Layers of the Earth

To understand the details of plate tectonics, one must first understand the layers of the Earth. Most of what we know about the inner layers is pieced together from models, seismic waves, and assumptions based on meteorite material. In general, the Earth can be divided into layers based on chemical composition and physical characteristics. (2 Plate Tectonics – An Introduction to Geology, n.d.)

Crust

The outermost chemical layer and the layer we humans occupy is known as the crust. The crust has two types: continental crust, which is relatively low density and has a composition similar to granite, and oceanic crust, which is relatively high density (especially when it is cold and old) and has a composition similar to basalt. In the lower part of the crust, rocks start to be more ductile and less brittle because of added heat. Earthquakes, therefore, generally occur in the upper crust.

At the base of the crust is a substantial change in seismic velocity called the Mohorovičić Discontinuity, or Moho for short, discovered by Andrija Mohorovičić (pronounced mo-ho-ro-vee-cheech) in 1909 by studying earthquake wave paths in his native Croatia. It is caused by the dramatic composition change between the mantle and the crust. Underneath the oceans, the Moho is about 5 km down. Under continents, the average is about 30-40 km, except near a sizeable mountain-building event, known as an orogeny, where that thickness is about doubled.

Mantle

The mantle is the layer below the crust and above the core. It is the most substantial layer by volume, extending from the base of the crust to a depth of about 2900 km. Most of what we know about the mantle comes from seismic waves, although some direct information has been gathered from parts of the ocean floor brought to the surface, known as ophiolites. Also carried within magma are xenoliths, which are small chunks of lower rock carried to the surface by eruptions. These xenoliths are made of the rock peridotite, which is ultramafic on the scale of igneous rocks. We assume the majority of the mantle is made of peridotite.

Core

The core of the Earth, which has both liquid and solid components, is made mostly of iron, nickel, and oxygen. It was first discovered in 1906 by looking into seismic data. It took the union of modelling, astronomical insight, and seismic data to discover that the core is mostly metallic iron. Meteorites contain much more iron than typical surface rocks. If meteoric material is what made the Earth, the core would have formed as dense material (including iron and nickel) sank to the center of the Earth via its weight as the planet formed, heating the Earth intensely.

Physical Layers

The Earth can also be broken down into five distinct physical layers based on how each layer responds to stress. While there is some overlap in the chemical and physical designations of layers, precisely the core-mantle boundary, there are significant differences between the two classification systems. (2 Plate Tectonics – An Introduction to Geology, n.d.)

Lithosphere

The lithosphere, with ‘litho’ meaning rock, is the outermost physical layer of the Earth. Including the crust, it has both an oceanic component and a continental component. The oceanic lithosphere, ranging from a thickness of zero (at the forming of new plates on the mid-ocean ridge) to 140 km, is thin and rigid. The continental lithosphere is more plastic (especially with depth) and thicker overall, from 40 to 280 km thick. Most importantly, the lithosphere is not continuous. It is broken into several segments that geologists call plates. A plate boundary is where two plates meet and move relative to each other. It is at and near plate boundaries where plate tectonics’ real action is seen, including mountain building, earthquakes, and volcanism.

Asthenosphere

The asthenosphere, with ‘astheno’ meaning weak, is the layer below the lithosphere. The most distinctive property of the asthenosphere is movement. While still solid, it will flow and move over geologic time scales because it is mechanically weak. In this layer, partly driven by convection of intense interior heat, movement allows the lithospheric plates to move. Since certain types of seismic waves pass through the asthenosphere, we know that it is solid, at least at the short time scales of the passage of seismic waves. The asthenosphere’s depth and occurrence depend on heat and can be very shallow at mid-ocean ridges and very deep in plate interiors and beneath mountains.

Outer Core

The outer core is the only liquid layer found within Earth’s interior. It starts at 2,890 km (1,795 mi) depth and extends to 5,150 km . It surrounds a solid inner core that is about 1,220 km (758 mi) thick, and the outer core is about 2,300 km (1,429 mi) thick.

The solid inner core can be explained by understanding that the immense pressure inhibits melting, though as the Earth cools by heat flowing outward, the inner core grows slightly larger over time. As the liquid iron and nickel in the outer core moves and convects, it becomes the most likely source for Earth’s magnetic field. This is critically important to maintaining the atmosphere and conditions on Earth that make it favourable to life. Loss of outer core convection and the Earth’s magnetic field could strip the atmosphere of most of the gases essential to life and dry out the planet, much like what has happened to Mars.

Continents

The oldest continental rocks are billions of years old. Constructive forces cause landforms’ physical features on Earth’s surface to grow. Crustal deformation – when crust compresses, and pulls apart, results in features like mountains, valleys, and seas. Mountains rise when continents collide or when one slab of ocean crust plunges beneath another slab of continental crust to create a chain of volcanoes. Sediments are deposited to form landforms, such as deltas. Volcanic eruptions can also be destructive forces that blow landforms apart. The destructive forces of weathering and erosion modify landforms. Water, wind, ice, and gravity are essential forces of erosion.

Oceanic Basins

The ocean basins are all younger than 180 million years. Although the ocean basins begin where the ocean meets the land, the continent extends downward to the seafloor, so the continental margin is made of continental crust.

The ocean floor itself is not flat. The most distinctive feature is the mountain range that runs through much of the ocean basin, known as the mid-oceanic ridge. The ocean trenches are the deepest places of the ocean, many of which are found around the edge of the Pacific Ocean. Chains of volcanoes are also found in the center of the oceans, such as around Hawaii. Flat plains are found on the ocean floor with their features covered by mud.