After Mohorovicic made his discovery, geologists began taking a closer look at the composition of Earth's crust. They found that the crust is not one uniform layer. Instead, there are many important
Crust (least dense)
Core (most dense)
Crust (least dense)
Core (most dense)
Lithosphère W 0-60 miles
Asthenosphere 60-220 miles (100 km-350 km)
Inner core 3,200 miles (5,155 km)
G Infobase Publishing
Figure 5.3 The Earth is made up of layers of varying depth.
differences between the crust that lies under the oceans and the crust that makes up the continents. The most obvious difference has to do with the thickness of the two. While the average thickness of
In the early 1900s, the science of seismology was considered a "man's field." Yet one of most important discoveries ever made about the composition of our planet was made by a woman. Inge Lehmann was a Danish scientist who refused to be stopped because of her gender. Born in 1888, Lehmann attended the first coeducational school in Denmark. Here, boys and girls were taught the same subjects and treated as equals. This was quite rare in those days. Because of her strong desire to learn and the fact that she had this unusual early education, Inge was not afraid to take on the male-dominated world of science when she became an adult.
In 1920, she earned her master's degree in mathematics from the University of Copenhagen. After several years of work during which she used her training in mathematics, Lehmann changed careers and began working with a scientist named N.E. Norlund. Norlund was setting up a series of seismic monitoring stations throughout Greenland and northern Europe, and Lehmann was delighted by the idea of traveling while working. She quickly became an expert at reading seismograms and soon returned to the University of Copenhagen to study seismology. After receiving a second master's degree, Lehmann was named chief of the seismology department of the newly created Royal Danish Geodetic Institute in 1928. Here she recorded hundreds of seismograms and published regular seismic bulletins.
While studying the seismograms of deep focus earthquakes, Lehmann noticed a pattern. Some of the P-waves that should have been bent as they passed through Earth's core seemed to be bouncing off of something
instead. Lehmann analyzed the problem further and concluded that this type of wave motion could only happen if Earth's core was really made up of two layers. Instead of having a single, large, solid core, Lehmann suggested that Earth has a liquid outer core surrounding a smaller solid inner core. She published her findings in a paper simply called "P'" ("P prime") in 1936. At first, few geologists accepted the idea of a double core, but as new data became available, it was clear that Lehmann was correct.
Lehmann continued working in the Danish Geodetic Institute until 1953, at which point she traveled to other labs around the world. In the United States, she conducted research at both the Lamont-Doherty Earth Observatory and the seismological lab at Cal Tech. In 1964, she received a Ph.D. from Columbia University. In 1971, the American Geophysical Union awarded her their highest honor, the William Bowie Medal. Inge Lehmann died in 1993, two months short of her 105th birthday. In 1997, the American Geophysical Union established the Lehmann Medal in her honor, but her greatest prize has to be the name given to the boundary between Earth's inner and outer core: the Lehmann Discontinuity.
the crust in the ocean is only about 3 miles (5 km), the continental crust is often found to be 18 to 25 miles (30 to 40 km) thick. In fact, under high mountains, the continental crust can be as much as 40 miles (65 km) thick, or more than 10 times thicker than the oceanic crust.
Another important difference between the oceanic and continental crust has to do with their individual composition. The crust under the continents appears to be made up of two distinct layers. The top layer is rich in the elements silica and aluminum and is often called the sial (derived from the first letters of silica and aluminum).
Andrija Mohorovicic was born in Volosko, Croatia, in 1857. The son of a carpenter, young Andrija showed his brilliance at an early age. By the time he was 15, he could speak 4 languages and was a whiz in math. After graduating from the University of Prague with a degree in physics and mathematics, he returned to Croatia and spent seven years teaching Earth and physical sciences at a high school. In 1882, he became a member of the faculty at the Nautical School in Bakar, Croatia, where his research interests took hold.
While at the Nautical School, Mohorovicic taught oceanography and meteorology. Like Wegener, he became fascinated with changes in the weather and climate, and he established a meteorological station at the school. He became so good at recording data that, in 1892, he became the head of the national meteorological observatory in Zagreb. In this new position, he was able to obtain better scientific instruments, including seismographs for recording earthquake data. Mohorovicic became fascinated with earthquakes and quickly became an expert in interpreting seismic data.
After a particularly large earthquake hit Europe in October 1909, he gathered seismograms from all around the region. After doing a detailed analysis of the wave patterns from many different seismic recordings, he made his most important discovery. Mohorovicic was able to show that there was a sudden change in density between the surface rocks and those located several miles below. He had discovered the boundary between the crust and the mantle! In honor of this achievement, this boundary is now called the "Mohorovicic Discontinuity" or Moho for short. As we will see a little later on, the Moho plays a major role in the movement of Earth's crust. Its discovery was another important piece in the plate tectonic puzzle.
The lower section of the crust has some silica, but it is also rich in the element magnesium and is called sima (si and ma). Because it is thinner, the crust under the ocean is made up of just a single layer of sima.
These differences in composition mean that rocks on land are much more variable than oceanic rock. For the most part, the rock on the seafloor is made of an igneous rock called basalt. Basalt contains a great deal of heavy elements like iron and magnesium. As a result, basalts tend to have a high density. Continental rocks, on the other hand, can be igneous, metamorphic, or sedimentary. For the most part, continental rocks have a much lower density than the basalts of the ocean. This difference in density plays an important role in how and why the continents move.
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