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2.1: Introduction to Earth's Surface

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Lesson Objectives

  • Describe how you can find a location and direction on Earth’s surface.
  • Describe topography.
  • Identify various landforms and briefly describe how they form.

Vocabulary

  • compass
  • compass rose
  • constructive forces
  • continent
  • destructive forces
  • elevation
  • relief
  • topography

Introduction

Beautiful mountain ranges, deep canyons, flat plains. These can all be seen on Earth's surface. Beneath the sea are other features that few people have seen directly. Understanding Earth's surface is one of the important things Earth scientists can do. Knowing where they are on the planet is one of the first things they need to describe.

Location

To describe your location wherever you are on Earth’s surface, you could use a coordinate system. For example, you could say that you are at 1234 Main Street, Springfield, Ohio. Or you could use a point of reference. If you want to meet up with a friend, you could tell him the distance and direction you are from the reference point. An example is, “I am at the corner of Maple Street and Main Street, about two blocks north of your apartment.”

When studying Earth’s surface, scientists must be able to pinpoint a feature they are interested in. Scientists and others have a system to describe the location of any feature. Usually they use latitude and longitude as a coordinate system. Lines of latitude and longitude form a grid. The grid is centered on a reference point. You will learn about this type of grid when we discuss maps later in this chapter.

Direction

When an object is moving, it is not enough to describe its location. We also need to know direction. Direction is important for describing moving objects. For example, a wind blows a storm over your school. Where is that storm coming from? Where is it going?

The most common way to describe direction is by using a compass. A compass is a device with a floating needle (Figure below). The needle is a small magnet that aligns itself with the Earth’s magnetic field. The compass needle always points to magnetic north. If you have a compass and you find north, you can then know any other direction. See the directions, such as east, south, west, etc., on a compass rose.

(A) A compass is a device that is used to determine direction. The needle points to Earth’s magnetic north pole. (B) A compass rose shows the four major directions plus intermediates between them.

A compass needle lines up with Earth’s magnetic north pole. This is different from Earth’s geographic north pole, or true north. The geographic north pole is the top of the imaginary axis around which Earth rotates. The geographic north pole is much like the spindle of a spinning top. The location of the geographic north pole does not change. However, the magnetic north pole shifts in location over time. Depending on where you live, you can correct for the difference between the two poles when you use a map and a compass (Figure below).

Earth’s magnetic north pole is about 11 degrees offset from its geographic north pole.

Some maps have a double compass rose. This allows users to make the corrections between magnetic north and true north. An example is a nautical chart that boaters use to chart their positions at sea (Figure below).

Nautical maps include a double compass rose that shows both magnetic directions (inner circle) and geographic compass directions (outer circle).

Topography

As you know, the surface of Earth is not flat. Some places are high and some places are low. For example, mountain ranges like the Sierra Nevada in California or the Andes in South America are high above the surrounding areas. We can describe the topography of a region by measuring the height or depth of that feature relative to sea level (Figure below). You might measure your height relative to your classmates. When your class lines up, some kids make high “mountains,” while others are more like small hills!

Topographical map

Topography of Earth showing North America and South America.

Relief, or terrain, includes all the landforms of a region. A topographic map shows the height, or elevation, of features in an area. This includes mountains, craters, valleys, and rivers. For example, Figure below shows the San Francisco Peaks in northern Arizona. Features on the map include mountains, hills and lava flows. You can recognize these features from the differences in elevation. We will talk about some different landforms in the next section.

San Francisco Mountain and surrounding areas

This image was made from data of the Landsat satellite. It shows the topography of the San Francisco Peaks and surrounding areas.

Continents and Landforms

If you take away the water in the oceans (Figure below), Earth looks really different. You see that the surface has two main features: continents and ocean basins. Continents are large land areas. Ocean basins extend from the edges of continents to the ocean floor and into deep trenches.

This image shows Earth with water removed. The red areas are high elevations (mountains). Yellow and green areas are lower elevations. Blue areas are the lowest on the ocean floor.

Continents are much older than ocean basins. Some rocks on the continents are billions of years old. Ocean basins are only millions of years old at their oldest. Because the continents are so old, a lot has happened to them!

As we view the land around us we see landforms. Landforms are physical features on Earth’s surface. Landforms are introduced in this section but will be discussed more in later chapters. Constructive forces cause landforms to grow. Lava flowing into the ocean can build land outward. A volcano can be a constructive force. Destructive forces may blow landforms apart. A volcano blowing its top off is a destructive force. The destructive forces of weathering and erosion change landforms more slowly. Over millions of years, mountains are worn down by rivers and streams.

Constructive and destructive forces work together to create landforms. Constructive forces create mountains and erosion may wear them away. Mountains are very large landforms. Mountains may wear away into a high flat area called a plateau, or a lower-lying plain. Interior plains are in the middle of continents. Coastal plains are on the edge of a continent, where it meets the ocean.

Features of continents include mountain ranges, plateaus, and plains.

Rivers and streams flow across continents. They cut away at rock, forming river valleys (Figure below). These are destructive forces. The bits and pieces of rock carried by rivers are deposited where rivers meet the oceans. These can form deltas, like the Mississippi River delta. They can also form barrier islands, like Padre Island in Texas. Rivers bring sand to the shore, which forms our beaches. These are constructive forces.

Summary of major landforms on continents and features of coastlines.

Ocean Basins

The ocean basin begins where the ocean meets the land. The continental margin begins at the shore and goes down to the ocean floor. It includes the continental shelf, slope, and rise. The continental shelf is part of the continent, but it is underwater today. It is about 100-200 meters deep, much shallower than the rest of the ocean. The continental shelf usually goes out about 100 to 200 kilometers from the shore (Figure below).

The continental shelf and slope of the southeastern United States goes down to the ocean floor.

The continental slope is the slope that forms the edge of the continent. It is seaward of the continental shelf. In some places, a large pile of sediments brought from rivers creates the continental rise. The continental rise ends at the ocean floor. Much of the ocean floor is called the abyssal plain.

The ocean floor is not totally flat. In many places, small hills rise above the ocean floor. These hills are undersea volcanoes, called seamounts (Figure above). Some rise more than 1000 m above the seafloor.

A chain of seamounts off the coast of New England (left). Oceanographers mapped one of these seamounts, called Bear Seamount, in great detail (right).

Besides seamounts, there are long, very tall (about 2 km) mountain ranges. These ranges are connected so that they form huge ridge systems called mid-ocean ridges (Figure below). The mid-ocean ridges form from volcanic eruptions. Lava from inside Earth breaks through the crust and creates the mountains.

Map of the mid-ocean ridge system (yellow-green) in Earth’s oceans.

The deepest places of the ocean are the ocean trenches. Many trenches line the edges of the Pacific Ocean. The Mariana Trench is the deepest place in the ocean. (Figure below). At about 11 km deep, it is the deepest place on Earth! To compare, the tallest place on Earth, Mount Everest, is less than 9 km tall.

Mariana Trench in the Pacific Ocean

The Mariana Trench is east of Guam in the Pacific Ocean.

Lesson Summary

  • Earth scientists must be able to describe the exact locations of features on Earth’s surface.
  • Locations often include distances and directions.
  • A compass has a tiny magnetic needle that points toward Earth’s magnetic North Pole. Once you have found north, you can find east, west, and south, using your compass for reference.
  • Topography describes how Earth’s surface varies in elevation.
  • Constructive forces create landforms. Destructive forces wear landforms down.

Lesson Review Questions

Recall

1. What information might you need to describe the location of a feature on the Earth’s surface?

2. On the continents, which landforms rise the highest?

3. What is topography?

Apply Concepts

4. Why would you need to know direction if an object is moving?

5. Why do nautical charts have two compass roses on them?

Think Critically

6. Why do you think that the ocean basins are younger than the continents?

7. Explain what landforms on the continents are created by erosion from wind and water. How does erosion create a landform?

Points to Consider

  • A new volcano rises in Mexico. How you would describe its position in a scientific report?
  • Can you devise a system to show low areas and high areas on a map?
  • Why do you think continents are higher areas on Earth than the ocean basins?

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