Earth’s Layered Structure Earth Science 8.4 Earth’s Layered Structure Earth’s Layered Structure

Earth Science 8.4 Earth’s Layered Structure Compared to the planets we see in the sky, Earth’s interior is close to us. However, we can not reach it. The deepest well drilled by modern man is only 12 kilometers below the Earth’s surface. We know what we know about earth’s interior largely from our study of seismic waves that travel through the earth.

Earth Science 8.4 Earth’s Layered Structure If Earth’s materials had the same properties in each layer, seismic waves would spread through it in straight lines at constant speed. This is not always the case. Sometimes, seismic waves reach seismographs located far away from an epicenter faster than locations close to an event.

Earth Science 8.4 Earth’s Layered Structure This increase in speed with depth is due to an increase in pressure deep down which changes the elastic properties of the deeper rock layers. As a result, the paths of the seismic waves through the earth are refracted, or bent, as they travel. Earth’s interior consists of three major layers defined by their chemical composition: the crust, the mantle, and the core.

Earth Science 8.4 Earth’s Layered Structure Earth’s Crust: Earth’s crust, the thin rocky outermost layer, is divided into oceanic crust and continental crust. The oceanic crust is roughly 7 kilometers thick and composed of igneous rocks basalt and gabbro. The continental crust is 8 – 75 kilometers thick ( but averages at 40 kilometers thick) and is made of several rock types.

Earth Science 8.4 Earth’s Layered Structure Earth’s Crust: The average composition of the continental crust is granitic rock called granodiorite. Continental rocks have an average density of about 2.7 -2.8 Continental crust rocks are over 4 billion years old in some areas. The rocks of the oceanic crust are younger (180 million years or less)

Earth Science 8.4 Earth’s Layered Structure Earth’s Mantle: Over 82 % of earth’s volume is contained in the mantle; a rocky shell that extends to a depth of 2890 kilometers. The boundary between the crust and mantle represents a change in the chemical composition. A common rock type in the uppermost layers of the mantle is peridotite, which has a density of about 3.4

Earth Science 8.4 Earth’s Layered Structure Earth’s Core: Earth’s core is a sphere composed mostly of iron-nickel alloy. At the extreme pressures found in the center core, the iron-rich material has an average density of 13 ( 13 times denser than water)

Earth Science 8.4 Earth’s Layered Structure Layer defined by Physical properties: Earth’s interior has gradual increases in temperature, pressure, and density with each increase in depth. When a substance is heated, the transfer of energy increases the vibrations of particles. If the temperature exceeds the melting point, the forces between particles are overcome and melting begins.

Earth Science 8.4 Earth’s Layered Structure Layer defined by Physical properties: Earth can be divided into layers based on the physical properties of each layer: The lithosphere The asthenosphere The lower mantle The outer core The inner core

Earth Science 8.4 Earth’s Layered Structure Layer defined by Physical properties: Lithosphere: Earth’s outermost layer consists of the crust and the uppermost mantle and forms a cool, rigid shell called the lithosphere. This layer averages about 100 kilometers in thickness.

Earth Science 8.4 Earth’s Layered Structure Layer defined by Physical properties: Asthenosphere Beneath the lithosphere lies a soft, comparatively weak layer known as the asthenosphere. Within the asthenosphere, the rocks are close enough to their melting point that they are easily deformed. The asthenosphere is weaker because the rock is near melting point; just like hot wax is weaker than cold wax. The lower lithosphere and asthenosphere are both part of the upper mantle.

Earth Science 8.4 Earth’s Layered Structure Layer defined by Physical properties: Lower Mantle: From a depth of about 660 kilometers down to near the base of mantle lies a more rigid layer called the lower mantle. Despite their increased strength, the rocks of the lower mantle are still very hot and capable of gradual flow.

Earth Science 8.4 Earth’s Layered Structure Layer defined by Physical properties: Inner and Outer Core: The core, which is composed of an iron-nickel alloy, is divided into two regions with different physical properties. The outer core is a liquid layer 2260 kilometers thick. The flow of metallic iron within this zone generates the Earth’s magnetic fields.

Earth Science 8.4 Earth’s Layered Structure Layer defined by Physical properties: Inner and Outer Core: The inner core is a sphere having a radius of about 1220 kilometers. Despite it’s high temperatures, the materials in the inner core are compressed into a solid state by the immense pressure.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Layers: Recall that seismic waves bend as they travel through Earth. During the 20th century, studies of P waves and S waves through earth helped scientists identify the boundaries of earth’s layers and determine that the outer core is liquid.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Layers: In 1909, Croatian seismologist Andrija Mohorovicic first presented evidence proving Earth’s layering. By studying seismic records, he found that the velocity of seismic waves increased abruptly about 50 kilometers below eastern Europe. This boundary separates the crust from the underlying mantle and is called the Mohorovicic discontinuity. The name of this boundary is usually shortened to the Moho.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Layers: Another boundary had been discovered in 1906 between the mantle and the core. Seismic waves from even small earthquakes can travel around the world. That is why a seismograph in Antarctica can record earthquakes that occurred beneath California. It was observed that P waves were bent around the liquid outer core. This zone where the P waves can not reach with direct waves is called the “P wave shadow zone”

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Layers: Where the “shadow zone” exists the P waves still reach but they are indirect waves, bounced once before arriving in the shadow zone.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Layers: Like sound echoing off a wall to reach around a corner; the P waves are refracted (bounced) off the Earth’s crust before they pass into the “shadow zone” . Inside the shadow zone, only refracted P waves can be sensed.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Layers: It was also shown that S waves did not travel through the core at all. From this information, it was concluded by scientists that the outer core was liquid.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Composition: To determine the composition of earth’s layers; scientists studied seismic data, rock samples from the crust and mantle, meteorites, and high pressure experiments on earth materials.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Composition: Scientists obtain data on rocks by performing high-pressure experiments. Small samples of rock and metal are heated and squeezed to the same conditions found deep within Earth’s interior. Seismic data and rock samples from drilling indicate that the continental crust is mostly made of low density granitic rock.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Composition: Until the late 60s, scientists had only seismic evidence to determine the oceanic crust makeup. Today, deep-sea drilling platforms make it possible to obtain rock samples from beneath the ocean floor. The crust beneath the ocean floor has a basalt composition.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Composition: The composition of the rocks of the mantle and core is known from indirect data. Some of the lava that reaches Earth’s surface comes from the partially melted asthenosphere within the mantle. In the laboratory, experiments show that partially melting the rock called peridotite produces something very similar to the lava that erupts from Hawaii.

Earth Science 8.4 Earth’s Layered Structure Discovering Earth’s Composition: Surprisingly, meteorites that collide with Earth provide evidence of earth’s inner composition. Meteorites are assumed to be made of the original material of which the Earth was formed. Their composition ranges from metallic meteorites made of iron and nickel to stony meteorites made of dense rock similar to peridotite. Iron meteorities