Reservoirs of Ancient Lava Shaped Earth

Geological history has periodically featured giant lava eruptions that coat large swaths of land or ocean floor with basaltic lava, which hardens into rock formations called flood basalt. New research from Matthew Jackson and Richard Carlson proposes that the remnants of six of the largest volcanic events of the past 250 million years contain traces of the ancient Earth's primitive mantle -- which existed before the largely differentiated mantle of today -- offering clues to the geochemical history of the planet.

Scientists recently discovered that an area in northern Canada and Greenland composed of flood basalt contains traces of ancient Earth's primitive mantle. Carlson and Jackson's research expanded these findings, in order to determine if other large volcanic rock deposits also derive from primitive sources.

Information about the primitive mantle reservoir -- which came into existence after Earth's core formed but before Earth's outer rocky shell differentiated into crust and depleted mantle -- would teach scientists about the geochemistry of early Earth and how our planet arrived at its present state.

Until recently, scientists believed that Earth's primitive mantle, such as the remnants found in northern Canada and Greenland, originated from a type of meteorite called carbonaceous chondrites. But comparisons of isotopes of the element neodymium between samples from Earth and samples from chondrites didn't produce the expected results, which suggested that modern mantle reservoirs may have evolved from something different.

Carlson, of Carnegie's Department of Terrestrial Magnetism, and Jackson, a former Carnegie fellow now at Boston University, examined the isotopic characteristics of flood basalts to determine whether they were created by a primitive mantle source, even if it wasn't a chondritic one.

They used geochemical techniques based on isotopes of neodymium and lead to compare basalts from the previously discovered 62-million-year-old primitive mantle source in northern Canada's Baffin Island and West Greenland to basalts from the South Pacific's Ontong-Java Plateau, which formed in the largest volcanic event in geologic history. They discovered minor differences in the isotopic compositions of the two basaltic provinces, but not beyond what could be expected in a primitive reservoir.

They compared these findings to basalts from four other large accumulations of lava-formed rocks in Botswana, Russia, India, and the Indian Ocean, and determined that lavas that have interacted with continental crust the least (and are thus less contaminated) have neodymium and lead isotopic compositions similar to an early-formed primitive mantle composition.

The presence of these early-earth signatures in the six flood basalts suggests that a significant fraction of the world's largest volcanic events originate from a modern mantle source that is similar to the primitive reservoir discovered in Baffin Island and West Greenland. This primitive mantle is hotter, due to a higher concentration of radioactive elements, and more easily melted than other mantle reservoirs. As a result, it could be more likely to generate the eruptions that form flood basalts.

So what is groundwater

Water stored underground: vital and vulnerable

So what is groundwater?

Rainwater percolates into the earth. Soil and rock are like a giant sponge, full of holes - typically tiny pores and cracks just millimetres in size. Below the water table, these holes are full of water. This is groundwater. Groundwater slowly travels through connected pores and cracks just centimetres to metres per year.

Water stored underground in cracks and pores

larger image [JPEG, 233.6 kb, 1000 X 876, notice]

Protecting the balance

Groundwater storage is like a bank account. The balance falls when withdrawals exceed deposits. Nature makes deposits through rainfall, and withdrawals through leakage of groundwater to streams and the ocean. Our wells represent further withdrawals. If total withdrawals exceed deposits, we deplete our groundwater storage. Do we know if we are draining our account?

Water table ups and downs through the seasons
The amount of water stored underground changes through the seasons. As winter and spring rains infiltrate the ground, stored groundwater increases and the water table rises. When the rains stop, the water table falls as groundwater leaks into streams and the ocean. Well pumping also removes water and lowers the water table. Excessive pumping of groundwater can result in long-term depletion of groundwater storage.

larger image [JPEG, 136.5 kb, 1000 X 535, notice]

Underground lakes and rivers?

Not on Bowen Island. Large underground streams and lakes only occur in limestone cave systems. Limestone is unique as it dissolves in water, allowing caves to form. Bowen Island's granitic and volcanic rocks do not dissolve in water and so lack cave systems.

Aquifers yield water via wells

larger image [JPEG, 216.5 kb, 1000 X 727, notice]

Tapping into water stored underground

Any body of rock or sediment that yields useful amounts of water is an aquifer. Bowen Island has two types of aquifer: fractured rock, and sand and gravel layers. The amount of water stored in fractured rock is typically limited, and these aquifers can run low during the summer drought. Sand and gravel can store more water and these aquifers are less likely to dry up in the summer. Shallow- dug wells can dry up as the water table falls during the summer.

Groundwater flows from upland recharge areas to valley discharge areas

larger image [JPEG, 178.2 kb, 1000 X 567, notice]

Ensuring our aquifers replenish

Most recharging of aquifers occurs in forested uplands and valley slopes, but land clearing, road building, and ditching reduce water infiltration by creating impermeable surfaces and diverting water into ditches and streams. Infiltration ponds along ditches can increase the return of water into the groundwater system.

Excessive pumping can reduce flow in streams

Okay

larger image [JPEG, 157.2 kb, 900 X 747, notice]

Oops! I dried up the stream
Groundwater springs feed streams year-round. They are the only source of stream water during the dry season. A pumped well draws down the nearby water table. Excessive pumping for an extended period of time can lower the water table over a broad area. This can divert groundwater from streams and even cause streams to dry up. Nothing damages a stream like taking away its water!

Overpumping

larger image [JPEG, 161.6 kb, 900 X 753, notice]

Are we depleting our groundwater?
To determine whether we are overpumping our island aquifers, we need a series of groundwater observation wells on Bowen Island. These are unused wells where water table levels can be regularly checked to determine long-term trends. Some groundwater monitoring has started on Bowen Island, but more observation wells are needed.

What is mean groundwater

What is groundwater?

When rain falls to the ground, the water does not stop moving. Some of it flows along the surface to streams or lakes, some of it is used by plants, some evaporates and returns to the atmosphere, and some sinks into the ground. Imagine pouring a glass of water onto a pile of sand. Where does the water go? The water moves into the spaces between the particles of sand.
Groundwater is water that is found underground in the cracks and spaces in soil, sand and rock. Groundwater is stored in--and moves slowly through--layers of soil, sand and rocks called aquifers. Aquifers typically consist of gravel, sand, sandstone, or fractured rock, like limestone. These materials are permeable because they have large connected spaces that allow water to flow through. The speed at which groundwater flows depends on the size of the spaces in the soil or rock and how well the spaces are connected.





The area where water fills the aquifer is called the saturated zone (or saturation zone). The top of this zone is called the water table. The water table may be located only a foot below the ground’s surface or it can sit hundreds of feet down.
Groundwater can be found almost everywhere. The water table may be deep or shallow; and may rise or fall depending on many factors. Heavy rains or melting snow may cause the water table to rise, or heavy pumping of groundwater supplies may cause the water table to fall.
Water in aquifers is brought to the surface naturally through a spring or can be discharged into lakes and streams. Groundwater can also be extracted through a well drilled into the aquifer. A well is a pipe in the ground that fills with groundwater. This water can be brought to the surface by a pump. Shallow wells may go dry if the water table falls below the bottom of the well. Some wells, called artesian wells, do not need a pump because of natural pressures that force the water up and out of the well.
Groundwater supplies are replenished, or recharged, by rain and snow melt. In some areas of the world, people face serious water shortages because groundwater is used faster than it is naturally replenished. In other areas groundwater is polluted by human activities.
In areas where material above the aquifer is permeable, pollutants can readily sink into groundwater supplies. Groundwater can be polluted by landfills, septic tanks, leaky underground gas tanks, and from overuse of fertilizers and pesticides. If groundwater becomes polluted, it will no longer be safe to drink.
Groundwater is used for drinking water by more than 50 percent of the people in the United States, including almost everyone who lives in rural areas. The largest use for groundwater is to irrigate crops.
It is important for all of us to learn to protect our groundwater because of its importance as a source of water for drinking and irrigation.