Earth's Layers, Volcanoes, Landforms, and External Agents

The Outer Layer of the Earth

The Earth is divided into three main layers:

• The Core: The innermost part, divided into the inner core and outer core.
• The Mantle: The layer surrounding the core. It is the thickest layer.
• The Crust: The outer layer of the Earth. It is very thin. Continents and the seabed are on the crust.

In the crust, we distinguish different forms of relief: mountains, valleys, depressions, and plains.

Tectonic plates are large blocks of land that form the Earth's crust.

• Contact zones between two plates are sites of high seismic activity and give rise to internal movements of the Earth: earthquakes and volcanoes.
• Ridges are underwater mountain ranges that separate two plates.

Agents of Volcanoes

• Crater: Opening at the end of the chimney where materials are expelled.
• Lateral Chimney: Smaller chimney on one side of the volcano.
• Main Chimney: Main duct through which magma rises.
• Volcanic Cone: Part of the volcano formed by the ejected material.
• Lava Flow: Course of materials coming down the volcano's slope.
• Magma Chamber: Pocket that lies inside the Earth, consisting of materials in a liquid state.

Important Concepts:

• Cape: Part of the coast that extends into the sea.
• Gulf: Rounded sea entrance.
• Archipelago: A set of islands that are close together (e.g., Canary Islands).
• Peninsula: Piece of land surrounded by water everywhere except for an isthmus.
• Plain: Flat or very gently sloping land.
• Plateau: More or less flat elevated terrain.
• Valley: Depression in the relief that usually has an elongated configuration.
• Mountain: Natural ground elevation.
• Range: A continuous chain of mountains.
• Depression: Depressed zone between mountains.

External Agents

Water, wind, vegetation, and humans are external agents.

• Rainwater flows through valleys, causing wear and tear, and carries rocks and sand, depositing them to form deltas and plains.
• Rain seeps through cracks in rocks, which can break if the water freezes, or chemically react and dissolve.
• Waves and currents erode the coastline and accumulate sand on beaches and in gulfs.
• Wind is a very important external agent that erodes and weathers rocks.
• Vegetation helps to break rocks or to fix the soil on some slopes.
• Humans transform the territory and, therefore, modify the relief.