hit the ground first and foremost. Hence, they are called primary waves.
Secondary (or “S”) waves arrive next. They can only travel through solid part of earth. Both P and S waves are referred to as body waves. The travel-time of seismic waves depending upon change in density with depth indicates that Earth is composed of several layers.
Surface (or “L”) waves are also known as “Love waves” and are formed when released energy comes on the surface of the earth. These are the horizontal and vertical waves on the surface of the earth. These are those waves which damage the most.
Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical motion, with no transverse, or perpendicular motion. The medium of S- waves and Rayleigh waves is almost same. However, speed of these waves is less than the speed of surface waves.
These waves move in the vertical plan in the direction of motion. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.
Table showing difference between different kinds of seismic waves.
|Primary Wave (P)||Secondary Wave (S)|
Surface Wave (L)
|Characteristics||P wave or compressional wave is the fastest kind of seismic wave, and, consequently, the first to ‘arrive’ at a seismic station.||S wave is s the second wave. we feel in an earthquake. It is slower than a P wave.||L wave is fastest surface wave and moves the ground from side-to-side.||Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.|
|Speed||The maximum speed of P- waves is around 14 Km/sec. Their average speed is 8 km/sec||The speed of these waves is 4 to 6 km/ sec.||Speed of these waves is around 2 to 3 km/sec.||Speed of these waves is less than 3 km/sec.|
|Movement||They cause matter to oscillate forward and backward, parallel to motion of the seismic wavefront.||They cause matter to oscillate side by side, perpendicular to motion of the seismic wavefront.||They cause matter to oscillate side by side||They show rolling motion, forward and up then backward and down|
As the molten rock moves and carries heat with it, convection currents occur. The molten rock rises and strikes the crust, but it cannot burst through. Instead, it moves horizontally along the bottom of the crust. Like a river’s current and its force on a boat, convection currents pull the tectonic plates along, causing them to move.
Earth’s surface contains a wide variety of geographic features, landforms, and bodies of water. Mountains rise miles above Earth’s surface, and basins and canyons extend deep into the earth. Geologists create topographic maps to show this wide variety of elevation and bodies of water on Earth’s surface. A topographic map is a detailed two-dimensional representation of natural and human-made features on the Earth’s surface (Figure 6). These maps are accurate and created using surveying equipment and aerial photography.
Topographic maps have contour lines that connect areas with the same elevation, or altitude (Figure 7). Contours are imaginary lines that connect locations of similar elevation. They represent the height of mountains and steepness of slopes on a two-dimensional map surface. Numbers on the contour lines indicate the elevation. Contour lines that are close together indicate the slope is steep and the elevation changes rapidly. Contour lines that are farther apart indicate the slope is gentle and the elevation changes gradually. Contour lines that V upwards indicate the presence of a river valley. Ridges are shown by contours that V downwards.
In the United States, topographic maps have been made by the United States Geological Survey (USGS) since 1879. Topographic coverage of the United States is available at scales of 1:24,000, 1:25,000 (metric), 1:62,250, 1:63,360 (Alaska only), 1:100,000 and 1:250,000.
A contour line is an isoline that connects points on a map that have the same elevation. Contours are often drawn on a map at a uniform vertical distance. This distance is called the contour interval.
Geologists, hikers, miners, and others use topographic maps to find information about existing bodies of water and landforms. Geologists monitor topographic variations over time to see how the land changes. If the elevation increases or new landforms are created, geologists infer that constructive processes are taking place. If the elevation decreases, destructive processes are taking place.