Metamorphic rock formation happens beneath the Earth’s surface where intense heat, pressure, and chemically active fluids transform existing rock types. At rockscapes.net, we want to help you understand the geological forces behind the creation of metamorphic rocks, their unique properties, and how you can use them to make beautiful and enduring landscapes. Learn how landscape architects and homeowners are utilizing these transformed stones to enhance their outdoor spaces and explore design ideas.
1. What Defines Metamorphic Rock Formation?
Metamorphic rock formation involves significant changes to pre-existing rocks, which include igneous rocks, sedimentary rocks, or even earlier metamorphic rocks. Rather than melting the rocks, the process of metamorphism alters them into denser, more compact forms. New minerals emerge through the reorganization of existing mineral components or reactions with fluids introduced into the rocks. Pressure and temperature fluctuations can further transform previously metamorphosed rocks into new variants.
1.1. How Does Metamorphism Transform Rocks?
Metamorphism changes rocks through intense heat, pressure, and chemically active fluids, without melting them.
The conditions that facilitate metamorphic rock formation are typically found deep within the Earth’s crust or at the convergence of tectonic plates. According to research from Arizona State University’s School of Earth and Space Exploration, the metamorphic process fundamentally alters the mineral composition and texture of rocks, leading to the creation of unique geological structures.
1.2. What Are the Key Conditions for Metamorphic Rock Formation?
Metamorphic rocks need high heat, intense pressure, and reactive chemical fluids, or a combination of these. These conditions, often found deep underground or where tectonic plates meet, change the rock structure without melting it.
2. What Are the Primary Types of Metamorphism?
There are two main types of metamorphism: regional and contact. Regional metamorphism occurs over large areas, typically associated with mountain building, where both high temperature and pressure play key roles. Contact metamorphism, conversely, occurs when magma intrudes into existing rock, causing changes primarily due to heat.
2.1. What Is Regional Metamorphism?
Regional metamorphism is large-scale and related to mountain building, featuring high temperature and pressure which leads to the creation of metamorphic rocks over extensive areas. The immense pressures and temperatures associated with tectonic movements cause widespread alterations in the rock’s mineralogy and texture.
2.2. How Does Contact Metamorphism Differ?
Contact metamorphism happens when magma heats nearby rock, causing changes mainly from the heat. The intensity of the metamorphism decreases with distance from the heat source, resulting in varied metamorphic zones around the intrusion.
2.3. What Role Do Fluids Play in Metamorphism?
Fluids, particularly water containing dissolved ions, act as catalysts that accelerate metamorphic reactions. These fluids facilitate the transport of ions, allowing minerals to react and recrystallize more rapidly. Hydrothermal metamorphism is a specific type where hot, chemically active fluids are the dominant factor.
3. What Are Common Examples of Metamorphic Rocks?
Common metamorphic rocks include phyllite, schist, gneiss, quartzite, and marble, each distinguished by its unique texture and mineral composition. These rocks are widely used in construction and landscaping due to their durability and aesthetic qualities.
3.1. What Makes Marble a Metamorphic Rock?
Marble forms when limestone is subjected to high temperature and pressure. This metamorphism results in a recrystallization of the original carbonate minerals, leading to a dense, durable rock prized for its beauty and workability. Marble is extensively used in sculptures, buildings, and decorative elements.
3.2. How Is Quartzite Formed?
Quartzite is created from sandstone through metamorphism. The process involves the recrystallization of quartz grains, resulting in a very hard and resistant rock. Because of its hardness and resistance to weathering, quartzite is ideal for paving, retaining walls, and other landscape applications.
3.3. What Distinguishes Gneiss From Other Metamorphic Rocks?
Gneiss is characterized by its banded appearance, which results from the segregation of minerals into distinct layers during metamorphism. High temperature and pressure cause the minerals to align, creating the characteristic striped pattern. Gneiss is often used in construction for its strength and unique visual texture.
3.4. What Are the Characteristics of Schist?
Schist is defined by its foliated texture, where platy minerals like mica are aligned, giving the rock a layered appearance. This foliation makes schist relatively easy to split into sheets. Because of its decorative appearance, schist is frequently used in landscaping and wall cladding.
4. What Is Foliation in Metamorphic Rocks?
Foliation in metamorphic rocks refers to the parallel alignment of minerals, which gives the rock a layered or banded appearance. This occurs when pressure squeezes flat or elongate minerals within a rock, causing them to align perpendicular to the direction of pressure.
4.1. How Does Pressure Affect Foliation?
Pressure is the primary driver of foliation. When a rock is subjected to directed pressure, minerals realign to minimize stress, with platy minerals aligning perpendicular to the direction of greatest pressure. This alignment results in the distinctive layered appearance of foliated rocks.
4.2. What Types of Rocks Exhibit Foliation?
Rocks like slate, phyllite, schist, and gneiss commonly exhibit foliation. Each rock type represents a different grade of metamorphism, with gneiss showing the most distinct banding due to the complete segregation of minerals into layers.
4.3. How Is Foliation Used in Identifying Rocks?
Foliation is a key characteristic used to identify and classify metamorphic rocks. The degree and type of foliation can indicate the intensity of metamorphism and the original composition of the parent rock. Geologists use foliation to understand the metamorphic history of a region.
5. What Are Non-Foliated Metamorphic Rocks?
Non-foliated metamorphic rocks lack a layered or banded appearance. This can occur because the original rock was composed of minerals that do not align easily, or because the metamorphism occurred without significant directed pressure, such as in contact metamorphism.
5.1. What Conditions Create Non-Foliated Rocks?
Non-foliated rocks form under conditions where pressure is uniform or absent, such as in contact metamorphism around igneous intrusions. Additionally, if the original rock is composed of equidimensional minerals like quartz or calcite, foliation will not develop.
5.2. Examples of Non-Foliated Rocks
Common examples of non-foliated rocks include marble and quartzite. Marble is metamorphosed limestone, and quartzite is metamorphosed sandstone. Both rocks are composed of minerals that recrystallize into a dense, interlocking mosaic without a preferred orientation.
5.3. How Do Non-Foliated Rocks Differ in Composition?
Non-foliated rocks typically consist of minerals that are not platy or elongate. For example, marble is primarily composed of calcite, and quartzite is composed of quartz. These minerals recrystallize during metamorphism but do not align to form layers, resulting in a uniform texture.
6. Where Can Metamorphic Rocks Be Found Geographically?
Metamorphic rocks are found worldwide, often in regions that have experienced significant tectonic activity, such as mountain ranges. Areas like the Appalachian Mountains in North America, the Alps in Europe, and the Himalayas in Asia are rich in metamorphic rocks.
6.1. Which Regions Are Known for Metamorphic Rock Formations?
Regions with a history of mountain-building, like the Himalayas, the Alps, and the Appalachians, are known for abundant metamorphic rock formations. These areas experienced the high pressures and temperatures needed for regional metamorphism.
6.2. How Does Local Geology Influence Metamorphic Rock Types?
Local geology significantly influences the types of metamorphic rocks found in a region. The composition of the original rocks, combined with the specific conditions of metamorphism, determines the resulting metamorphic rock type. For instance, a region with abundant shale may produce slate and phyllite, while a region with limestone may produce marble.
6.3. What Role Do Tectonic Plates Play?
Tectonic plates play a crucial role in the formation of metamorphic rocks. The convergence of tectonic plates generates the high pressures and temperatures needed for regional metamorphism. Subduction zones, where one plate slides beneath another, are particularly active areas for metamorphic rock formation.
7. How Are Metamorphic Rocks Used in Landscaping?
Metamorphic rocks are popular in landscaping due to their durability, unique textures, and aesthetic appeal. They are used for various purposes, including retaining walls, pathways, decorative accents, and water features.
7.1. What Types of Metamorphic Rocks Are Best for Landscaping?
Gneiss, quartzite, and slate are great for landscaping, because they are strong, and visually appealing. Gneiss is used for retaining walls and pathways, quartzite for decorative gravel and stone, and slate for paving and wall cladding.
7.2. How Can Metamorphic Rocks Enhance Garden Design?
Metamorphic rocks add natural beauty and texture to garden designs. Their varied colors and patterns provide visual interest, while their durability ensures long-lasting performance. Rocks can be arranged to create focal points, define garden beds, and provide habitat for plants and wildlife.
7.3. What Are Some Creative Landscaping Ideas With Metamorphic Rocks?
Creative landscaping ideas include using metamorphic rocks to build rock gardens, construct dry creek beds, and create naturalistic water features. Large boulders can serve as focal points, while smaller stones can be used as mulch or edging. Slate can be used to create elegant pathways and patios.
8. What Are the Benefits of Using Metamorphic Rocks?
The benefits of using metamorphic rocks include their durability, aesthetic appeal, and low maintenance requirements. These rocks withstand weathering, resist erosion, and provide long-lasting beauty to landscapes.
8.1. How Durable Are Metamorphic Rocks in Outdoor Settings?
Metamorphic rocks are highly durable in outdoor settings, thanks to their dense, interlocking mineral structure. They resist weathering from rain, wind, and temperature changes, making them ideal for use in retaining walls, pathways, and other landscape features.
8.2. How Do Metamorphic Rocks Contribute to Aesthetic Value?
Metamorphic rocks add aesthetic value to landscapes by providing natural colors, textures, and patterns. Their unique appearance enhances the visual appeal of gardens, pathways, and water features, creating a sense of timeless beauty.
8.3. What Maintenance Do Metamorphic Rocks Require?
Metamorphic rocks require very little maintenance. They do not decompose or fade, and they naturally resist pests and diseases. Occasional cleaning with water may be needed to remove dirt or algae, but overall, metamorphic rocks provide a low-maintenance landscaping solution.
9. How Do Experts Study Metamorphic Rock Formations?
Experts study metamorphic rock formations through field observations, laboratory analysis, and computer modeling. Field observations involve mapping rock outcrops, collecting samples, and documenting geological structures.
9.1. What Techniques Are Used to Analyze Metamorphic Rocks?
Techniques for analyzing metamorphic rocks include petrographic microscopy, X-ray diffraction, and geochemical analysis. Petrographic microscopy involves studying thin sections of rocks under a microscope to identify minerals and textures. X-ray diffraction is used to determine the mineral composition of rocks, and geochemical analysis provides information about the chemical elements present in the rocks.
9.2. How Does Petrographic Microscopy Aid in Understanding?
Petrographic microscopy is essential for understanding the mineralogy and texture of metamorphic rocks. By examining thin sections under a microscope, geologists can identify the minerals present, their size, shape, and arrangement, and any evidence of deformation or alteration. This information helps to reconstruct the metamorphic history of the rock.
9.3. What Role Does Geochemical Analysis Play?
Geochemical analysis plays a vital role in understanding the origin and evolution of metamorphic rocks. By measuring the concentrations of various elements in the rock, geologists can determine the composition of the original rock, the conditions of metamorphism, and the source of any fluids involved in the process.
10. What Are the Latest Trends in Metamorphic Rock Use?
Latest trends in metamorphic rock use include incorporating locally sourced stones, using rocks in sustainable landscaping designs, and creating naturalistic, low-maintenance landscapes. Landscape architects and homeowners are increasingly seeking ways to use metamorphic rocks in environmentally friendly ways.
10.1. How Are Locally Sourced Rocks Becoming More Popular?
Locally sourced rocks are becoming more popular as people seek to reduce the environmental impact of their landscaping projects. Using rocks from nearby quarries reduces transportation costs and emissions, supports local economies, and helps to preserve the unique geological character of a region.
10.2. What Is Sustainable Landscaping With Metamorphic Rocks?
Sustainable landscaping with metamorphic rocks involves using rocks in ways that minimize environmental impact and promote ecological health. This includes using permeable paving materials to reduce runoff, creating habitats for wildlife, and selecting rocks that are durable and require little maintenance.
10.3. How Are Designers Creating Naturalistic Landscapes?
Designers are creating naturalistic landscapes by using metamorphic rocks to mimic natural rock formations and ecosystems. This involves arranging rocks in ways that resemble outcrops, creating dry creek beds, and incorporating native plants that thrive in rocky environments.
11. What Factors Influence the Choice of Metamorphic Rock?
The choice of metamorphic rock for a project depends on several factors, including the desired aesthetic, the intended use, the local climate, and the availability of materials. Each type of metamorphic rock has unique properties that make it suitable for different applications.
11.1. How Does Aesthetic Preference Impact Rock Selection?
Aesthetic preference is a major factor in rock selection. Some people prefer the bold patterns of gneiss, while others prefer the subtle colors of slate or the uniform texture of quartzite. The choice depends on the overall design style and the desired look and feel of the landscape.
11.2. What Role Does Climate Play in Choosing Rocks?
Climate plays a crucial role in choosing rocks for landscaping. In areas with freeze-thaw cycles, it is important to select rocks that are resistant to cracking and spalling. Porous rocks may absorb water, which can freeze and expand, causing the rock to deteriorate over time.
11.3. How Does Availability Affect Metamorphic Rock Choice?
The availability of metamorphic rocks can significantly affect the choice of materials. Locally sourced rocks are often more affordable and environmentally friendly than rocks that must be transported long distances. Additionally, the local geology may limit the types of rocks that are readily available.
12. What Is the Environmental Impact of Quarrying Metamorphic Rocks?
Quarrying metamorphic rocks can have significant environmental impacts, including habitat destruction, soil erosion, and water pollution. Sustainable quarrying practices aim to minimize these impacts through careful planning, responsible extraction methods, and reclamation of disturbed areas.
12.1. How Does Quarrying Affect Local Ecosystems?
Quarrying can destroy habitats, disrupt wildlife, and alter natural drainage patterns. The removal of vegetation and topsoil can lead to soil erosion and sedimentation of nearby streams. Sustainable quarrying practices include minimizing the footprint of the quarry, preserving topsoil for reclamation, and restoring disturbed areas with native plants.
12.2. What Are Sustainable Quarrying Practices?
Sustainable quarrying practices include using efficient extraction methods, minimizing waste, and reclaiming disturbed areas. This involves careful planning, responsible management of water resources, and restoration of habitats with native plants.
12.3. How Can Consumers Support Eco-Friendly Rock Use?
Consumers can support eco-friendly rock use by choosing locally sourced materials, selecting rocks from quarries that adhere to sustainable practices, and using rocks in ways that minimize environmental impact. This includes using permeable paving materials, creating habitats for wildlife, and reducing the use of synthetic materials.
At rockscapes.net, we are committed to providing information and resources that promote the responsible and sustainable use of metamorphic rocks in landscaping.
13. What Are Some Myths About Metamorphic Rocks?
Common myths about metamorphic rocks include the idea that they are all very old and that they are all formed in extreme conditions. While many metamorphic rocks are indeed ancient, metamorphism can occur at various times in Earth’s history.
13.1. Do All Metamorphic Rocks Form Under Extreme Conditions?
Not all metamorphic rocks form under extreme conditions. While high temperature and pressure are important factors, metamorphism can also occur at relatively low temperatures and pressures, especially in the presence of chemically active fluids.
13.2. Are Metamorphic Rocks Always Banded?
Not all metamorphic rocks are banded. Foliation, the alignment of minerals that creates banding, is only present in rocks that have been subjected to directed pressure. Non-foliated rocks, such as marble and quartzite, lack this layered appearance.
13.3. Can Metamorphic Rocks Revert to Igneous or Sedimentary?
Metamorphic rocks do not revert back to igneous or sedimentary rocks, but they can become the parent material for these rock types. For example, metamorphic rocks can be uplifted and eroded to form sediments, which can then be lithified into sedimentary rocks. Alternatively, metamorphic rocks can be subducted and melted to form magma, which can then cool and solidify into igneous rocks.
14. How Do Metamorphic Rocks Contribute to Soil Formation?
Metamorphic rocks contribute to soil formation through weathering and erosion. As rocks break down into smaller particles, they release minerals that enrich the soil and provide nutrients for plants.
14.1. What Minerals Do Metamorphic Rocks Contribute to Soil?
Metamorphic rocks contribute a variety of minerals to the soil, including quartz, feldspar, mica, and various clay minerals. These minerals provide essential nutrients for plant growth and influence the physical and chemical properties of the soil.
14.2. How Does Rock Composition Affect Soil Fertility?
The composition of the parent rock significantly affects soil fertility. Rocks that are rich in essential nutrients, such as phosphorus, potassium, and calcium, produce soils that are more fertile and better able to support plant growth.
14.3. How Does Weathering Influence Soil Structure?
Weathering influences soil structure by breaking down rocks into smaller particles, which then combine with organic matter to form aggregates. These aggregates improve soil porosity, aeration, and water-holding capacity, creating a more favorable environment for plant roots.
15. How Are Metamorphic Rocks Important in Construction?
Metamorphic rocks are important in construction due to their strength, durability, and aesthetic appeal. They are used for a variety of purposes, including building foundations, walls, paving, and roofing.
15.1. Which Metamorphic Rocks Are Used in Building Foundations?
Gneiss and quartzite are used for building foundations, because they are very strong, and resist weathering. Their high compressive strength and resistance to water absorption make them ideal for supporting heavy loads and withstanding harsh environmental conditions.
15.2. How Is Slate Used in Roofing?
Slate is used in roofing due to its durability, water resistance, and aesthetic appeal. Because of its ability to be split into thin, flat sheets, slate is a long-lasting and attractive roofing material.
15.3. How Does Marble Enhance Interior Design?
Marble enhances interior design with its beauty, texture, and versatility. It is used for countertops, flooring, wall cladding, and decorative elements, adding a touch of luxury and elegance to any space.
16. Are There Any Famous Landmarks Made of Metamorphic Rock?
Yes, several famous landmarks are made of metamorphic rock, including the Washington Monument (marble), the Lincoln Memorial (marble), and many buildings in the Alps (gneiss and schist).
16.1. Why Was Marble Chosen for the Washington Monument?
Marble was chosen for the Washington Monument because of its beauty, durability, and symbolic significance. The white marble symbolizes purity and strength, reflecting the values of the nation’s founders.
16.2. How Does Gneiss Contribute to Alpine Architecture?
Gneiss contributes to Alpine architecture with its strength, durability, and unique appearance. The local gneiss is used to build houses, churches, and other structures, creating a distinctive regional style.
16.3. What Other Structures Use Metamorphic Rocks Prominently?
Other structures using metamorphic rocks prominently include the Parthenon in Athens (marble), the Taj Mahal in India (marble), and numerous government buildings and monuments around the world.
17. How Can You Identify Metamorphic Rocks?
You can identify metamorphic rocks by observing their texture, mineral composition, and geological context. Foliated rocks have a layered appearance, while non-foliated rocks have a uniform texture.
17.1. What Tools Are Needed for Rock Identification?
Tools for rock identification include a hand lens, a streak plate, a hardness scale, and a field guide. A hand lens is used to examine the texture and mineral composition of rocks, a streak plate is used to determine the color of a mineral’s powder, and a hardness scale is used to assess a mineral’s resistance to scratching.
17.2. How Does Texture Aid in Identification?
Texture aids in identification by revealing the arrangement of minerals within the rock. Foliated rocks have a layered texture, while non-foliated rocks have a uniform texture. The size, shape, and orientation of the mineral grains can provide clues about the rock’s origin and metamorphic history.
17.3. What Role Does Color Play in Rock Identification?
Color plays a role in rock identification, but it is not always a reliable indicator. The color of a rock can be influenced by the presence of minor impurities or weathering, so it is important to consider other factors, such as texture and mineral composition, when identifying rocks.
18. What Are Some Research Institutions Studying Metamorphism?
Research institutions studying metamorphism include Arizona State University’s School of Earth and Space Exploration, the U.S. Geological Survey, and various universities with strong geology programs.
18.1. What Kind of Research Is Conducted at These Institutions?
Research conducted at these institutions includes field studies of metamorphic terranes, laboratory experiments on mineral reactions, and computer modeling of metamorphic processes. Scientists at these institutions are working to better understand the mechanisms of metamorphism, the conditions under which it occurs, and its role in the Earth’s evolution.
18.2. How Do These Studies Benefit the Public?
These studies benefit the public by providing a better understanding of the Earth’s resources, hazards, and history. Knowledge of metamorphic processes can help to locate valuable mineral deposits, assess the stability of geological formations, and predict the effects of climate change on the landscape.
18.3. What Are Some Notable Publications on Metamorphic Rocks?
Notable publications on metamorphic rocks include textbooks on petrology and metamorphism, research articles in scientific journals, and geological maps and reports. These resources provide detailed information about the formation, composition, and distribution of metamorphic rocks.
19. How Does Climate Change Affect Metamorphic Rock Landscapes?
Climate change affects metamorphic rock landscapes through increased weathering, changes in vegetation patterns, and shifts in water availability. These changes can alter the appearance and stability of rock formations and impact the ecosystems they support.
19.1. How Does Increased Weathering Impact Rock Formations?
Increased weathering, driven by higher temperatures and more extreme weather events, can accelerate the breakdown of rock formations. This can lead to increased erosion, landslides, and changes in the appearance of the landscape.
19.2. What Role Does Vegetation Play in Rock Stability?
Vegetation plays a vital role in rock stability by anchoring soil, reducing erosion, and intercepting rainfall. Changes in vegetation patterns, driven by climate change, can affect the stability of rock formations and the health of surrounding ecosystems.
19.3. How Does Water Availability Influence Metamorphic Landscapes?
Water availability influences metamorphic landscapes by controlling weathering rates, vegetation patterns, and the formation of soil. Changes in precipitation patterns, driven by climate change, can alter these processes and impact the appearance and stability of the landscape.
20. What Future Innovations Are Expected in Rock Landscaping?
Future innovations in rock landscaping include the development of new sustainable materials, the use of advanced technologies for design and installation, and the integration of ecological principles into landscaping practices.
20.1. What New Materials Are Being Developed?
New materials being developed for rock landscaping include recycled aggregates, manufactured stones with enhanced durability, and bio-based binders for creating permeable paving materials. These materials offer sustainable alternatives to traditional rock products.
20.2. How Are Technologies Improving Landscaping Practices?
Technologies improving landscaping practices include 3D modeling software for design, GPS-guided equipment for installation, and remote sensing techniques for monitoring the health of landscapes. These technologies enable more efficient, precise, and sustainable landscaping practices.
20.3. What Ecological Principles Are Being Integrated?
Ecological principles being integrated into landscaping practices include the use of native plants, the creation of habitats for wildlife, and the management of water resources. These practices promote biodiversity, reduce environmental impact, and create more resilient landscapes.
Understanding metamorphic rock formation is crucial for creating beautiful and sustainable landscapes. With the right knowledge and materials, you can transform your outdoor spaces into stunning reflections of nature’s artistry.
For more inspiration, detailed information on various rock types, and expert advice on landscaping projects, visit rockscapes.net. Explore our extensive collection of design ideas and discover how to bring the enduring beauty of metamorphic rocks to your landscape. Contact us today to start planning your dream outdoor space. Address: 1151 S Forest Ave, Tempe, AZ 85281, United States. Phone: +1 (480) 965-9011. Website: rockscapes.net.
FAQ Section
1. What exactly is a metamorphic rock?
A metamorphic rock is one that has been transformed from an existing rock type—sedimentary, igneous, or another metamorphic rock—due to changes in temperature, pressure, or chemical environment.
2. Where does metamorphic rock formation typically occur?
Metamorphic rock formation commonly occurs deep beneath the Earth’s surface where conditions of high heat and pressure are prevalent, or in areas where tectonic plates collide.
3. What are the main types of metamorphism?
The primary types of metamorphism are regional metamorphism, which affects large areas, and contact metamorphism, which occurs when rock is heated by nearby magma.
4. How does regional metamorphism differ from contact metamorphism?
Regional metamorphism involves widespread changes due to large-scale tectonic activity, while contact metamorphism is localized, resulting from the heat of magma altering adjacent rocks.
5. What role do fluids play in the process of metamorphism?
Fluids, especially water containing dissolved ions, act as catalysts, speeding up chemical reactions and aiding in the transport of elements during the metamorphic process.
6. Can you name a few common examples of metamorphic rocks?
Examples include marble, which comes from limestone; quartzite, from sandstone; and gneiss and schist, which can originate from various rock types.
7. What is foliation, and why is it important in metamorphic rocks?
Foliation is the layering or banding seen in some metamorphic rocks, resulting from the alignment of minerals under pressure. It helps in identifying the type and intensity of metamorphism.
8. What are non-foliated metamorphic rocks, and how do they form?
Non-foliated metamorphic rocks like marble and quartzite do not have layers. They form when the original rock lacked platy minerals or when pressure was uniform.
9. What makes metamorphic rocks useful in landscaping?
Metamorphic rocks are highly valued in landscaping for their durability, unique textures, and aesthetic appeal, making them ideal for walls, pathways, and decorative features.
10. How do scientists study metamorphic rocks to understand Earth’s history?
Scientists study metamorphic rocks through field observations, laboratory analyses, and computer modeling, which help reveal the conditions and processes that shaped the Earth’s crust over millions of years.
