Internal Structure of the Earth Lesson: Layers and Components

Lesson Overview

• Overview of Earth's Layers
• The Crust: Earth's Outer Skin
• The Mantle: The Largest Layer
• The Core: Earth's Metallic Center
• Discovering Earth's Internal Structure
• Plate Tectonics: Earth's Moving Crust
• Volcanoes: A Window into Earth's Interior

Earth is a layered planet with a structured interior composed of different materials. The internal structure is divided into three main layers: crust, mantle, and core, each differing in composition, physical state, and density. Scientists determine these layers through indirect methods, such as seismic wave behavior, gravitational data, and meteorite studies.

Overview of Earth's Layers

The three primary layers of the Earth are:

• Crust – The outermost solid layer where life exists.
• Mantle – The thickest layer, composed of dense silicate rocks, responsible for convection currents.
• Core – The innermost metallic layer, divided into a liquid outer core and a solid inner core.

Each layer has distinct physical properties and plays a crucial role in Earth's geological and geodynamic activities.

Summary of Earth's Internal Layers

The Crust: Earth's Outer Skin

Composition & Structure

The crust is Earth's outermost layer, forming the continents and ocean floors. It is composed mainly of silicate minerals, including granite (continental crust) and basalt (oceanic crust).

Types of Crust

1. Continental Crust – Thicker (20–70 km), composed mainly of granite, less dense (~2.7 g/cm³).
2. Oceanic Crust – Thinner (5–10 km), composed mainly of basalt, denser (~3.0 g/cm³).

The crust "floats" on the denser mantle below. The boundary between the crust and mantle is called the Mohorovičić Discontinuity (Moho).

Key Facts

• The crust is solid rock formed beneath both land and sea.

• It is the least dense layer of the Earth.

• The crust and the uppermost mantle form the lithosphere, which is broken into tectonic plates.

The Mantle: The Largest Layer

Composition & Density

The mantle makes up about 84% of Earth's volume and extends from the Moho to about 2,900 km depth. It is composed of iron- and magnesium-rich silicate minerals like peridotite.

Key Properties

• State: Solid but capable of slow plastic flow (over millions of years).

• Density: Ranges from 3.3 g/cm³ (upper mantle) to 5.6 g/cm³ (lower mantle).

• Convection Currents: The mantle moves via convection, where hot material rises and cool material sinks, driving plate tectonics.

Layers of the Mantle

• Upper Mantle – Includes the asthenosphere, a weak, partially molten layer that allows tectonic plates to move.

• Lower Mantle – More rigid due to high pressure but still flows slowly.

Key Facts

• The mantle is denser than the crust.
• The asthenosphere is where plate motion occurs.
• Mantle rock slowly circulates, driving tectonic activity.

The Core: Earth's Metallic Center

Composition & Structure

The core is the densest and hottest part of Earth, mainly composed of iron and nickel. It is divided into:

• Outer Core – A liquid layer where flowing iron generates Earth's magnetic field.

• Inner Core – A solid sphere due to extreme pressure despite high temperatures.

Core Characteristics

Key Facts

• The core is responsible for Earth's magnetic field.
• The inner core is the densest layer.
• The outer core is liquid, allowing convection currents.

Discovering Earth's Internal Structure

Since no one has directly seen the Earth's interior, scientists rely on indirect methods such as:

• Seismic Waves – Earthquake-generated waves change speed and direction, revealing layers.
• Density & Gravity Studies – Earth's overall density suggests a metallic core.
• Magnetic Field – A convecting liquid outer core produces Earth's magnetic field.
• Meteorite Studies – Iron meteorites provide clues about Earth's core composition.

Plate Tectonics: Earth's Moving Crust

What Are Tectonic Plates?

Tectonic plates are rigid sections of the lithosphere that move slowly over the asthenosphere. There are seven major plates, including the Pacific, Eurasian, and African Plates.

Types of Plate Boundaries

1. Divergent Boundaries – Plates move apart (e.g., Mid-Atlantic Ridge).
2. Convergent Boundaries – Plates collide, forming mountains or subduction zones (e.g., Himalayas).
3. Transform Boundaries – Plates slide past each other (e.g., San Andreas Fault).

Key Facts

• Earthquakes and volcanoes occur mainly at plate boundaries.
• Plate movements are driven by mantle convection.

Volcanoes: A Window into Earth's Interior

How Do Volcanoes Form?

Magma from the mantle rises through the crust and erupts as lava. Most volcanoes form at:

• Subduction Zones – Magma forms as an oceanic plate sinks beneath another.
• Divergent Boundaries – Magma rises at spreading centers.
• Hotspots – Plumes of hot mantle material create volcanoes like Hawaii.

Key Facts

• Magma comes from the mantle, not the core.
• Most volcanoes occur along tectonic plate boundaries.
• Volcanic eruptions release gases, lava, and ash, shaping Earth's surface.

By understanding Earth's internal structure, students can grasp why earthquakes, volcanic eruptions, and plate movements occur. This knowledge provides a foundation for geology, seismology, and Earth sciences.