The Earth’s crust is its outermost solid layer. This rocky shell is not uniform in its thickness; it varies significantly across the globe. These variations in crustal thickness help us comprehend the planet’s geological processes and its dynamic nature. The crust is a relatively thin layer compared to the Earth’s overall size, making up less than one percent of the planet’s radius and volume.
The Earth’s Outer Layer
The Earth’s crust is composed of two types: continental crust and oceanic crust. Continental crust underlies landmasses and continental shelves, made of granitic rocks. This crust is less dense, averaging 2.7 grams per cubic centimeter. Its thickness ranges from 20 to 70 kilometers, and some parts date back billions of years.
Oceanic crust, in contrast, forms the ocean basins and is thinner and denser than its continental counterpart. It is made of basaltic rocks, rich in iron and magnesium. The density of oceanic crust averages 2.9 to 3 grams per cubic centimeter. This crust is considerably thinner, ranging from 5 to 10 kilometers, and is geologically much younger, the oldest being about 200 million years old. The difference in density explains why oceanic crust sits lower, forming ocean basins, while continental crust floats higher.
Where Crustal Thickness Peaks
The Earth’s crust reaches its greatest thickness beneath major mountain ranges and high plateaus, regions of continental crust. The Himalayas, particularly the Tibetan Plateau, represent a prime example of great crustal thickness. In this region, the crust can be deep, with thicknesses ranging from 70 to 75 kilometers in southern Tibet, and 60 to 65 kilometers in northern, northeastern, and southeastern Tibet. Estimates suggest the crust can reach 80 kilometers in parts of the Tibetan Plateau.
Another area of thickened crust lies beneath the Andes Mountains in South America. The central Andes exhibit crustal thicknesses varying from 49 to 80 kilometers across the Altiplano and Puna regions. The Western and Eastern Cordilleras of the Andes have crustal depths of 70 to 74 kilometers. These areas of crustal depth are directly linked to geological forces that shape Earth’s surface.
How Thick Crust Forms
Thick crust forms primarily from plate tectonics at convergent plate boundaries. A primary mechanism for crustal thickening is the collision of two continental plates. Because continental crust is buoyant and does not readily subduct, when two continental landmasses collide, compressional forces cause the crust to buckle, fold, and stack upon itself. This process, known as crustal shortening, leads to an increase in vertical thickness.
The collision between the Indian and Eurasian plates, forming the Himalayas and Tibetan Plateau, is a prime example. Here, the continental crust has been shortened horizontally and thickened vertically over millions of years. Another way crust can thicken is through the subduction of oceanic crust beneath continental crust. While oceanic crust typically subducts, in some cases, a shallow subduction angle causes the overriding continental plate to experience compression, which results in crustal thickening and mountain building. This process is observed in the Andes, where the Nazca plate subducts beneath the South American plate, leading to crustal shortening and thickening, forming the high mountain ranges.
Uncovering Crustal Depths
Scientists determine the thickness of the Earth’s crust primarily through seismic methods. These techniques involve studying how seismic waves from earthquakes or artificial sources travel through and reflect off different layers within the Earth. As seismic waves encounter boundaries between materials of varying density and elasticity, such as the crust-mantle boundary (known as the Mohorovičić discontinuity or Moho), their speed and direction change.
By measuring the travel times of these waves and analyzing their reflections and refractions, geophysicists can create images of the subsurface structure. This allows them to map the depth of the Moho and, consequently, the thickness of the crust. While seismic data provides the most direct measurements, other geophysical methods, such as gravity anomaly studies, also help understand crustal thickness variations by detecting differences in the Earth’s gravitational field caused by variations in subsurface mass.