Question

What is the Moho? How was it first recognized? Describe the differences between continental crust and oceanic crust.

Short answer

The Moho, or Mohorovičić discontinuity, is the boundary between Earth's crust and mantle, first recognized in 1909 by Croatian seismologist Andrija Mohorovičić. It lies at a depth of about 5-10 km beneath the oceanic crust and 25-65 km beneath the continental crust. The continental crust is primarily composed of granitic rocks, thicker (about 35-65 km), and older (up to four billion years) than oceanic crust, which is predominantly basaltic, thinner (about 5-10 km), and younger (less than 200 million years). Differences in composition, thickness, and age contribute to variations in overall structure between these two types of crusts.

Step-by-step solution

Definition of Moho

The Moho, or Mohorovičić discontinuity, is the boundary between Earth's crust and the mantle. It is characterized by a sudden change in seismic wave velocities, which indicates a transition between different rock compositions in the crust and mantle. The Moho typically lies at a depth of about 5-10 km beneath the oceanic crust and 25-65 km beneath the continental crust.

Moho's Discovery

The Moho was first recognized in 1909 by Croatian seismologist Andrija Mohorovičić. He analyzed the arrival times of different seismic waves from an earthquake and observed that some waves traveled much faster below a certain depth, suggesting a boundary between two distinct layers of Earth's interior. This boundary was later named the Mohorovičić discontinuity in his honor.

Differences Between Continental Crust and Oceanic Crust: Composition

Continental crust and oceanic crust have different compositions. Continental crust is primarily composed of granitic rocks, which are less dense and have higher silica content (SiO2). In contrast, oceanic crust predominantly consists of basaltic rocks, which are denser and have lower silica content. This difference in composition contributes to variations in thickness, density and, ultimately, overall structure between the two types of crust.

Differences Between Continental Crust and Oceanic Crust: Thickness

The thickness of the crust also varies significantly between continental and oceanic regions. Continental crust is typically much thicker than oceanic crust, with an average thickness of about 35 km, but it can be as thick as 65 km in some mountainous regions. On the other hand, oceanic crust has an average thickness of about 5-10 km.

Differences Between Continental Crust and Oceanic Crust: Age

Lastly, continental and oceanic crusts differ in age. Continental crust is generally older, ranging from a few million to over four billion years old. In contrast, oceanic crust is much younger, with an average age of fewer than 200 million years. The reason for this age difference is that oceanic crust is continuously being created at mid-ocean ridges and destroyed at subduction zones, whereas continental crust is more stable and long-lasting. By thoroughly understanding the Moho and its discovery, as well as the differences between continental and oceanic crusts, you will have a better grasp of Earth's internal structure and the mechanisms that shape it.

Key concepts

Continental Crust vs Oceanic Crust

When we compare the continental crust to the oceanic crust, we encounter a striking contrast in several key attributes. The continental crust is what we live on; it's the landmasses that form the continents. It consists of various rocks with a dominant component of granite, giving it a lighter density and a higher concentration of silica, making it buoyant.

On the other hand, the oceanic crust underlies our oceans and is primarily composed of basalt. This type of crust is thinner, denser, and comparatively young; it continually cycles back into the Earth's mantle through the process of subduction, essentially recycling material on a geologic timescale. Conversely, the continental crust is much older, some parts dating to the early history of Earth, and does not subduct as easily due to its buoyancy.

Additionally, the differences in thickness and density between these two types of crust play crucial roles in earth's geological activity, such as mountain formation and volcanic activity. The continents can also host a complex array of life due to their stable and diverse geological features.

Seismic Wave Velocities

Seismic waves are like Earth's X-rays, providing insights into its internal structure. These waves have different velocities depending on the material they travel through. In seismology, the term 'velocity' refers to the speed at which seismic waves propagate through different layers of the Earth.

In general, seismic waves travel faster through denser materials. This fact underlies how the Mohorovičić discontinuity, or 'Moho', was discovered - a sudden increase in seismic wave speeds was noted as they passed from the crust into the much denser mantle. The velocity of P-waves (primary waves) and S-waves (secondary waves) in oceanic crust can differ from those in continental crust due to the variations in density and composition.

Moreover, this characterization of seismic wave velocities is not limited to discerning crust from mantle; it is also an invaluable tool for locating oil deposits, understanding earthquake mechanics, and mapping the interior structure of the Earth on a broader scale.

Earth's Internal Structure

The Earth is like a layered sphere, each layer with its own unique properties and compositions. The internal structure of Earth can be visualized starting from the exterior crust, a relatively thin layer comprised of the continental and oceanic varieties discussed above. The crust transitions into the solid, silica-rich upper mantle at the Mohorovičić discontinuity.

Below the upper mantle lies a transition zone before reaching the lower mantle, which extends down to the outer core. The outer core is liquid and primarily composed of iron and nickel, which creates Earth's magnetic field. The innermost layer is the solid inner core, also rich in iron and nickel but solid due to immense pressures.

Each layer has different physical and chemical characteristics, which affect seismic wave velocities and are thus key to understanding the dynamic process of Earth, including plate tectonics, volcanic activity, and even the creation of natural resources.