Let's explore each of these in turn. Decompression melting occurs at mid-ocean ridges. When two plates move apart, they create a space that can be filled by hot rock that rises buoyantly from below. As long as this hot rock rises faster than the temperature can cool off, the rock can melt because the pressure is decreasing as the rock gets closer to the surface. See pencast sketch of decompression melting at a midocean ridge! Let's visualize what decompression melting looks like as a plot in Pressure-Temperature space!
You can construct Pressure-Temperature plots to show melting curves for all kinds of substances, not just lava at a mid-ocean ridge. For example, the plot below shows data for table salt. Note that these scientists put temperature on the y axis, and pressure on the x axis. We did it the other way around and had pressure increasing downwards on the y axis because we wanted pressure to be analogous to depth in the Earth in our plot.
Rocks melt at a lower temperature in the presence of volatiles such as water and carbon dioxide. How do you get water underneath a volcano? The most common way to do it is to send it down a subduction zone. When a subducting plate sinks under the overriding plate, the water-saturated upper part of the lithosphere goes down, too. As the cold slab sinks, water is forced out and percolates upward into the overlaying hot, dry mantle rock.
This sudden addition of water lowers the melting point of that mantle rock, and it begins to melt. For example, sedimentary rock shale becomes slate when heat and pressure are added. The more heat and pressure you add, the further the rock metamorphoses until it becomes gneiss. If it is heated further, the rock will melt completely and reform as an igneous rock. Empower your students to learn about the rock cycle with this collection of resources.
According to the United States Geologic Survey, there are approximately 1, potentially active volcanoes worldwide.
Most are located around the Pacific Ocean in what is commonly called the Ring of Fire. A volcano is defined as an opening in the Earth's crust through which lava, ash, and gases erupt. The term also includes the cone-shaped landform built by repeated eruptions over time. Teach your students about volcanoes with this collection of engaging material. The structure of the earth is divided into four major components: the crust, the mantle, the outer core, and the inner core.
Each layer has a unique chemical composition, physical state, and can impact life on Earth's surface. Movement in the mantle caused by variations in heat from the core, cause the plates to shift, which can cause earthquakes and volcanic eruptions.
These natural hazards then change our landscape, and in some cases, threaten lives and property. Learn more about how the earth is constructed with these classroom resources. Igneous rocks are one of three main types of rocks along with sedimentary and metamorphic , and they include both intrusive and extrusive rocks.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Article Vocabulary. Friday, October 31, Magma is a molten and semi-molten rock mixture found under the surface of the Earth. This mixture is usually made up of four parts: a hot liquid base, called the melt ; mineral s crystal lized by the melt; solid rock s incorporate d into the melt from the surrounding confine s; and dissolve d gas es.
When magma is eject ed by a volcano or other vent , the material is called lava. Magma that has cooled into a solid is called igneous rock. This heat makes magma a very fluid and dynamic substance, able to create new landform s and engage physical and chemical transform ations in a variety of different environment s.
Earth is divided into three general layers. The core is the superheated center, the mantle is the thick, middle layer, and the crust is the top layer on which we live. Most of the mantle and crust are solid, so the presence of magma is crucial to understanding the geology and morphology of the mantle. Differences in temperature , pressure , and structural formations in the mantle and crust cause magma to form in different ways. Decompression melting involves the upward movement of Earth's mostly-solid mantle.
This hot material rises to an area of lower pressure through the process of convection. Areas of lower pressure always have a lower melting point than areas of high pressure. This reduction in overlying pressure, or decompression, enables the mantle rock to melt and form magma. Decompression melting often occurs at divergent boundaries, where tectonic plate s separate.
The rift ing movement causes the buoyant magma below to rise and fill the space of lower pressure. The rock then cools into new crust. When located beneath the ocean, these plumes, also known as hot spot s, push magma onto the seafloor. These volcanic mounds can grow into volcanic island s over millions of years of activity. As the liquid rock solidifies, it loses its heat to the surrounding crust.
Transfer of heat often happens at convergent boundaries, where tectonic plates are crashing together. As the dense r tectonic plate subduct s, or sinks below, or the less-dense tectonic plate, hot rock from below can intrude into the cooler plate above. This process transfers heat and creates magma. Over millions of years, the magma in this subduction zone can create a series of active volcanoes known as a volcanic arc.
Flux melting occurs when water or carbon dioxide are added to rock. These compounds cause the rock to melt at lower temperatures.
This creates magma in places where it originally maintained a solid structure. For instance, as two tectonic plates collide, one plate may get forced under the other plate. As it does so, the plate that is forced down subducted releases water into the upper mantle which lowers the pressure enough to melt the rock. Localized regions of magma form in the mantle near subduction zones.
The mantle can then rise and create volcanoes. The point is that magma is created in small pockets small relative to the size of the earth as part of the tectonic plate movement, and does not exist as a global sea of magma just under the crust. The confusion about the state of the upper mantle perhaps arises from the way diagrams are presented.
For instance, the image above shows the mantle in a glowing orange color. This coloring can be confused to mean that this layer is hot liquid rock, like lava.
0コメント