# Gravity drives the world around us

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Anyone who has ever slipped on a banana peel knows all too well the force of gravity. A fundamental force of nature, gravity is the attraction of one body of mass (such as a person) to another body of mass (such as the Earth). The gravitational pull that each body feels gets stronger the closer they are to each other.

The gravitational force keeps the planets in orbit about the Sun.

(Public domain.)

Learning to walk acquaints us all with the pull of the Earth's gravity. Another force that we experience every day is the gravitational pull of the moon. As the moon orbits the Earth, it pulls on the Earth's surface and everything on it. The most visible example of this is the rising and falling of the ocean's tide as it tries to keep up with this pull from the moon.

The moon's gravity not only moves the oceans, but also moves the ground beneath our feet. As the moon orbits, it pulls the Earth's crust slowly up and down, generating Earth tides. The pull from the sun's gravity also contributes to this motion, but not as much as the moon's, because it is so much farther away. We can't feel this motion (which can be over 10 inches, depending on how the sun and moon are aligned), because it happens so slowly over the course of a day, but very sensitive scientific instruments such as GPS (Global Positioning System) or gravimeters can measure these slight motions of the Earth's surface.

Gravity acts differently on different substances, depending on how dense the substance is (the amount of mass in a given volume). Air is much less dense than water, so bubbles under water feel a much smaller downward pull from the Earth's gravity, and they quickly make their way to the surface. Similarly, magma at depth can be much less dense than the surrounding rock, so it tries to rapidly escape to the surface and erupt.

The density of a substance can also be affected by its temperature. Most substances expand and become less dense as they heat up. In substances that can easily flow like liquids and gases, the cooler areas that are more dense will feel more gravitational pull and will sink, while the hotter, less dense areas will rise. This process is known as convection and is nature's most efficient way of distributing heat.

We can feel examples of convection in our atmosphere every day. Cool air sinks and warm air rises, creating areas of high and low barometric pressure, respectively. As the air flows from areas of high to low pressure, we feel the wind and sometimes suffer through the resulting storms if the atmospheric convection is strong enough. Hurricanes, typhoons, and tornadoes are all an expression of this process.

Examples of convection in liquids are as close as your stove top. When water is heated in a pan, the water in the bottom of the pan near the burner becomes hot, expands, and becomes less dense. It then rises to the top of the pan as cooler water flows down to take its place. This process starts slowly at first, but becomes more and more vigorous as the water approaches boiling.

On a much larger scale, a similar process happens in the world's oceans. In regions where oceanic waters are heated (for example, closer to the equator), water rises toward the surface, and in regions where oceanic waters are cooled, water sinks. The resulting convection produces global oceanic currents which are nature's way of evening out the ocean's temperature.

Deep within the Earth, a similar process occurs. The core of the Earth is very hot and heats the base of the Earth's mantle. As regions heat up, they expand and begin to flow toward the Earth's surface. This flow occurs very slowly, because the Earth's mantle is not really fluid, but very hot rock that can deform slowly under high pressure. This slow convection in the mantle transfers the extreme heat out of the Earth's core and provides one of the driving forces of plate tectonics.

Gravity is a force we can't see that affects our lives daily. It is a driving force behind geology, oceanic currents, weather, and, unfortunately, accidents involving banana peels.

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### Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. The summit caldera has been expanding, indicating inflation, since the beginning of 2007. The number of earthquakes located in the summit area is at low levels (usually fewer than 10 per day are large enough to locate).

Eruptive activity at Puu Oo continues. On clear nights, glow is visible from several vents within the crater. There have been a number of small lava flows in the last several weeks from vents on the southwest flank of Puu Oo, and from a hornito at the head of the PKK lava tube. Lava, however, continues to flow through the PKK lava tube and is feeding a persistent breakout that is often visible after dark streaming down the face of the pali. About 1 km south of Puu Oo, the Campout tube branches off from the PKK tube. The Campout tube carries lava to the ocean at Kamokuna located inside Hawaii Volcanoes National Park.

In the last week, intermittent breakouts from the Campout tube have also been seen on the slope of Pulama pali and on the coastal plain. An eastern branch of the Campout tube continues to send lava northeastward along the base of the pali where it is burning trees at the base of the Royal Gardens Subdivision. A western branch of the Campout tube hosts minor surface flows inland from the sea cliff at East Laeapuki.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

One earthquake beneath Hawaii Island was reported felt within the past week. A magnitude-1.9 earthquake occurred at 10:27 a.m. H.s.t. on Sunday, April 8, and was located 8 km (5 miles) southeast of Waimea at a depth of 12 km (7 miles).

Mauna Loa is not erupting. One earthquake was located beneath the summit. Extension of distances between locations spanning the summit, indicating inflation, continues at steady slow rates.