Read the passage. Circle the incorrect articles. Discuss in class the correct articles.
Mount St Helens in Washington, USA,
had been quiet for over the century.
Suddenly, in March 1980, small earth
tremors and eruptions of the gas
and rocks began. An bulge appeared
on its northern side, which grew 90
metres high by mid-May. There were
the frequent earth tremors. Authorities
kept sightseers away.
Answers
Answer:
Seismic station installation in Mount St. Helens's crater 1981 lava dome. USGS, in conjunction with the University of Washington, maintain seismic stations at Mount St. Helens. An increase in seismicity (earthquakes) is often the first precursor to an approaching eruption. (Credit: Lyn Topinka. Public domain.)
In an average month, 22 events are located by the Pacific Northwest Seismic Network (PNSN), with the number going far higher during eruptive periods. Although a few seismic stations were installed near Mount St. Helens in the 1970s, the first complete network of stations was installed in 1980 in response to unrest starting in March of that year. Since then millions of earthquakes, as well as other non-earthquake signals (e.g., rockfalls, explosions, avalanches, glacier quakes, helicopters) have been recorded. The PNSN earthquake catalog encompasses periods of precursory activity (preceding eruptions in 1980-1986 and 2004), seismicity associated with episodic explosive eruptions (1980) and dome growth (1980-1986 and 2004-2008), and times of relative quiet between eruptions (1987 - 2004 and 2008 to present). Seismic data recorded by this network have been used in many studies, including:
forecasting eruptions and detecting explosions
determining eruption dynamics
developing models of the magmatic system beneath Mount St. Helens
determining the on-the-ground processes responsible for various types of seismic signals
detecting repetitive events (or earthquakefamilies), including the first published study of repetitive events in a volcanic setting.
1980-1986
One of the intriguing aspects of volcano seismology is trying to determine the cause, volcanologic or otherwise, of a particular earthquake. This is illustrated by reviewing a plot of earthquake depths over time (right).
The May 18, 1980, eruption is marked by a vertical streak of earthquakes extending down well below 15 km (9.3 miles). Scientists believe that these earthquakes occurred when the May 18 eruption drained magma from deeper parts of the magmatic system, leaving voids of unsupported rock that then failed and produced earthquakes.
In contrast, the 1981-1986 time period was dominated by shallow earthquakes, mainly less than 2 km (1.2 mi) deep. These earthquakes occurred primarily as precursors to individual dome-building eruptions, and are thought to have occurred as a result of stress accumulations associated with magmamovement.
Earthquakes at Mount St. Helens from 1980-2012, plot represents dep...
Earthquakes at Mount St. Helens from 1980-2012, plot represents depth of earthquakes over time. Circle size relative to earthquake size. (Credit: Moran, Seth. Public domain.)
1987-2004
Although eruptions stopped after 1986, earthquakes continued to occur, including several years- long dense groups (swarms) of "deep" earthquakes (depths greater than 3 km, or 1.9 mi), which hadn't been seen since 1980. Scientists think these particular "deep" earthquakes probably occurred when new magma entered the system from below and caused the pressure to increase on the system. In contrast, shallow earthquakes from 1987-2004 probably occurred due to a combination of factors: magma left- over from the 1980-86 eruptions cooled, contracted, and created earthquakes; heated groundwater (hydrothermal fluids) circulated and added stress to the volcanic structure; and magma potentially accumulated below 3 km (1.8 mi), which added stress to the system.
Drumbeat pattern of earthquakes on a webicorder plot (digital seism...
The station recording this pattern of seismicity was called Yellow Rock (YEL), which was approximately 1 km north of the 2004-2008 vent. YEL was removed in 2007 before the advancing Crater Glacier reached it. The YEL site is now under a hundred or so feet of ice. (Credit: Moran, Seth. Public domain.)
2004-2008
The 2004-2008 eruption produced many earthquakes, with well over one million occurring in association with the construction of a new lava domecomplex. A notable phenomena observed during the eruption was the occurrence of very regularly spaced patterns of small earthquakes, which were dubbed "drumbeat" earthquakes. These consistently spaced seismic events accompanied the steady eruption of lava spines as they emerged from the Mount St. Helens conduit, and have since been reported at other volcanoes (e.g., Augustine Volcano in 2006).
Owing to the robust seismic data set for Mount St. Helens, scientists understand the character of typical eruptive and non-eruptive earthquake patterns at this volcano. These perspectives are vital assets when interpreting the significance of earthquake activity at the volcano, and they can help scientists to determine when a future eruption may occur. The PNSN's website is an excellent resource for viewing and mapping earthquakes at Mount St. Helens.