2.1+Earth's+Surface

If you’ve seen the Grand Canyon or a photo of the Grand Canyon, you’ll know that this is not the Grand Canyon. Or is it? The rock colors are not the way they look to a person standing at the rim. From the rim, the Grand Canyon is mostly red with the prominent white stripe of the Coconino Sandstone near the top. The cliffs are are more angular than the cliffs pictured here. NASA produced this image using data from the Advanced Spaceborne Thermal Emissions and Reflection Radiometer (ASTER), one of five remote sensing devices aboard the Terra spacecraft. ASTER measures 14 different wavelengths of the electromagnetic spectrum, ranging from visible to infrared light, to give information on land surface temperature, reflectance, and elevation. The resolution is between 15 and 90 meters. So, is this the Grand Canyon? Any image or map of the Earth’s surface is just a representation. All maps show the data the map maker intended at the best level of accuracy possible. Each representation is valuable in its own way but has limitations. The only real Grand Canyon is the one you are looking at from the rim of the canyon. Or is that image altered by the tint of your sunglasses?

Lesson Objectives

 * Briefly identify different features of continents and ocean basins.
 * Define constructive forces and give a few examples.
 * Define destructive forces and give a few examples.

Introduction
Earth’s surface features are the result of constructive and destructive forces. Constructive forces cause landforms to grow. The eruption of a new volcano creates a new landform. Destructive forces wear landforms down. The slow processes of mechanical and chemical weathering and erosion work over time to change once high mountains into smooth flat plateaus.

Earth’s Features
The **Figure** [|below] is a slice through a relief map of Earth’s surface without the water in the oceans. What are its two most distinctive features?
 * The **continents** are large land areas extending from high mountain tops to sea level.
 * The **ocean basins** extend from the edges of the continents down steep slopes to the ocean floor and into deep trenches.

In this figure, color indicates elevation. Red represents the highest mountains with orange, yellow, and green indicating lower elevations. Light blue to darker blue to bluish-purple descends to the deepest ocean floor.

Continents
The oldest continental rocks are billions of years old, so the continents have had a lot of time for things to happen to them. **Constructive forces** cause physical features on Earth’s surface known as **landforms** to grow. Crustal deformation – when crust compresses, pulls apart or slides past other crust – results in hills, valleys, and other landforms. Mountains rise when continents collide or when one slab of ocean crust plunges beneath another or a slab of continental crust creates 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 important forces of erosion (**Figure** [|below]).

Landforms in this radar image of the Crater Highlands of Tanzania are accentuated by 2x vertical relief and also by color. The highest elevations are white and lowest elevations are green. Image from NASA/JPL/NGA Shuttle Radar Topography team.

Look for constructive and destructive landforms in the image above. This scene is within the East African Rift where crust is being pulled apart to form a large valley. Which features result from constructive forces? Volcanoes have been constructed within the valley by rising magma. Which features result from destructive forces? Volcanic explosions or collapses have destroyed volcanic mountains to form craters. Fractures caused by the rifting in the valley are breaking apart features. Streams are eroding downward into the slopes of the volcanoes. Landslides erode the steep volcanoes. A landslide scar is seen on left side of the small, very steep volcanic cone and landslide deposits have traveled outward from the scar.

Ocean 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 totally flat, as illustrated in the **Figure** [|below]. The most distinctive feature is the mountain range that runs through much of the ocean basin, known as the **mid-ocean ridge**. The deepest places of the ocean are the **ocean trenches**, many of which are located around the edge of the Pacific Ocean. Chains of volcanoes are also found in the center of the oceans, such as in the area of Hawaii. Flat plains are found on the ocean floor with their features covered by mud.

Major features of the world

Changing Earth
Earth’s surface changes over short and long periods of time. Constructive forces cause new features to form by volcanic activity or uplift of the crust. Existing landforms are modified by destructive forces, perhaps even eroded away by water, wind, ice, and gravity. Beneath the oceans, volcanic activity forms new seafloor while old seafloor is destroyed at the trenches. You will explore many ways that the Earth’s surface changes as you proceed through this book.

Lesson Summary

 * For the most part, continents are much older than ocean basins.
 * Both the continents and ocean basins are covered by many types of landforms, including mountains and flat plains.
 * Constructive forces cause landforms to grow.
 * Destructive forces modify or even destroy landforms.
 * Earth’s surface is constantly changing. Change can happen rapidly, as when a volcano blows itself apart, or slowly, as in the grain by grain erosion of a stream into a canyon.

Review Questions

 * 1) What are constructive forces and what landforms do they create?
 * 2) What are destructive forces and what landforms do they create?
 * 3) In a single region, are only constructive or only destructive forces at work?
 * 4) In terms of Earth’s surface, what is the only thing that is constant?
 * 5) What are some of the landforms found in the ocean basins?
 * 6) Until recently, scientists thought the seafloor was just flat and muddy. Why do you think they thought this? What do they think now?

Further Reading / Supplemental Links

 * Current ocean research with videos and explanations is found at [|http://oceanexplorer.noaa.gov/.]

Vocabulary
ocean trench The deepest parts of the ocean basins. ocean basin Areas covered by ocean water. The three major ocean basins are the Pacific, Atlantic, and Indian. mid-ocean ridge A large, continuous mountain range found within an ocean basin. landform A physical feature that is part of the landscape, such as a hill or peninsula. destructive forces Forces that modify or destroy landforms. Agents of erosion include water, wind, ice, and gravity. continental margin Submerged, outer edge of the continent. continent Land mass above sea level. constructive forces Forces that cause landforms to grow. Crustal deformation and volcanic eruptions are two examples.

Points to Consider

 * If erosion is constantly eating away at landforms, why isn’t Earth’s land surface completely flat?
 * Why do you think some regions of some continents, such as the middle part of the United States, are almost entirely flat?
 * Why are continents higher than ocean basins?