Earth Materials

Science background provides teachers with more in-depth information on the phenomena students explore in this unit. Below is an excerpt of the science background for teachers from this lesson on Earth materials. 

Earth’s Structure

All rocks on Earth’s surface experience weathering and erosion. A rock is a mixture of minerals that heat and pressure have pressed together. Minerals are inorganic solids that form naturally from elements and compounds. But rocks aren’t just found on Earth’s surface.

If you sliced Earth in half, you would see four layers based on the density of each layer. The inner core is Earth’s hottest layer. It is made up of a mixture of solid iron and nickel metal. It is surrounded by an outer core that is made up of a less dense mixture of liquid iron and nickel metal.

Surrounding the inner and outer core is the mantle, which is mostly molten, semi- solid rock called magma. The mantle makes up almost two thirds of the Earth's mass (around 83 percent) and is about 2,900 km (1,802 miles) thick. Earth’s crust is the hard rock layer of the planet that makes up the continents and holds the oceans. The crust is Earth’s thinnest and coolest layer. The crust is part of Earth’s outermost 100 kilometers called the lithosphere. The lithosphere also includes the part of the upper mantle, where the rocks are cooler and stronger than the rest of the mantle. The lithosphere is broken into slabs of solid rock called tectonic plates, and they are constantly moving and interacting to create many of the landforms of the planet. The lithosphere floats in many pieces on a zone of weak, melted rock called the asthenosphere.

The tectonic plates move because the mantle is in constant motion. Because of Earth’s structure, heat is continuously flowing from Earth’s center outward. Remember that heat is energy that has transferred whenever two substances are at different temperatures, and it always flows from faster- moving atoms (a higher temperature) to slower-moving atoms (a cooler temperature) until both substances reach the same temperature.

Convection in the Mantle

Heat from Earth’s core warms the lower part of the mantle through conduction (heat transfer that occurs when molecules collide). When molecules of the mantle near the core are heated, their particles move faster.

As the particles move faster, they become more spread out and less dense than the cooler, upper mantle rocks, whose particles are much slower. The cooler particles clump together, becoming denser. The warmer, less-dense rocks rise while the cooler, denser rocks sink, creating slow currents within the mantle. This motion describes a form of heat transfer called convection. Convection is the same process that powers deep ocean currents and wind.

Plate Boundaries

As the mantle moves because of convection, the tectonic plates on top of the mantle also move. As they move, they interact with each other in different ways. Scientists continue to study the complex behaviors of the plates, which sometimes crash together, pull apart, or sideswipe each other over time. There are three primary ways that plates can interact:

• When two plates move toward one another perpendicular to the fault line, they form a convergent boundary. When an oceanic plate converges on a continental plate, the oceanic plate sinks under the continental plate because it is denser. This is called subduction— the process that occurs when two plates crash into each other, and the edge of the heavier plate slides beneath the lighter one. The heavier crust melts back into the Earth's mantle, becoming magma. As it melts, some of it returns to the surface as volcanoes and lava flows, creating volcanic mountain ranges. When two continental plates converge on one another, neither is subducted because both plates are relatively light. Instead, the force is so powerful that the land is uplifted and mountain ranges form.
• When two plates move away from each other perpendicular to the fault line, they form a divergent boundary. This creates ocean ridges and rift valleys, and may result in earthquakes and magma welling up onto the surface. Earthquakes last less than a minute, but the moving plates can shift Earth’s surface up to 18 meters in any direction in that time.
• Sometimes plates slide past one another. This happens at transform plate boundaries, and it results in the plates grinding along their sides as they go. Earthquakes can occur along these boundaries.